An Introduction to Programming in Emacs Lisp
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This is An Introduction to Programming in Emacs Lisp, for people who are not programmers.
Distributed with Emacs version 29.1.90.
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This master menu first lists each chapter and index; then it lists every node in every chapter.
Table of Contents
- Preface
- 1 List Processing
- 2 Practicing Evaluation
- 3 How To Write Function Definitions
- An Aside about Primitive Functions
- 3.1 The
defun
Macro - 3.2 Install a Function Definition
- 3.3 Make a Function Interactive
- 3.4 Different Options for
interactive
- 3.5 Install Code Permanently
- 3.6
let
- 3.7 The
if
Special Form - 3.8 If–then–else Expressions
- 3.9 Truth and Falsehood in Emacs Lisp
- 3.10
save-excursion
- 3.11 Review
- 3.12 Exercises
- 4 A Few Buffer-Related Functions
- 5 A Few More Complex Functions
- 5.1 The Definition of
copy-to-buffer
- 5.2 The Definition of
insert-buffer
- 5.3 Complete Definition of
beginning-of-buffer
- 5.4 Review
- 5.5
optional
Argument Exercise
- 5.1 The Definition of
- 6 Narrowing and Widening
- 7
car
,cdr
,cons
: Fundamental Functions - 8 Cutting and Storing Text
- 9 How Lists are Implemented
- 10 Yanking Text Back
- 11 Loops and Recursion
- 11.1
while
- 11.2 Save your time:
dolist
anddotimes
- 11.3 Recursion
- 11.4 Looping Exercise
- 11.1
- 12 Regular Expression Searches
- 13 Counting via Repetition and Regexps
- 14 Counting Words in a
defun
- Divide and Conquer
- 14.1 What to Count?
- 14.2 What Constitutes a Word or Symbol?
- 14.3 The
count-words-in-defun
Function - 14.4 Count Several
defuns
Within a File - 14.5 Find a File
- 14.6
lengths-list-file
in Detail - 14.7 Count Words in
defuns
in Different Files - 14.8 Recursively Count Words in Different Files
- 14.9 Prepare the Data for Display in a Graph
- 15 Readying a Graph
- 16 Your .emacs File
- Emacs’s Default Configuration
- 16.1 Site-wide Initialization Files
- 16.2 Specifying Variables using
defcustom
- 16.3 Beginning a .emacs File
- 16.4 Text and Auto Fill Mode
- 16.5 Mail Aliases
- 16.6 Indent Tabs Mode
- 16.7 Some Key Bindings
- 16.8 Keymaps
- 16.9 Loading Files
- 16.10 Autoloading
- 16.11 A Simple Extension:
line-to-top-of-window
- 16.12 X11 Colors
- 16.13 Miscellaneous Settings for a .emacs File
- 16.14 A Modified Mode Line
- 17 Debugging
- 18 Conclusion
- Appendix A The
the-the
Function - Appendix B Handling the Kill Ring
- Appendix C A Graph with Labeled Axes
- Appendix D Free Software and Free Manuals
- Appendix E GNU Free Documentation License
- Index
- About the Author
Short Table of Contents
- Preface
- 1 List Processing
- 2 Practicing Evaluation
- 3 How To Write Function Definitions
- 4 A Few Buffer-Related Functions
- 5 A Few More Complex Functions
- 6 Narrowing and Widening
- 7
car
,cdr
,cons
: Fundamental Functions - 8 Cutting and Storing Text
- 9 How Lists are Implemented
- 10 Yanking Text Back
- 11 Loops and Recursion
- 12 Regular Expression Searches
- 13 Counting via Repetition and Regexps
- 14 Counting Words in a
defun
- 15 Readying a Graph
- 16 Your .emacs File
- 17 Debugging
- 18 Conclusion
- Appendix A The
the-the
Function - Appendix B Handling the Kill Ring
- Appendix C A Graph with Labeled Axes
- Appendix D Free Software and Free Manuals
- Appendix E GNU Free Documentation License
- Index
- About the Author
Next: List Processing, Previous: An Introduction to Programming in Emacs Lisp, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
Preface
Most of the GNU Emacs integrated environment is written in the programming language called Emacs Lisp. The code written in this programming language is the software—the sets of instructions—that tell the computer what to do when you give it commands. Emacs is designed so that you can write new code in Emacs Lisp and easily install it as an extension to the editor.
(GNU Emacs is sometimes called an “extensible editor”, but it does much more than provide editing capabilities. It is better to refer to Emacs as an “extensible computing environment”. However, that phrase is quite a mouthful. It is easier to refer to Emacs simply as an editor. Moreover, everything you do in Emacs—find the Mayan date and phases of the moon, simplify polynomials, debug code, manage files, read letters, write books—all these activities are kinds of editing in the most general sense of the word.)
- Why Study Emacs Lisp?
- On Reading this Text
- For Whom This is Written
- Lisp History
- A Note for Novices
- Thank You
Next: On Reading this Text, Up: Preface [Contents][Index]
Why Study Emacs Lisp?
Although Emacs Lisp is usually thought of in association only with Emacs, it is a full computer programming language. You can use Emacs Lisp as you would any other programming language.
Perhaps you want to understand programming; perhaps you want to extend Emacs; or perhaps you want to become a programmer. This introduction to Emacs Lisp is designed to get you started: to guide you in learning the fundamentals of programming, and more importantly, to show you how you can teach yourself to go further.
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On Reading this Text
All through this document, you will see little sample programs you can run inside of Emacs. If you read this document in Info inside of GNU Emacs, you can run the programs as they appear. (This is easy to do and is explained when the examples are presented.) Alternatively, you can read this introduction as a printed book while sitting beside a computer running Emacs. (This is what I like to do; I like printed books.) If you don’t have a running Emacs beside you, you can still read this book, but in this case, it is best to treat it as a novel or as a travel guide to a country not yet visited: interesting, but not the same as being there.
Much of this introduction is dedicated to walkthroughs or guided tours of code used in GNU Emacs. These tours are designed for two purposes: first, to give you familiarity with real, working code (code you use every day); and, second, to give you familiarity with the way Emacs works. It is interesting to see how a working environment is implemented. Also, I hope that you will pick up the habit of browsing through source code. You can learn from it and mine it for ideas. Having GNU Emacs is like having a dragon’s cave of treasures.
In addition to learning about Emacs as an editor and Emacs Lisp as a
programming language, the examples and guided tours will give you an
opportunity to get acquainted with Emacs as a Lisp programming
environment. GNU Emacs supports programming and provides tools that
you will want to become comfortable using, such as M-. (the key
which invokes the xref-find-definitions
command). You will
also learn about buffers and other objects that are part of the
environment. Learning about these features of Emacs is like learning
new routes around your home town.
Finally, I hope to convey some of the skills for using Emacs to learn aspects of programming that you don’t know. You can often use Emacs to help you understand what puzzles you or to find out how to do something new. This self-reliance is not only a pleasure, but an advantage.
Next: Lisp History, Previous: On Reading this Text, Up: Preface [Contents][Index]
For Whom This is Written
This text is written as an elementary introduction for people who are not programmers. If you are a programmer, you may not be satisfied with this primer. The reason is that you may have become expert at reading reference manuals and be put off by the way this text is organized.
An expert programmer who reviewed this text said to me:
I prefer to learn from reference manuals. I “dive into” each paragraph, and “come up for air” between paragraphs.
When I get to the end of a paragraph, I assume that subject is done, finished, that I know everything I need (with the possible exception of the case when the next paragraph starts talking about it in more detail). I expect that a well written reference manual will not have a lot of redundancy, and that it will have excellent pointers to the (one) place where the information I want is.
This introduction is not written for this person!
Firstly, I try to say everything at least three times: first, to introduce it; second, to show it in context; and third, to show it in a different context, or to review it.
Secondly, I hardly ever put all the information about a subject in one place, much less in one paragraph. To my way of thinking, that imposes too heavy a burden on the reader. Instead I try to explain only what you need to know at the time. (Sometimes I include a little extra information so you won’t be surprised later when the additional information is formally introduced.)
When you read this text, you are not expected to learn everything the first time. Frequently, you need make only a nodding acquaintance with some of the items mentioned. My hope is that I have structured the text and given you enough hints that you will be alert to what is important, and concentrate on it.
You will need to dive into some paragraphs; there is no other way to read them. But I have tried to keep down the number of such paragraphs. This book is intended as an approachable hill, rather than as a daunting mountain.
This book, An Introduction to Programming in Emacs Lisp, has a companion document, The GNU Emacs Lisp Reference Manual in The GNU Emacs Lisp Reference Manual. The reference manual has more detail than this introduction. In the reference manual, all the information about one topic is concentrated in one place. You should turn to it if you are like the programmer quoted above. And, of course, after you have read this Introduction, you will find the Reference Manual useful when you are writing your own programs.
Next: A Note for Novices, Previous: For Whom This is Written, Up: Preface [Contents][Index]
Lisp History
Lisp was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it superior for other purposes as well, such as writing editor commands and integrated environments.
GNU Emacs Lisp is largely inspired by Maclisp, which was written at MIT in the 1960s. It is somewhat inspired by Common Lisp, which became a standard in the 1980s. However, Emacs Lisp is much simpler than Common Lisp. (The standard Emacs distribution contains an optional extensions file, cl-lib.el, that adds many Common Lisp features to Emacs Lisp.)
Next: Thank You, Previous: Lisp History, Up: Preface [Contents][Index]
A Note for Novices
If you don’t know GNU Emacs, you can still read this document profitably. However, I recommend you learn Emacs, if only to learn to move around your computer screen. You can teach yourself how to use Emacs with the built-in tutorial. To use it, type C-h t. (This means you press and release the CTRL key and the h at the same time, and then press and release t.)
Also, I often refer to one of Emacs’s standard commands by listing the
keys which you press to invoke the command and then giving the name of
the command in parentheses, like this: M-C-\
(indent-region
). What this means is that the
indent-region
command is customarily invoked by typing
M-C-\. (You can, if you wish, change the keys that are typed to
invoke the command; this is called rebinding. See Keymaps.) The abbreviation M-C-\ means that you type your
META key, CTRL key and \ key all at the same time.
(On many modern keyboards the META key is labeled
ALT.)
Sometimes a combination like this is called a keychord, since it is
similar to the way you play a chord on a piano. If your keyboard does
not have a META key, the ESC key prefix is used in place
of it. In this case, M-C-\ means that you press and release your
ESC key and then type the CTRL key and the \ key at
the same time. But usually M-C-\ means press the CTRL key
along with the key that is labeled ALT and, at the same time,
press the \ key.
In addition to typing a lone keychord, you can prefix what you type with C-u, which is called the universal argument. The C-u keychord passes an argument to the subsequent command. Thus, to indent a region of plain text by 6 spaces, mark the region, and then type C-u 6 M-C-\. (If you do not specify a number, Emacs either passes the number 4 to the command or otherwise runs the command differently than it would otherwise.) See Numeric Arguments in The GNU Emacs Manual.
If you are reading this in Info using GNU Emacs, you can read through this whole document just by pressing the space bar, SPC. (To learn about Info, type C-h i and then select Info.)
A note on terminology: when I use the word Lisp alone, I often am referring to the various dialects of Lisp in general, but when I speak of Emacs Lisp, I am referring to GNU Emacs Lisp in particular.
Previous: A Note for Novices, Up: Preface [Contents][Index]
Thank You
My thanks to all who helped me with this book. My especial thanks to Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Frank Ritter, Randy Smith, Richard M. Stallman, and Melissa Weisshaus. My thanks also go to both Philip Johnson and David Stampe for their patient encouragement. My mistakes are my own.
Robert J. Chassell bob@gnu.org
Next: Practicing Evaluation, Previous: Preface, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
1 List Processing
To the untutored eye, Lisp is a strange programming language. In Lisp code there are parentheses everywhere. Some people even claim that the name stands for “Lots of Isolated Silly Parentheses”. But the claim is unwarranted. Lisp stands for LISt Processing, and the programming language handles lists (and lists of lists) by putting them between parentheses. The parentheses mark the boundaries of the list. Sometimes a list is preceded by an apostrophe ‘'’, called a single-quote in Lisp.1 Lists are the basis of Lisp.
- Lisp Lists
- Run a Program
- Generate an Error Message
- Symbol Names and Function Definitions
- The Lisp Interpreter
- Evaluation
- Variables
- Arguments
- Setting the Value of a Variable
- Summary
- Exercises
Next: Run a Program, Up: List Processing [Contents][Index]
1.1 Lisp Lists
In Lisp, a list looks like this: '(rose violet daisy buttercup)
.
This list is preceded by a single apostrophe. It could just as well be
written as follows, which looks more like the kind of list you are likely
to be familiar with:
'(rose violet daisy buttercup)
The elements of this list are the names of the four different flowers, separated from each other by whitespace and surrounded by parentheses, like flowers in a field with a stone wall around them.
Next: Lisp Atoms, Up: Lisp Lists [Contents][Index]
Numbers, Lists inside of Lists
Lists can also have numbers in them, as in this list: (+ 2 2)
.
This list has a plus-sign, ‘+’, followed by two ‘2’s, each
separated by whitespace.
In Lisp, both data and programs are represented the same way; that is, they are both lists of words, numbers, or other lists, separated by whitespace and surrounded by parentheses. (Since a program looks like data, one program may easily serve as data for another; this is a very powerful feature of Lisp.) (Incidentally, these two parenthetical remarks are not Lisp lists, because they contain ‘;’ and ‘.’ as punctuation marks.)
Here is another list, this time with a list inside of it:
'(this list has (a list inside of it))
The components of this list are the words ‘this’, ‘list’, ‘has’, and the list ‘(a list inside of it)’. The interior list is made up of the words ‘a’, ‘list’, ‘inside’, ‘of’, ‘it’.
Next: Whitespace in Lists, Previous: Numbers, Lists inside of Lists, Up: Lisp Lists [Contents][Index]
1.1.1 Lisp Atoms
In Lisp, what we have been calling words are called atoms. This
term comes from the historical meaning of the word atom, which means
“indivisible”. As far as Lisp is concerned, the words we have been
using in the lists cannot be divided into any smaller parts and still
mean the same thing as part of a program; likewise with numbers and
single character symbols like ‘+’. On the other hand, unlike an
ancient atom, a list can be split into parts. (See car
cdr
& cons
Fundamental Functions.)
In a list, atoms are separated from each other by whitespace. They can be right next to a parenthesis.
Technically speaking, a list in Lisp consists of parentheses surrounding
atoms separated by whitespace or surrounding other lists or surrounding
both atoms and other lists. A list can have just one atom in it or
have nothing in it at all. A list with nothing in it looks like this:
()
, and is called the empty list. Unlike anything else, an
empty list is considered both an atom and a list at the same time.
The printed representation of both atoms and lists are called symbolic expressions or, more concisely, s-expressions. The word expression by itself can refer to either the printed representation, or to the atom or list as it is held internally in the computer. Often, people use the term expression indiscriminately. (Also, in many texts, the word form is used as a synonym for expression.)
Incidentally, the atoms that make up our universe were named such when they were thought to be indivisible; but it has been found that physical atoms are not indivisible. Parts can split off an atom or it can fission into two parts of roughly equal size. Physical atoms were named prematurely, before their truer nature was found. In Lisp, certain kinds of atom, such as an array, can be separated into parts; but the mechanism for doing this is different from the mechanism for splitting a list. As far as list operations are concerned, the atoms of a list are unsplittable.
As in English, the meanings of the component letters of a Lisp atom are different from the meaning the letters make as a word. For example, the word for the South American sloth, the ‘ai’, is completely different from the two words, ‘a’, and ‘i’.
There are many kinds of atom in nature but only a few in Lisp: for example, numbers, such as 37, 511, or 1729, and symbols, such as ‘+’, ‘foo’, or ‘forward-line’. The words we have listed in the examples above are all symbols. In everyday Lisp conversation, the word “atom” is not often used, because programmers usually try to be more specific about what kind of atom they are dealing with. Lisp programming is mostly about symbols (and sometimes numbers) within lists. (Incidentally, the preceding three word parenthetical remark is a proper list in Lisp, since it consists of atoms, which in this case are symbols, separated by whitespace and enclosed by parentheses, without any non-Lisp punctuation.)
Text between double quotation marks—even sentences or paragraphs—is also an atom. Here is an example:
'(this list includes "text between quotation marks.")
In Lisp, all of the quoted text including the punctuation mark and the blank spaces is a single atom. This kind of atom is called a string (for “string of characters”) and is the sort of thing that is used for messages that a computer can print for a human to read. Strings are a different kind of atom than numbers or symbols and are used differently.
Next: GNU Emacs Helps You Type Lists, Previous: Lisp Atoms, Up: Lisp Lists [Contents][Index]
1.1.2 Whitespace in Lists
The amount of whitespace in a list does not matter. From the point of view of the Lisp language,
'(this list looks like this)
is exactly the same as this:
'(this list looks like this)
Both examples show what to Lisp is the same list, the list made up of the symbols ‘this’, ‘list’, ‘looks’, ‘like’, and ‘this’ in that order.
Extra whitespace and newlines are designed to make a list more readable by humans. When Lisp reads the expression, it gets rid of all the extra whitespace (but it needs to have at least one space between atoms in order to tell them apart.)
Odd as it seems, the examples we have seen cover almost all of what Lisp lists look like! Every other list in Lisp looks more or less like one of these examples, except that the list may be longer and more complex. In brief, a list is between parentheses, a string is between quotation marks, a symbol looks like a word, and a number looks like a number. (For certain situations, square brackets, dots and a few other special characters may be used; however, we will go quite far without them.)
Previous: Whitespace in Lists, Up: Lisp Lists [Contents][Index]
1.1.3 GNU Emacs Helps You Type Lists
When you type a Lisp expression in GNU Emacs using either Lisp Interaction mode or Emacs Lisp mode, you have available to you several commands to format the Lisp expression so it is easy to read. For example, pressing the TAB key automatically indents the line the cursor is on by the right amount. A command to properly indent the code in a region is customarily bound to M-C-\. Indentation is designed so that you can see which elements of a list belong to which list—elements of a sub-list are indented more than the elements of the enclosing list.
In addition, when you type a closing parenthesis, Emacs momentarily jumps the cursor back to the matching opening parenthesis, so you can see which one it is. This is very useful, since every list you type in Lisp must have its closing parenthesis match its opening parenthesis. (See Major Modes in The GNU Emacs Manual, for more information about Emacs’s modes.)
Next: Generate an Error Message, Previous: Lisp Lists, Up: List Processing [Contents][Index]
1.2 Run a Program
A list in Lisp—any list—is a program ready to run. If you run it (for which the Lisp jargon is evaluate), the computer will do one of three things: do nothing except return to you the list itself; send you an error message; or, treat the first symbol in the list as a command to do something. (Usually, of course, it is the last of these three things that you really want!)
The single apostrophe, '
, that I put in front of some of the
example lists in preceding sections is called a quote; when it
precedes a list, it tells Lisp to do nothing with the list, other than
take it as it is written. But if there is no quote preceding a list,
the first item of the list is special: it is a command for the computer
to obey. (In Lisp, these commands are called functions.) The list
(+ 2 2)
shown above did not have a quote in front of it, so Lisp
understands that the +
is an instruction to do something with the
rest of the list: add the numbers that follow.
If you are reading this inside of GNU Emacs in Info, here is how you can evaluate such a list: place your cursor immediately after the right hand parenthesis of the following list and then type C-x C-e:
(+ 2 2)
You will see the number 4
appear in the echo area2. (What you have just done is evaluate the list. The echo area is
the line at the bottom of the screen that displays or echoes text.)
Now try the same thing with a quoted list: place the cursor right
after the following list and type C-x C-e:
'(this is a quoted list)
You will see (this is a quoted list)
appear in the echo area.
In both cases, what you are doing is giving a command to the program inside of GNU Emacs called the Lisp interpreter—giving the interpreter a command to evaluate the expression. The name of the Lisp interpreter comes from the word for the task done by a human who comes up with the meaning of an expression—who interprets it.
You can also evaluate an atom that is not part of a list—one that is not surrounded by parentheses; again, the Lisp interpreter translates from the humanly readable expression to the language of the computer. But before discussing this (see Variables), we will discuss what the Lisp interpreter does when you make an error.
Next: Symbol Names and Function Definitions, Previous: Run a Program, Up: List Processing [Contents][Index]
1.3 Generate an Error Message
Partly so you won’t worry if you do it accidentally, we will now give a command to the Lisp interpreter that generates an error message. This is a harmless activity; and indeed, we will often try to generate error messages intentionally. Once you understand the jargon, error messages can be informative. Instead of being called “error” messages, they should be called “help” messages. They are like signposts to a traveler in a strange country; deciphering them can be hard, but once understood, they can point the way.
The error message is generated by a built-in GNU Emacs debugger. We
will enter the debugger. You get out of the debugger by typing q
.
What we will do is evaluate a list that is not quoted and does not have a meaningful command as its first element. Here is a list almost exactly the same as the one we just used, but without the single-quote in front of it. Position the cursor right after it and type C-x C-e:
(this is an unquoted list)
A *Backtrace* window will open up and you should see the following in it:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function this) (this is an unquoted list) eval((this is an unquoted list) nil) elisp--eval-last-sexp(nil) eval-last-sexp(nil) funcall-interactively(eval-last-sexp nil) call-interactively(eval-last-sexp nil nil) command-execute(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
Your cursor will be in this window (you may have to wait a few seconds before it becomes visible). To quit the debugger and make the debugger window go away, type:
q
Please type q right now, so you become confident that you can get out of the debugger. Then, type C-x C-e again to re-enter it.
Based on what we already know, we can almost read this error message.
You read the *Backtrace* buffer from the bottom up; it tells
you what Emacs did. When you typed C-x C-e, you made an
interactive call to the command eval-last-sexp
. eval
is
an abbreviation for “evaluate” and sexp
is an abbreviation for
“symbolic expression”. The command means “evaluate last symbolic
expression”, which is the expression just before your cursor.
Each line above tells you what the Lisp interpreter evaluated next. The most recent action is at the top. The buffer is called the *Backtrace* buffer because it enables you to track Emacs backwards.
At the top of the *Backtrace* buffer, you see the line:
Debugger entered--Lisp error: (void-function this)
The Lisp interpreter tried to evaluate the first atom of the list, the word ‘this’. It is this action that generated the error message ‘void-function this’.
The message contains the words ‘void-function’ and ‘this’.
The word ‘function’ was mentioned once before. It is a very important word. For our purposes, we can define it by saying that a function is a set of instructions to the computer that tell the computer to do something.
Now we can begin to understand the error message: ‘void-function this’. The function (that is, the word ‘this’) does not have a definition of any set of instructions for the computer to carry out.
The slightly odd word, ‘void-function’, is designed to cover the way Emacs Lisp is implemented, which is that when a symbol does not have a function definition attached to it, the place that should contain the instructions is void.
On the other hand, since we were able to add 2 plus 2 successfully, by
evaluating (+ 2 2)
, we can infer that the symbol +
must
have a set of instructions for the computer to obey and those
instructions must be to add the numbers that follow the +
.
It is possible to prevent Emacs entering the debugger in cases like this. We do not explain how to do that here, but we will mention what the result looks like, because you may encounter a similar situation if there is a bug in some Emacs code that you are using. In such cases, you will see only one line of error message; it will appear in the echo area and look like this:
Symbol's function definition is void: this
The message goes away as soon as you type a key, even just to move the cursor.
We know the meaning of the word ‘Symbol’. It refers to the first atom of the list, the word ‘this’. The word ‘function’ refers to the instructions that tell the computer what to do. (Technically, the symbol tells the computer where to find the instructions, but this is a complication we can ignore for the moment.)
The error message can be understood: ‘Symbol's function definition is void: this’. The symbol (that is, the word ‘this’) lacks instructions for the computer to carry out.
Next: The Lisp Interpreter, Previous: Generate an Error Message, Up: List Processing [Contents][Index]
1.4 Symbol Names and Function Definitions
We can articulate another characteristic of Lisp based on what we have
discussed so far—an important characteristic: a symbol, like
+
, is not itself the set of instructions for the computer to
carry out. Instead, the symbol is used, perhaps temporarily, as a way
of locating the definition or set of instructions. What we see is the
name through which the instructions can be found. Names of people
work the same way. I can be referred to as ‘Bob’; however, I am
not the letters ‘B’, ‘o’, ‘b’ but am, or was, the
consciousness consistently associated with a particular life-form.
The name is not me, but it can be used to refer to me.
In Lisp, one set of instructions can be attached to several names.
For example, the computer instructions for adding numbers can be
linked to the symbol plus
as well as to the symbol +
(and are in some dialects of Lisp). Among humans, I can be referred
to as ‘Robert’ as well as ‘Bob’ and by other words as well.
On the other hand, a symbol can have only one function definition attached to it at a time. Otherwise, the computer would be confused as to which definition to use. If this were the case among people, only one person in the world could be named ‘Bob’. However, the function definition to which the name refers can be changed readily. (See Install a Function Definition.)
Since Emacs Lisp is large, it is customary to name symbols in a way that identifies the part of Emacs to which the function belongs. Thus, all the names for functions that deal with Texinfo start with ‘texinfo-’ and those for functions that deal with reading mail start with ‘rmail-’.
Next: Evaluation, Previous: Symbol Names and Function Definitions, Up: List Processing [Contents][Index]
1.5 The Lisp Interpreter
Based on what we have seen, we can now start to figure out what the Lisp interpreter does when we command it to evaluate a list. First, it looks to see whether there is a quote before the list; if there is, the interpreter just gives us the list. On the other hand, if there is no quote, the interpreter looks at the first element in the list and sees whether it has a function definition. If it does, the interpreter carries out the instructions in the function definition. Otherwise, the interpreter prints an error message.
This is how Lisp works. Simple. There are added complications which we will get to in a minute, but these are the fundamentals. Of course, to write Lisp programs, you need to know how to write function definitions and attach them to names, and how to do this without confusing either yourself or the computer.
Next: Byte Compiling, Up: The Lisp Interpreter [Contents][Index]
Complications
Now, for the first complication. In addition to lists, the Lisp interpreter can evaluate a symbol that is not quoted and does not have parentheses around it. The Lisp interpreter will attempt to determine the symbol’s value as a variable. This situation is described in the section on variables. (See Variables.)
The second complication occurs because some functions are unusual and do not work in the usual manner. Those that don’t are called special forms. They are used for special jobs, like defining a function, and there are not many of them. In the next few chapters, you will be introduced to several of the more important special forms.
As well as special forms, there are also macros. A macro is a construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression that is to be evaluated in place of the original expression. (See Lisp macro.)
For the purposes of this introduction, you do not need to worry too much
about whether something is a special form, macro, or ordinary function.
For example, if
is a special form (see The if
Special Form), but when
is a macro (see Lisp macro). In earlier versions of Emacs,
defun
was a special form, but now it is a macro (see The defun
Macro).
It still behaves in the same way.
The final complication is this: if the function that the Lisp interpreter is looking at is not a special form, and if it is part of a list, the Lisp interpreter looks to see whether the list has a list inside of it. If there is an inner list, the Lisp interpreter first figures out what it should do with the inside list, and then it works on the outside list. If there is yet another list embedded inside the inner list, it works on that one first, and so on. It always works on the innermost list first. The interpreter works on the innermost list first, to evaluate the result of that list. The result may be used by the enclosing expression.
Otherwise, the interpreter works left to right, from one expression to the next.
Previous: Complications, Up: The Lisp Interpreter [Contents][Index]
1.5.1 Byte Compiling
One other aspect of interpreting: the Lisp interpreter is able to interpret two kinds of entity: humanly readable code, on which we will focus exclusively, and specially processed code, called byte compiled code, which is not humanly readable. Byte compiled code runs faster than humanly readable code.
You can transform humanly readable code into byte compiled code by
running one of the compile commands such as byte-compile-file
.
Byte compiled code is usually stored in a file that ends with a
.elc extension rather than a .el extension. You will
see both kinds of file in the emacs/lisp directory; the files
to read are those with .el extensions.
As a practical matter, for most things you might do to customize or extend Emacs, you do not need to byte compile; and I will not discuss the topic here. See Byte Compilation in The GNU Emacs Lisp Reference Manual, for a full description of byte compilation.
Next: Variables, Previous: The Lisp Interpreter, Up: List Processing [Contents][Index]
1.6 Evaluation
When the Lisp interpreter works on an expression, the term for the activity is called evaluation. We say that the interpreter “evaluates the expression”. I’ve used this term several times before. The word comes from its use in everyday language, “to ascertain the value or amount of; to appraise”, according to Webster’s New Collegiate Dictionary.
Next: Evaluating Inner Lists, Up: Evaluation [Contents][Index]
How the Lisp Interpreter Acts
After evaluating an expression, the Lisp interpreter will most likely return the value that the computer produces by carrying out the instructions it found in the function definition, or perhaps it will give up on that function and produce an error message. (The interpreter may also find itself tossed, so to speak, to a different function or it may attempt to repeat continually what it is doing for ever and ever in an infinite loop. These actions are less common; and we can ignore them.) Most frequently, the interpreter returns a value.
At the same time the interpreter returns a value, it may do something else as well, such as move a cursor or copy a file; this other kind of action is called a side effect. Actions that we humans think are important, such as printing results, are often side effects to the Lisp interpreter. It is fairly easy to learn to use side effects.
In summary, evaluating a symbolic expression most commonly causes the Lisp interpreter to return a value and perhaps carry out a side effect; or else produce an error.
Previous: How the Lisp Interpreter Acts, Up: Evaluation [Contents][Index]
1.6.1 Evaluating Inner Lists
If evaluation applies to a list that is inside another list, the outer list may use the value returned by the first evaluation as information when the outer list is evaluated. This explains why inner expressions are evaluated first: the values they return are used by the outer expressions.
We can investigate this process by evaluating another addition example. Place your cursor after the following expression and type C-x C-e:
(+ 2 (+ 3 3))
The number 8 will appear in the echo area.
What happens is that the Lisp interpreter first evaluates the inner
expression, (+ 3 3)
, for which the value 6 is returned; then it
evaluates the outer expression as if it were written (+ 2 6)
, which
returns the value 8. Since there are no more enclosing expressions to
evaluate, the interpreter prints that value in the echo area.
Now it is easy to understand the name of the command invoked by the
keystrokes C-x C-e: the name is eval-last-sexp
. The
letters sexp
are an abbreviation for “symbolic expression”, and
eval
is an abbreviation for “evaluate”. The command
evaluates the last symbolic expression.
As an experiment, you can try evaluating the expression by putting the cursor at the beginning of the next line immediately following the expression, or inside the expression.
Here is another copy of the expression:
(+ 2 (+ 3 3))
If you place the cursor at the beginning of the blank line that
immediately follows the expression and type C-x C-e, you will
still get the value 8 printed in the echo area. Now try putting the
cursor inside the expression. If you put it right after the next to
last parenthesis (so it appears to sit on top of the last parenthesis),
you will get a 6 printed in the echo area! This is because the command
evaluates the expression (+ 3 3)
.
Now put the cursor immediately after a number. Type C-x C-e and
you will get the number itself. In Lisp, if you evaluate a number, you
get the number itself—this is how numbers differ from symbols. If you
evaluate a list starting with a symbol like +
, you will get a
value returned that is the result of the computer carrying out the
instructions in the function definition attached to that name. If a
symbol by itself is evaluated, something different happens, as we will
see in the next section.
Next: Arguments, Previous: Evaluation, Up: List Processing [Contents][Index]
1.7 Variables
In Emacs Lisp, a symbol can have a value attached to it just as it can have a function definition attached to it. The two are different. The function definition is a set of instructions that a computer will obey. A value, on the other hand, is something, such as number or a name, that can vary (which is why such a symbol is called a variable). The value of a symbol can be any expression in Lisp, such as a symbol, number, list, or string. A symbol that has a value is often called a variable.
A symbol can have both a function definition and a value attached to it at the same time. Or it can have just one or the other. The two are separate. This is somewhat similar to the way the name Cambridge can refer to the city in Massachusetts and have some information attached to the name as well, such as “great programming center”.
Another way to think about this is to imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice versa.
fill-column
, an Example Variable- Error Message for a Symbol Without a Function
- Error Message for a Symbol Without a Value
Next: Error Message for a Symbol Without a Function, Up: Variables [Contents][Index]
fill-column
, an Example Variable
The variable fill-column
illustrates a symbol with a value
attached to it: in every GNU Emacs buffer, this symbol is set to some
value, usually 72 or 70, but sometimes to some other value. To find the
value of this symbol, evaluate it by itself. If you are reading this in
Info inside of GNU Emacs, you can do this by putting the cursor after
the symbol and typing C-x C-e:
fill-column
After I typed C-x C-e, Emacs printed the number 72 in my echo
area. This is the value for which fill-column
is set for me as I
write this. It may be different for you in your Info buffer. Notice
that the value returned as a variable is printed in exactly the same way
as the value returned by a function carrying out its instructions. From
the point of view of the Lisp interpreter, a value returned is a value
returned. What kind of expression it came from ceases to matter once
the value is known.
A symbol can have any value attached to it or, to use the jargon, we can
bind the variable to a value: to a number, such as 72; to a
string, "such as this"
; to a list, such as (spruce pine
oak)
; we can even bind a variable to a function definition.
A symbol can be bound to a value in several ways. See Setting the Value of a Variable, for information about one way to do this.
Next: Error Message for a Symbol Without a Value, Previous: fill-column
, an Example Variable, Up: Variables [Contents][Index]
1.7.1 Error Message for a Symbol Without a Function
When we evaluated fill-column
to find its value as a variable,
we did not place parentheses around the word. This is because we did
not intend to use it as a function name.
If fill-column
were the first or only element of a list, the
Lisp interpreter would attempt to find the function definition
attached to it. But fill-column
has no function definition.
Try evaluating this:
(fill-column)
You will create a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function fill-column) (fill-column) eval((fill-column) nil) elisp--eval-last-sexp(nil) eval-last-sexp(nil) funcall-interactively(eval-last-sexp nil) call-interactively(eval-last-sexp nil nil) command-execute(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(Remember, to quit the debugger and make the debugger window go away, type q in the *Backtrace* buffer.)
Previous: Error Message for a Symbol Without a Function, Up: Variables [Contents][Index]
1.7.2 Error Message for a Symbol Without a Value
If you attempt to evaluate a symbol that does not have a value bound to
it, you will receive an error message. You can see this by
experimenting with our 2 plus 2 addition. In the following expression,
put your cursor right after the +
, before the first number 2,
type C-x C-e:
(+ 2 2)
In GNU Emacs 22, you will create a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-variable +) eval(+) elisp--eval-last-sexp(nil) eval-last-sexp(nil) funcall-interactively(eval-last-sexp nil) call-interactively(eval-last-sexp nil nil) command-execute(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(Again, you can quit the debugger by typing q in the *Backtrace* buffer.)
This backtrace is different from the very first error message we saw, which said, ‘Debugger entered--Lisp error: (void-function this)’. In this case, the function does not have a value as a variable; while in the other error message, the function (the word ‘this’) did not have a definition.
In this experiment with the +
, what we did was cause the Lisp
interpreter to evaluate the +
and look for the value of the
variable instead of the function definition. We did this by placing the
cursor right after the symbol rather than after the parenthesis of the
enclosing list as we did before. As a consequence, the Lisp interpreter
evaluated the preceding s-expression, which in this case was
+
by itself.
Since +
does not have a value bound to it, just the function
definition, the error message reported that the symbol’s value as a
variable was void.
Next: Setting the Value of a Variable, Previous: Variables, Up: List Processing [Contents][Index]
1.8 Arguments
To see how information is passed to functions, let’s look again at our old standby, the addition of two plus two. In Lisp, this is written as follows:
(+ 2 2)
If you evaluate this expression, the number 4 will appear in your echo
area. What the Lisp interpreter does is add the numbers that follow
the +
.
The numbers added by +
are called the arguments of the
function +
. These numbers are the information that is given to
or passed to the function.
The word “argument” comes from the way it is used in mathematics and
does not refer to a disputation between two people; instead it refers to
the information presented to the function, in this case, to the
+
. In Lisp, the arguments to a function are the atoms or lists
that follow the function. The values returned by the evaluation of
these atoms or lists are passed to the function. Different functions
require different numbers of arguments; some functions require none at
all.3
- Arguments’ Data Types
- An Argument as the Value of a Variable or List
- Variable Number of Arguments
- Using the Wrong Type Object as an Argument
- The
message
Function
Next: An Argument as the Value of a Variable or List, Up: Arguments [Contents][Index]
1.8.1 Arguments’ Data Types
The type of data that should be passed to a function depends on what
kind of information it uses. The arguments to a function such as
+
must have values that are numbers, since +
adds numbers.
Other functions use different kinds of data for their arguments.
For example, the concat
function links together or unites two or
more strings of text to produce a string. The arguments are strings.
Concatenating the two character strings abc
, def
produces
the single string abcdef
. This can be seen by evaluating the
following:
(concat "abc" "def")
The value produced by evaluating this expression is "abcdef"
.
A function such as substring
uses both a string and numbers as
arguments. The function returns a part of the string, a substring of
the first argument. This function takes three arguments. Its first
argument is the string of characters, the second and third arguments
are numbers that indicate the beginning (inclusive) and end
(exclusive) of the substring. The numbers are a count of the number
of characters (including spaces and punctuation) from the beginning of
the string. Note that the characters in a string are numbered from
zero, not one.
For example, if you evaluate the following:
(substring "The quick brown fox jumped." 16 19)
you will see "fox"
appear in the echo area. The arguments are the
string and the two numbers.
Note that the string passed to substring
is a single atom even
though it is made up of several words separated by spaces. Lisp counts
everything between the two quotation marks as part of the string,
including the spaces. You can think of the substring
function as
a kind of atom smasher since it takes an otherwise indivisible atom
and extracts a part. However, substring
is only able to extract
a substring from an argument that is a string, not from another type of
atom such as a number or symbol.
Next: Variable Number of Arguments, Previous: Arguments’ Data Types, Up: Arguments [Contents][Index]
1.8.2 An Argument as the Value of a Variable or List
An argument can be a symbol that returns a value when it is evaluated.
For example, when the symbol fill-column
by itself is evaluated,
it returns a number. This number can be used in an addition.
Position the cursor after the following expression and type C-x C-e:
(+ 2 fill-column)
The value will be a number two more than what you get by evaluating
fill-column
alone. For me, this is 74, because my value of
fill-column
is 72.
As we have just seen, an argument can be a symbol that returns a value
when evaluated. In addition, an argument can be a list that returns a
value when it is evaluated. For example, in the following expression,
the arguments to the function concat
are the strings
"The "
and " red foxes."
and the list
(number-to-string (+ 2 fill-column))
.
(concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")
If you evaluate this expression—and if, as with my Emacs,
fill-column
evaluates to 72—"The 74 red foxes."
will
appear in the echo area. (Note that you must put spaces after the
word ‘The’ and before the word ‘red’ so they will appear in
the final string. The function number-to-string
converts the
integer that the addition function returns to a string.
number-to-string
is also known as int-to-string
.)
Next: Using the Wrong Type Object as an Argument, Previous: An Argument as the Value of a Variable or List, Up: Arguments [Contents][Index]
1.8.3 Variable Number of Arguments
Some functions, such as concat
, +
or *
, take any
number of arguments. (The *
is the symbol for multiplication.)
This can be seen by evaluating each of the following expressions in
the usual way. What you will see in the echo area is printed in this
text after ‘⇒’, which you may read as “evaluates to”.
In the first set, the functions have no arguments:
(+) ⇒ 0 (*) ⇒ 1
In this set, the functions have one argument each:
(+ 3) ⇒ 3 (* 3) ⇒ 3
In this set, the functions have three arguments each:
(+ 3 4 5) ⇒ 12 (* 3 4 5) ⇒ 60
Next: The message
Function, Previous: Variable Number of Arguments, Up: Arguments [Contents][Index]
1.8.4 Using the Wrong Type Object as an Argument
When a function is passed an argument of the wrong type, the Lisp
interpreter produces an error message. For example, the +
function expects the values of its arguments to be numbers. As an
experiment we can pass it the quoted symbol hello
instead of a
number. Position the cursor after the following expression and type
C-x C-e:
(+ 2 'hello)
When you do this you will generate an error message. What has happened
is that +
has tried to add the 2 to the value returned by
'hello
, but the value returned by 'hello
is the symbol
hello
, not a number. Only numbers can be added. So +
could not carry out its addition.
You will create and enter a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p hello) +(2 hello) eval((+ 2 'hello) nil) elisp--eval-last-sexp(t) eval-last-sexp(nil) funcall-interactively(eval-print-last-sexp nil) call-interactively(eval-print-last-sexp nil nil) command-execute(eval-print-last-sexp) ---------- Buffer: *Backtrace* ----------
As usual, the error message tries to be helpful and makes sense after you learn how to read it.4
The first part of the error message is straightforward; it says
‘wrong type argument’. Next comes the mysterious jargon word
‘number-or-marker-p’. This word is trying to tell you what
kind of argument the +
expected.
The symbol number-or-marker-p
says that the Lisp interpreter is
trying to determine whether the information presented it (the value of
the argument) is a number or a marker (a special object representing a
buffer position). What it does is test to see whether the +
is
being given numbers to add. It also tests to see whether the
argument is something called a marker, which is a specific feature of
Emacs Lisp. (In Emacs, locations in a buffer are recorded as markers.
When the mark is set with the C-@ or C-SPC command,
its position is kept as a marker. The mark can be considered a
number—the number of characters the location is from the beginning
of the buffer.) In Emacs Lisp, +
can be used to add the
numeric value of marker positions as numbers.
The ‘p’ of number-or-marker-p
is the embodiment of a
practice started in the early days of Lisp programming. The ‘p’
stands for predicate. In the jargon used by the early Lisp
researchers, a predicate refers to a function to determine whether some
property is true or false. So the ‘p’ tells us that
number-or-marker-p
is the name of a function that determines
whether it is true or false that the argument supplied is a number or
a marker. Other Lisp symbols that end in ‘p’ include zerop
,
a function that tests whether its argument has the value of zero, and
listp
, a function that tests whether its argument is a list.
Finally, the last part of the error message is the symbol hello
.
This is the value of the argument that was passed to +
. If the
addition had been passed the correct type of object, the value passed
would have been a number, such as 37, rather than a symbol like
hello
. But then you would not have got the error message.
Previous: Using the Wrong Type Object as an Argument, Up: Arguments [Contents][Index]
1.8.5 The message
Function
Like +
, the message
function takes a variable number of
arguments. It is used to send messages to the user and is so useful
that we will describe it here.
A message is printed in the echo area. For example, you can print a message in your echo area by evaluating the following list:
(message "This message appears in the echo area!")
The whole string between double quotation marks is a single argument
and is printed in toto. (Note that in this example, the message
itself will appear in the echo area within double quotes; that is
because you see the value returned by the message
function. In
most uses of message
in programs that you write, the text will
be printed in the echo area as a side-effect, without the quotes.
See multiply-by-seven
in
detail, for an example of this.)
However, if there is a ‘%s’ in the quoted string of characters, the
message
function does not print the ‘%s’ as such, but looks
to the argument that follows the string. It evaluates the second
argument and prints the value at the location in the string where the
‘%s’ is.
You can see this by positioning the cursor after the following expression and typing C-x C-e:
(message "The name of this buffer is: %s." (buffer-name))
In Info, "The name of this buffer is: *info*."
will appear in the
echo area. The function buffer-name
returns the name of the
buffer as a string, which the message
function inserts in place
of %s
.
To print a value as an integer, use ‘%d’ in the same way as
‘%s’. For example, to print a message in the echo area that
states the value of the fill-column
, evaluate the following:
(message "The value of fill-column is %d." fill-column)
On my system, when I evaluate this list, "The value of
fill-column is 72."
appears in my echo area5.
If there is more than one ‘%s’ in the quoted string, the value of the first argument following the quoted string is printed at the location of the first ‘%s’ and the value of the second argument is printed at the location of the second ‘%s’, and so on.
For example, if you evaluate the following,
(message "There are %d %s in the office!" (- fill-column 14) "pink elephants")
a rather whimsical message will appear in your echo area. On my system
it says, "There are 58 pink elephants in the office!"
.
The expression (- fill-column 14)
is evaluated and the resulting
number is inserted in place of the ‘%d’; and the string in double
quotes, "pink elephants"
, is treated as a single argument and
inserted in place of the ‘%s’. (That is to say, a string between
double quotes evaluates to itself, like a number.)
Finally, here is a somewhat complex example that not only illustrates the computation of a number, but also shows how you can use an expression within an expression to generate the text that is substituted for ‘%s’:
(message "He saw %d %s" (- fill-column 32) (concat "red " (substring "The quick brown foxes jumped." 16 21) " leaping."))
In this example, message
has three arguments: the string,
"He saw %d %s"
, the expression, (- fill-column 32)
, and
the expression beginning with the function concat
. The value
resulting from the evaluation of (- fill-column 32)
is inserted
in place of the ‘%d’; and the value returned by the expression
beginning with concat
is inserted in place of the ‘%s’.
When your fill column is 70 and you evaluate the expression, the
message "He saw 38 red foxes leaping."
appears in your echo
area.
Next: Summary, Previous: Arguments, Up: List Processing [Contents][Index]
1.9 Setting the Value of a Variable
There are several ways by which a variable can be given a value.
One of the ways is to use the special form setq
. Another way
is to use let
(see let
). (The jargon for this process is
to bind a variable to a value.)
The following sections not only describe how setq
works but
also illustrate how arguments are passed.
Next: Counting, Up: Setting the Value of a Variable [Contents][Index]
1.9.1 Using setq
To set the value of the symbol flowers
to the list (rose
violet daisy buttercup)
, evaluate the following expression by
positioning the cursor after the expression and typing C-x C-e.
(setq flowers '(rose violet daisy buttercup))
The list (rose violet daisy buttercup)
will appear in the echo
area. This is what is returned by the setq
special
form. As a side effect, the symbol flowers
is bound to the
list; that is, the symbol flowers
, which can be viewed as
a variable, is given the list as its value. (This process, by the
way, illustrates how a side effect to the Lisp interpreter, setting
the value, can be the primary effect that we humans are interested in.
This is because every Lisp function must return a value if it does not
get an error, but it will only have a side effect if it is designed to
have one.)
After evaluating the setq
expression, you can evaluate the
symbol flowers
and it will return the value you just set.
Here is the symbol. Place your cursor after it and type C-x C-e.
flowers
When you evaluate flowers
, the list
(rose violet daisy buttercup)
appears in the echo area.
Incidentally, if you evaluate 'flowers
, the variable with a quote
in front of it, what you will see in the echo area is the symbol itself,
flowers
. Here is the quoted symbol, so you can try this:
'flowers
Also, as an added convenience, setq
permits you to set several
different variables to different values, all in one expression.
To set the value of the variable carnivores
to the list
'(lion tiger leopard)
using setq
, the following expression
is used:
(setq carnivores '(lion tiger leopard))
Also, setq
can be used to assign different values to
different variables. The first argument is bound to the value
of the second argument, the third argument is bound to the value of the
fourth argument, and so on. For example, you could use the following to
assign a list of trees to the symbol trees
and a list of herbivores
to the symbol herbivores
:
(setq trees '(pine fir oak maple) herbivores '(gazelle antelope zebra))
(The expression could just as well have been on one line, but it might not have fit on a page; and humans find it easier to read nicely formatted lists.)
Although I have been using the term “assign”, there is another way
of thinking about the workings of setq
; and that is to say that
setq
makes the symbol point to the list. This latter
way of thinking is very common and in forthcoming chapters we shall
come upon at least one symbol that has “pointer” as part of its
name. The name is chosen because the symbol has a value, specifically
a list, attached to it; or, expressed another way, the symbol is set
to point to the list.
Previous: Using setq
, Up: Setting the Value of a Variable [Contents][Index]
1.9.2 Counting
Here is an example that shows how to use setq
in a counter. You
might use this to count how many times a part of your program repeats
itself. First set a variable to zero; then add one to the number each
time the program repeats itself. To do this, you need a variable that
serves as a counter, and two expressions: an initial setq
expression that sets the counter variable to zero; and a second
setq
expression that increments the counter each time it is
evaluated.
(setq counter 0) ; Let’s call this the initializer. (setq counter (+ counter 1)) ; This is the incrementer. counter ; This is the counter.
(The text following the ‘;’ are comments. See Change a Function Definition.)
If you evaluate the first of these expressions, the initializer,
(setq counter 0)
, and then evaluate the third expression,
counter
, the number 0
will appear in the echo area. If
you then evaluate the second expression, the incrementer, (setq
counter (+ counter 1))
, the counter will get the value 1. So if you
again evaluate counter
, the number 1
will appear in the
echo area. Each time you evaluate the second expression, the value of
the counter will be incremented.
When you evaluate the incrementer, (setq counter (+ counter 1))
,
the Lisp interpreter first evaluates the innermost list; this is the
addition. In order to evaluate this list, it must evaluate the variable
counter
and the number 1
. When it evaluates the variable
counter
, it receives its current value. It passes this value and
the number 1
to the +
which adds them together. The sum
is then returned as the value of the inner list and passed to the
setq
which sets the variable counter
to this new value.
Thus, the value of the variable, counter
, is changed.
Next: Exercises, Previous: Setting the Value of a Variable, Up: List Processing [Contents][Index]
1.10 Summary
Learning Lisp is like climbing a hill in which the first part is the steepest. You have now climbed the most difficult part; what remains becomes easier as you progress onwards.
In summary,
- Lisp programs are made up of expressions, which are lists or single atoms.
- Lists are made up of zero or more atoms or inner lists, separated by whitespace and surrounded by parentheses. A list can be empty.
- Atoms are multi-character symbols, like
forward-paragraph
, single character symbols like+
, strings of characters between double quotation marks, or numbers. - A number evaluates to itself.
- A string between double quotes also evaluates to itself.
- When you evaluate a symbol by itself, its value is returned.
- When you evaluate a list, the Lisp interpreter looks at the first symbol in the list and then at the function definition bound to that symbol. Then the instructions in the function definition are carried out.
- A single-quote ‘'’ tells the Lisp interpreter that it should return the following expression as written, and not evaluate it as it would if the quote were not there.
- Arguments are the information passed to a function. The arguments to a function are computed by evaluating the rest of the elements of the list of which the function is the first element.
- A function always returns a value when it is evaluated (unless it gets an error); in addition, it may also carry out some action that is a side effect. In many cases, a function’s primary purpose is to create a side effect.
Previous: Summary, Up: List Processing [Contents][Index]
1.11 Exercises
A few simple exercises:
- Generate an error message by evaluating an appropriate symbol that is not within parentheses.
- Generate an error message by evaluating an appropriate symbol that is between parentheses.
- Create a counter that increments by two rather than one.
- Write an expression that prints a message in the echo area when evaluated.
Next: How To Write Function Definitions, Previous: List Processing, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
2 Practicing Evaluation
Before learning how to write a function definition in Emacs Lisp, it is useful to spend a little time evaluating various expressions that have already been written. These expressions will be lists with the functions as their first (and often only) element. Since some of the functions associated with buffers are both simple and interesting, we will start with those. In this section, we will evaluate a few of these. In another section, we will study the code of several other buffer-related functions, to see how they were written.
- How to Evaluate
- Buffer Names
- Getting Buffers
- Switching Buffers
- Buffer Size and the Location of Point
- Exercise
Next: Buffer Names, Up: Practicing Evaluation [Contents][Index]
How to Evaluate
Whenever you give an editing command to Emacs Lisp, such as the command to move the cursor or to scroll the screen, you are evaluating an expression, the first element of which is a function. This is how Emacs works.
When you type keys, you cause the Lisp interpreter to evaluate an
expression and that is how you get your results. Even typing plain text
involves evaluating an Emacs Lisp function, in this case, one that uses
self-insert-command
, which simply inserts the character you
typed. The functions you evaluate by typing keystrokes are called
interactive functions, or commands; how you make a function
interactive will be illustrated in the chapter on how to write function
definitions. See Making a Function Interactive.
In addition to typing keyboard commands, we have seen a second way to evaluate an expression: by positioning the cursor after a list and typing C-x C-e. This is what we will do in the rest of this section. There are other ways to evaluate an expression as well; these will be described as we come to them.
Besides being used for practicing evaluation, the functions shown in the next few sections are important in their own right. A study of these functions makes clear the distinction between buffers and files, how to switch to a buffer, and how to determine a location within it.
Next: Getting Buffers, Previous: How to Evaluate, Up: Practicing Evaluation [Contents][Index]
2.1 Buffer Names
The two functions, buffer-name
and buffer-file-name
, show
the difference between a file and a buffer. When you evaluate the
following expression, (buffer-name)
, the name of the buffer
appears in the echo area. When you evaluate (buffer-file-name)
,
the name of the file to which the buffer refers appears in the echo
area. Usually, the name returned by (buffer-name)
is the same as
the name of the file to which it refers, and the name returned by
(buffer-file-name)
is the full path-name of the file.
A file and a buffer are two different entities. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). Usually, a buffer contains information that you have copied from a file; we say the buffer is visiting that file. This copy is what you work on and modify. Changes to the buffer do not change the file, until you save the buffer. When you save the buffer, the buffer is copied to the file and is thus saved permanently.
If you are reading this in Info inside of GNU Emacs, you can evaluate each of the following expressions by positioning the cursor after it and typing C-x C-e.
(buffer-name) (buffer-file-name)
When I do this in Info, the value returned by evaluating
(buffer-name)
is "*info*", and the value returned by
evaluating (buffer-file-name)
is nil.
On the other hand, while I am writing this document, the value
returned by evaluating (buffer-name)
is
"introduction.texinfo", and the value returned by evaluating
(buffer-file-name)
is
"/gnu/work/intro/introduction.texinfo".
The former is the name of the buffer and the latter is the name of the
file. In Info, the buffer name is "*info*". Info does not
point to any file, so the result of evaluating
(buffer-file-name)
is nil. The symbol nil
is
from the Latin word for “nothing”; in this case, it means that the
buffer is not associated with any file. (In Lisp, nil
is also
used to mean “false” and is a synonym for the empty list, ()
.)
When I am writing, the name of my buffer is "introduction.texinfo". The name of the file to which it points is "/gnu/work/intro/introduction.texinfo".
(In the expressions, the parentheses tell the Lisp interpreter to
treat buffer-name
and buffer-file-name
as
functions; without the parentheses, the interpreter would attempt to
evaluate the symbols as variables. See Variables.)
In spite of the distinction between files and buffers, you will often find that people refer to a file when they mean a buffer and vice versa. Indeed, most people say, “I am editing a file,” rather than saying, “I am editing a buffer which I will soon save to a file.” It is almost always clear from context what people mean. When dealing with computer programs, however, it is important to keep the distinction in mind, since the computer is not as smart as a person.
The word “buffer”, by the way, comes from the meaning of the word as a cushion that deadens the force of a collision. In early computers, a buffer cushioned the interaction between files and the computer’s central processing unit. The drums or tapes that held a file and the central processing unit were pieces of equipment that were very different from each other, working at their own speeds, in spurts. The buffer made it possible for them to work together effectively. Eventually, the buffer grew from being an intermediary, a temporary holding place, to being the place where work is done. This transformation is rather like that of a small seaport that grew into a great city: once it was merely the place where cargo was warehoused temporarily before being loaded onto ships; then it became a business and cultural center in its own right.
Not all buffers are associated with files. For example, a *scratch* buffer does not visit any file. Similarly, a *Help* buffer is not associated with any file.
In the old days, when you lacked a ~/.emacs file and started an
Emacs session by typing the command emacs
alone, without naming
any files, Emacs started with the *scratch* buffer visible.
Nowadays, you will see a splash screen. You can follow one of the
commands suggested on the splash screen, visit a file, or press q
to quit the splash screen and reach the *scratch* buffer.
If you switch to the *scratch* buffer, type
(buffer-name)
, position the cursor after it, and then type
C-x C-e to evaluate the expression. The name "*scratch*"
will be returned and will appear in the echo area. "*scratch*"
is the name of the buffer. When you type (buffer-file-name)
in
the *scratch* buffer and evaluate that, nil
will appear
in the echo area, just as it does when you evaluate
(buffer-file-name)
in Info.
Incidentally, if you are in the *scratch* buffer and want the value returned by an expression to appear in the *scratch* buffer itself rather than in the echo area, type C-u C-x C-e instead of C-x C-e. This causes the value returned to appear after the expression. The buffer will look like this:
(buffer-name)"*scratch*"
You cannot do this in Info since Info is read-only and it will not allow you to change the contents of the buffer. But you can do this in any buffer you can edit; and when you write code or documentation (such as this book), this feature is very useful.
Next: Switching Buffers, Previous: Buffer Names, Up: Practicing Evaluation [Contents][Index]
2.2 Getting Buffers
The buffer-name
function returns the name of the buffer;
to get the buffer itself, a different function is needed: the
current-buffer
function. If you use this function in code, what
you get is the buffer itself.
A name and the object or entity to which the name refers are different
from each other. You are not your name. You are a person to whom
others refer by name. If you ask to speak to George and someone hands you
a card with the letters ‘G’, ‘e’, ‘o’, ‘r’,
‘g’, and ‘e’ written on it, you might be amused, but you would
not be satisfied. You do not want to speak to the name, but to the
person to whom the name refers. A buffer is similar: the name of the
scratch buffer is *scratch*, but the name is not the buffer. To
get a buffer itself, you need to use a function such as
current-buffer
.
However, there is a slight complication: if you evaluate
current-buffer
in an expression on its own, as we will do here,
what you see is a printed representation of the name of the buffer
without the contents of the buffer. Emacs works this way for two
reasons: the buffer may be thousands of lines long—too long to be
conveniently displayed; and, another buffer may have the same contents
but a different name, and it is important to distinguish between them.
Here is an expression containing the function:
(current-buffer)
If you evaluate this expression in Info in Emacs in the usual way, #<buffer *info*> will appear in the echo area. The special format indicates that the buffer itself is being returned, rather than just its name.
Incidentally, while you can type a number or symbol into a program, you
cannot do that with the printed representation of a buffer: the only way
to get a buffer itself is with a function such as current-buffer
.
A related function is other-buffer
. This returns the most
recently selected buffer other than the one you are in currently, not
a printed representation of its name. If you have recently switched
back and forth from the *scratch* buffer, other-buffer
will return that buffer.
You can see this by evaluating the expression:
(other-buffer)
You should see #<buffer *scratch*> appear in the echo area, or the name of whatever other buffer you switched back from most recently6.
Next: Buffer Size and the Location of Point, Previous: Getting Buffers, Up: Practicing Evaluation [Contents][Index]
2.3 Switching Buffers
The other-buffer
function actually provides a buffer when it is
used as an argument to a function that requires one. We can see this
by using other-buffer
and switch-to-buffer
to switch to a
different buffer.
But first, a brief introduction to the switch-to-buffer
function. When you switched back and forth from Info to the
*scratch* buffer to evaluate (buffer-name)
, you most
likely typed C-x b and then typed *scratch*7 when prompted in the minibuffer for the name of
the buffer to which you wanted to switch. The keystrokes, C-x
b, cause the Lisp interpreter to evaluate the interactive function
switch-to-buffer
. As we said before, this is how Emacs works:
different keystrokes call or run different functions. For example,
C-f calls forward-char
, M-e calls
forward-sentence
, and so on.
By writing switch-to-buffer
in an expression, and giving it a
buffer to switch to, we can switch buffers just the way C-x b
does:
(switch-to-buffer (other-buffer))
The symbol switch-to-buffer
is the first element of the list,
so the Lisp interpreter will treat it as a function and carry out the
instructions that are attached to it. But before doing that, the
interpreter will note that other-buffer
is inside parentheses
and work on that symbol first. other-buffer
is the first (and
in this case, the only) element of this list, so the Lisp interpreter
calls or runs the function. It returns another buffer. Next, the
interpreter runs switch-to-buffer
, passing to it, as an
argument, the other buffer, which is what Emacs will switch to. If
you are reading this in Info, try this now. Evaluate the expression.
(To get back, type C-x b RET.)8
In the programming examples in later sections of this document, you will
see the function set-buffer
more often than
switch-to-buffer
. This is because of a difference between
computer programs and humans: humans have eyes and expect to see the
buffer on which they are working on their computer terminals. This is
so obvious, it almost goes without saying. However, programs do not
have eyes. When a computer program works on a buffer, that buffer does
not need to be visible on the screen.
switch-to-buffer
is designed for humans and does two different
things: it switches the buffer to which Emacs’s attention is directed; and
it switches the buffer displayed in the window to the new buffer.
set-buffer
, on the other hand, does only one thing: it switches
the attention of the computer program to a different buffer. The buffer
on the screen remains unchanged (of course, normally nothing happens
there until the command finishes running).
Also, we have just introduced another jargon term, the word call. When you evaluate a list in which the first symbol is a function, you are calling that function. The use of the term comes from the notion of the function as an entity that can do something for you if you call it—just as a plumber is an entity who can fix a leak if you call him or her.
Next: Exercise, Previous: Switching Buffers, Up: Practicing Evaluation [Contents][Index]
2.4 Buffer Size and the Location of Point
Finally, let’s look at several rather simple functions,
buffer-size
, point
, point-min
, and
point-max
. These give information about the size of a buffer and
the location of point within it.
The function buffer-size
tells you the size of the current
buffer; that is, the function returns a count of the number of
characters in the buffer.
(buffer-size)
You can evaluate this in the usual way, by positioning the cursor after the expression and typing C-x C-e.
In Emacs, the current position of the cursor is called point.
The expression (point)
returns a number that tells you where the
cursor is located as a count of the number of characters from the
beginning of the buffer up to point.
You can see the character count for point in this buffer by evaluating the following expression in the usual way:
(point)
As I write this, the value of point is 65724. The point
function is frequently used in some of the examples later in this
book.
The value of point depends, of course, on its location within the buffer. If you evaluate point in this spot, the number will be larger:
(point)
For me, the value of point in this location is 66043, which means that there are 319 characters (including spaces) between the two expressions. (Doubtless, you will see different numbers, since I will have edited this since I first evaluated point.)
The function point-min
is somewhat similar to point
, but
it returns the value of the minimum permissible value of point in the
current buffer. This is the number 1 unless narrowing is in
effect. (Narrowing is a mechanism whereby you can restrict yourself,
or a program, to operations on just a part of a buffer.
See Narrowing and Widening.) Likewise, the
function point-max
returns the value of the maximum permissible
value of point in the current buffer.
Previous: Buffer Size and the Location of Point, Up: Practicing Evaluation [Contents][Index]
2.5 Exercise
Find a file with which you are working and move towards its middle. Find its buffer name, file name, length, and your position in the file.
Next: A Few Buffer-Related Functions, Previous: Practicing Evaluation, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
3 How To Write Function Definitions
When the Lisp interpreter evaluates a list, it looks to see whether the first symbol on the list has a function definition attached to it; or, put another way, whether the symbol points to a function definition. If it does, the computer carries out the instructions in the definition. A symbol that has a function definition is called, simply, a function (although, properly speaking, the definition is the function and the symbol refers to it.)
- An Aside about Primitive Functions
- The
defun
Macro - Install a Function Definition
- Make a Function Interactive
- Different Options for
interactive
- Install Code Permanently
let
- The
if
Special Form - If–then–else Expressions
- Truth and Falsehood in Emacs Lisp
save-excursion
- Review
- Exercises
Next: The defun
Macro, Up: How To Write Function Definitions [Contents][Index]
An Aside about Primitive Functions
All functions are defined in terms of other functions, except for a few primitive functions that are written in the C programming language. When you write functions’ definitions, you will write them in Emacs Lisp and use other functions as your building blocks. Some of the functions you will use will themselves be written in Emacs Lisp (perhaps by you) and some will be primitives written in C. The primitive functions are used exactly like those written in Emacs Lisp and behave like them. They are written in C so we can easily run GNU Emacs on any computer that has sufficient power and can run C.
Let me re-emphasize this: when you write code in Emacs Lisp, you do not distinguish between the use of functions written in C and the use of functions written in Emacs Lisp. The difference is irrelevant. I mention the distinction only because it is interesting to know. Indeed, unless you investigate, you won’t know whether an already-written function is written in Emacs Lisp or C.
Next: Install a Function Definition, Previous: An Aside about Primitive Functions, Up: How To Write Function Definitions [Contents][Index]
3.1 The defun
Macro
In Lisp, a symbol such as mark-whole-buffer
has code attached to
it that tells the computer what to do when the function is called.
This code is called the function definition and is created by
evaluating a Lisp expression that starts with the symbol defun
(which is an abbreviation for define function).
In subsequent sections, we will look at function definitions from the
Emacs source code, such as mark-whole-buffer
. In this section,
we will describe a simple function definition so you can see how it
looks. This function definition uses arithmetic because it makes for a
simple example. Some people dislike examples using arithmetic; however,
if you are such a person, do not despair. Hardly any of the code we
will study in the remainder of this introduction involves arithmetic or
mathematics. The examples mostly involve text in one way or another.
A function definition has up to five parts following the word
defun
:
- The name of the symbol to which the function definition should be attached.
- A list of the arguments that will be passed to the function. If no
arguments will be passed to the function, this is an empty list,
()
. - Documentation describing the function. (Technically optional, but strongly recommended.)
- Optionally, an expression to make the function interactive so you can use it by typing M-x and then the name of the function; or by typing an appropriate key or keychord.
- The code that instructs the computer what to do: the body of the function definition.
It is helpful to think of the five parts of a function definition as being organized in a template, with slots for each part:
(defun function-name (arguments…)
"optional-documentation…"
(interactive argument-passing-info) ; optional
body…)
As an example, here is the code for a function that multiplies its argument by 7. (This example is not interactive. See Making a Function Interactive, for that information.)
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
This definition begins with a parenthesis and the symbol defun
,
followed by the name of the function.
The name of the function is followed by a list that contains the
arguments that will be passed to the function. This list is called
the argument list. In this example, the list has only one
element, the symbol, number
. When the function is used, the
symbol will be bound to the value that is used as the argument to the
function.
Instead of choosing the word number
for the name of the argument,
I could have picked any other name. For example, I could have chosen
the word multiplicand
. I picked the word “number” because it
tells what kind of value is intended for this slot; but I could just as
well have chosen the word “multiplicand” to indicate the role that the
value placed in this slot will play in the workings of the function. I
could have called it foogle
, but that would have been a bad
choice because it would not tell humans what it means. The choice of
name is up to the programmer and should be chosen to make the meaning of
the function clear.
Indeed, you can choose any name you wish for a symbol in an argument
list, even the name of a symbol used in some other function: the name
you use in an argument list is private to that particular definition.
In that definition, the name refers to a different entity than any use
of the same name outside the function definition. Suppose you have a
nick-name “Shorty” in your family; when your family members refer to
“Shorty”, they mean you. But outside your family, in a movie, for
example, the name “Shorty” refers to someone else. Because a name in an
argument list is private to the function definition, you can change the
value of such a symbol inside the body of a function without changing
its value outside the function. The effect is similar to that produced
by a let
expression. (See let
.)
The argument list is followed by the documentation string that
describes the function. This is what you see when you type
C-h f and the name of a function. Incidentally, when you
write a documentation string like this, you should make the first line
a complete sentence since some commands, such as apropos
, print
only the first line of a multi-line documentation string. Also, you
should not indent the second line of a documentation string, if you
have one, because that looks odd when you use C-h f
(describe-function
). The documentation string is optional, but
it is so useful, it should be included in almost every function you
write.
The third line of the example consists of the body of the function
definition. (Most functions’ definitions, of course, are longer than
this.) In this function, the body is the list, (* 7 number)
, which
says to multiply the value of number by 7. (In Emacs Lisp,
*
is the function for multiplication, just as +
is the
function for addition.)
When you use the multiply-by-seven
function, the argument
number
evaluates to the actual number you want used. Here is an
example that shows how multiply-by-seven
is used; but don’t try
to evaluate this yet!
(multiply-by-seven 3)
The symbol number
, specified in the function definition in the
next section, is bound to the value 3 in the actual use of
the function. Note that although number
was inside parentheses
in the function definition, the argument passed to the
multiply-by-seven
function is not in parentheses. The
parentheses are written in the function definition so the computer can
figure out where the argument list ends and the rest of the function
definition begins.
If you evaluate this example, you are likely to get an error message. (Go ahead, try it!) This is because we have written the function definition, but not yet told the computer about the definition—we have not yet loaded the function definition in Emacs. Installing a function is the process that tells the Lisp interpreter the definition of the function. Installation is described in the next section.
Next: Make a Function Interactive, Previous: The defun
Macro, Up: How To Write Function Definitions [Contents][Index]
3.2 Install a Function Definition
If you are reading this inside of Info in Emacs, you can try out the
multiply-by-seven
function by first evaluating the function
definition and then evaluating (multiply-by-seven 3)
. A copy of
the function definition follows. Place the cursor after the last
parenthesis of the function definition and type C-x C-e. When you
do this, multiply-by-seven
will appear in the echo area. (What
this means is that when a function definition is evaluated, the value it
returns is the name of the defined function.) At the same time, this
action installs the function definition.
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
By evaluating this defun
, you have just installed
multiply-by-seven
in Emacs. The function is now just as much a
part of Emacs as forward-word
or any other editing function you
use. (multiply-by-seven
will stay installed until you quit
Emacs. To reload code automatically whenever you start Emacs, see
Installing Code Permanently.)
The effect of installation
You can see the effect of installing multiply-by-seven
by
evaluating the following sample. Place the cursor after the following
expression and type C-x C-e. The number 21 will appear in the
echo area.
(multiply-by-seven 3)
If you wish, you can read the documentation for the function by typing
C-h f (describe-function
) and then the name of the
function, multiply-by-seven
. When you do this, a
*Help* window will appear on your screen that says:
multiply-by-seven is a Lisp function. (multiply-by-seven NUMBER) Multiply NUMBER by seven.
(To return to a single window on your screen, type C-x 1.)
Previous: The effect of installation, Up: Install a Function Definition [Contents][Index]
3.2.1 Change a Function Definition
If you want to change the code in multiply-by-seven
, just rewrite
it. To install the new version in place of the old one, evaluate the
function definition again. This is how you modify code in Emacs. It is
very simple.
As an example, you can change the multiply-by-seven
function to
add the number to itself seven times instead of multiplying the number
by seven. It produces the same answer, but by a different path. At
the same time, we will add a comment to the code; a comment is text
that the Lisp interpreter ignores, but that a human reader may find
useful or enlightening. The comment is that this is the second
version.
(defun multiply-by-seven (number) ; Second version.
"Multiply NUMBER by seven."
(+ number number number number number number number))
The comment follows a semicolon, ‘;’. In Lisp, everything on a line that follows a semicolon is a comment. The end of the line is the end of the comment. To stretch a comment over two or more lines, begin each line with a semicolon.
See Beginning a .emacs File, and Comments in The GNU Emacs Lisp Reference Manual, for more about comments.
You can install this version of the multiply-by-seven
function by
evaluating it in the same way you evaluated the first function: place
the cursor after the last parenthesis and type C-x C-e.
In summary, this is how you write code in Emacs Lisp: you write a function; install it; test it; and then make fixes or enhancements and install it again.
Next: Different Options for interactive
, Previous: Install a Function Definition, Up: How To Write Function Definitions [Contents][Index]
3.3 Make a Function Interactive
You make a function interactive by placing a list that begins with
the special form interactive
immediately after the
documentation. A user can invoke an interactive function by typing
M-x and then the name of the function; or by typing the keys to
which it is bound, for example, by typing C-n for
next-line
or C-x h for mark-whole-buffer
.
Interestingly, when you call an interactive function interactively, the value returned is not automatically displayed in the echo area. This is because you often call an interactive function for its side effects, such as moving forward by a word or line, and not for the value returned. If the returned value were displayed in the echo area each time you typed a key, it would be very distracting.
An Interactive multiply-by-seven
, An Overview
Both the use of the special form interactive
and one way to
display a value in the echo area can be illustrated by creating an
interactive version of multiply-by-seven
.
Here is the code:
(defun multiply-by-seven (number) ; Interactive version.
"Multiply NUMBER by seven."
(interactive "p")
(message "The result is %d" (* 7 number)))
You can install this code by placing your cursor after it and typing C-x C-e. The name of the function will appear in your echo area. Then, you can use this code by typing C-u and a number and then typing M-x multiply-by-seven and pressing RET. The phrase ‘The result is …’ followed by the product will appear in the echo area.
Speaking more generally, you invoke a function like this in either of two ways:
- By typing a prefix argument that contains the number to be passed, and then typing M-x and the name of the function, as with C-u 3 M-x forward-sentence; or,
- By typing whatever key or keychord the function is bound to, as with C-u 3 M-e.
Both the examples just mentioned work identically to move point forward
three sentences. (Since multiply-by-seven
is not bound to a key,
it could not be used as an example of key binding.)
(See Some Key Bindings, to learn how to bind a command to a key.)
A prefix argument is passed to an interactive function by typing the META key followed by a number, for example, M-3 M-e, or by typing C-u and then a number, for example, C-u 3 M-e (if you type C-u without a number, it defaults to 4).
Previous: An Interactive multiply-by-seven
, An Overview, Up: Make a Function Interactive [Contents][Index]
3.3.1 An Interactive multiply-by-seven
Let’s look at the use of the special form interactive
and then at
the function message
in the interactive version of
multiply-by-seven
. You will recall that the function definition
looks like this:
(defun multiply-by-seven (number) ; Interactive version.
"Multiply NUMBER by seven."
(interactive "p")
(message "The result is %d" (* 7 number)))
In this function, the expression, (interactive "p")
, is a list of
two elements. The "p"
tells Emacs to pass the prefix argument to
the function and use its value for the argument of the function.
The argument will be a number. This means that the symbol
number
will be bound to a number in the line:
(message "The result is %d" (* 7 number))
For example, if your prefix argument is 5, the Lisp interpreter will evaluate the line as if it were:
(message "The result is %d" (* 7 5))
(If you are reading this in GNU Emacs, you can evaluate this expression
yourself.) First, the interpreter will evaluate the inner list, which
is (* 7 5)
. This returns a value of 35. Next, it
will evaluate the outer list, passing the values of the second and
subsequent elements of the list to the function message
.
As we have seen, message
is an Emacs Lisp function especially
designed for sending a one line message to a user. (See The message
function.) In summary, the message
function prints its first argument in the echo area as is, except for
occurrences of ‘%d’ or ‘%s’ (and various other %-sequences
which we have not mentioned). When it sees a control sequence, the
function looks to the second or subsequent arguments and prints the
value of the argument in the location in the string where the control
sequence is located.
In the interactive multiply-by-seven
function, the control string
is ‘%d’, which requires a number, and the value returned by
evaluating (* 7 5)
is the number 35. Consequently, the number 35
is printed in place of the ‘%d’ and the message is ‘The result
is 35’.
(Note that when you call the function multiply-by-seven
, the
message is printed without quotes, but when you call message
, the
text is printed in double quotes. This is because the value returned by
message
is what appears in the echo area when you evaluate an
expression whose first element is message
; but when embedded in a
function, message
prints the text as a side effect without
quotes.)
Next: Install Code Permanently, Previous: Make a Function Interactive, Up: How To Write Function Definitions [Contents][Index]
3.4 Different Options for interactive
In the example, multiply-by-seven
used "p"
as the
argument to interactive
. This argument told Emacs to interpret
your typing either C-u followed by a number or META
followed by a number as a command to pass that number to the function
as its argument. Emacs has more than twenty characters predefined for
use with interactive
. In almost every case, one of these
options will enable you to pass the right information interactively to
a function. (See Code Characters for
interactive
in The GNU Emacs Lisp Reference Manual.)
Consider the function zap-to-char
. Its interactive expression
is
(interactive "p\ncZap to char: ")
The first part of the argument to interactive
is ‘p’, with
which you are already familiar. This argument tells Emacs to
interpret a prefix, as a number to be passed to the function. You
can specify a prefix either by typing C-u followed by a number
or by typing META followed by a number. The prefix is the
number of specified characters. Thus, if your prefix is three and the
specified character is ‘x’, then you will delete all the text up
to and including the third next ‘x’. If you do not set a prefix,
then you delete all the text up to and including the specified
character, but no more.
The ‘c’ tells the function the name of the character to which to delete.
More formally, a function with two or more arguments can have
information passed to each argument by adding parts to the string that
follows interactive
. When you do this, the information is
passed to each argument in the same order it is specified in the
interactive
list. In the string, each part is separated from
the next part by a ‘\n’, which is a newline. For example, you
can follow ‘p’ with a ‘\n’ and an ‘cZap to char: ’.
This causes Emacs to pass the value of the prefix argument (if there
is one) and the character.
In this case, the function definition looks like the following, where
arg
and char
are the symbols to which interactive
binds the prefix argument and the specified character:
(defun name-of-function (arg char) "documentation…" (interactive "p\ncZap to char: ") body-of-function…)
(The space after the colon in the prompt makes it look better when you
are prompted. See The Definition of
copy-to-buffer
, for an example.)
When a function does not take arguments, interactive
does not
require any. Such a function contains the simple expression
(interactive)
. The mark-whole-buffer
function is like
this.
Alternatively, if the special letter-codes are not right for your
application, you can pass your own arguments to interactive
as
a list.
See The Definition of append-to-buffer
,
for an example. See Using Interactive
in The GNU Emacs Lisp Reference Manual, for a more complete
explanation about this technique.
Next: let
, Previous: Different Options for interactive
, Up: How To Write Function Definitions [Contents][Index]
3.5 Install Code Permanently
When you install a function definition by evaluating it, it will stay installed until you quit Emacs. The next time you start a new session of Emacs, the function will not be installed unless you evaluate the function definition again.
At some point, you may want to have code installed automatically whenever you start a new session of Emacs. There are several ways of doing this:
- If you have code that is just for yourself, you can put the code for the function definition in your .emacs initialization file. When you start Emacs, your .emacs file is automatically evaluated and all the function definitions within it are installed. See Your .emacs File.
- Alternatively, you can put the function definitions that you want
installed in one or more files of their own and use the
load
function to cause Emacs to evaluate and thereby install each of the functions in the files. See Loading Files. - Thirdly, if you have code that your whole site will use, it is usual to put it in a file called site-init.el that is loaded when Emacs is built. This makes the code available to everyone who uses your machine. (See the INSTALL file that is part of the Emacs distribution.)
Finally, if you have code that everyone who uses Emacs may want, you can post it on a computer network or send a copy to the Free Software Foundation. (When you do this, please license the code and its documentation under a license that permits other people to run, copy, study, modify, and redistribute the code and which protects you from having your work taken from you.) If you send a copy of your code to the Free Software Foundation, and properly protect yourself and others, it may be included in the next release of Emacs. In large part, this is how Emacs has grown over the past years, by donations.
Next: The if
Special Form, Previous: Install Code Permanently, Up: How To Write Function Definitions [Contents][Index]
3.6 let
The let
expression is a special form in Lisp that you will need
to use in most function definitions.
let
is used to attach or bind a symbol to a value in such a way
that the Lisp interpreter will not confuse the variable with a
variable of the same name that is not part of the function.
To understand why the let
special form is necessary, consider
the situation in which you own a home that you generally refer to as
“the house”, as in the sentence, “The house needs painting.” If you
are visiting a friend and your host refers to “the house”, he is
likely to be referring to his house, not yours, that is, to a
different house.
If your friend is referring to his house and you think he is referring
to your house, you may be in for some confusion. The same thing could
happen in Lisp if a variable that is used inside of one function has
the same name as a variable that is used inside of another function,
and the two are not intended to refer to the same value. The
let
special form prevents this kind of confusion.
let
Prevents Confusion- The Parts of a
let
Expression - Sample
let
Expression - Uninitialized Variables in a
let
Statement - How
let
Binds Variables
Next: The Parts of a let
Expression, Up: let
[Contents][Index]
let
Prevents Confusion
The let
special form prevents confusion. let
creates a
name for a local variable that overshadows any use of the same
name outside the let
expression (in computer science jargon, we
call this binding the variable). This is like understanding
that in your host’s home, whenever he refers to “the house”, he
means his house, not yours. (The symbols used to name function
arguments are bound as local variables in exactly the same way.
See The defun
Macro.)
Another way to think about let
is that it defines a special
region in your code: within the body of the let
expression, the
variables you’ve named have their own local meaning. Outside of the
let
body, they have other meanings (or they may not be defined
at all). This means that inside the let
body, calling
setq
for a variable named by the let
expression will set
the value of the local variable of that name. However, outside
of the let
body (such as when calling a function that was
defined elsewhere), calling setq
for a variable named by the
let
expression will not affect that local
variable.9
let
can create more than one variable at once. Also,
let
gives each variable it creates an initial value, either a
value specified by you, or nil
. (In the jargon, this is
binding the variable to the value.) After let
has created
and bound the variables, it executes the code in the body of the
let
, and returns the value of the last expression in the body,
as the value of the whole let
expression. (“Execute” is a jargon
term that means to evaluate a list; it comes from the use of the word
meaning “to give practical effect to” (Oxford English
Dictionary). Since you evaluate an expression to perform an action,
“execute” has evolved as a synonym to “evaluate”.)
Next: Sample let
Expression, Previous: let
Prevents Confusion, Up: let
[Contents][Index]
3.6.1 The Parts of a let
Expression
A let
expression is a list of three parts. The first part is
the symbol let
. The second part is a list, called a
varlist, each element of which is either a symbol by itself or a
two-element list, the first element of which is a symbol. The third
part of the let
expression is the body of the let
. The
body usually consists of one or more lists.
A template for a let
expression looks like this:
(let varlist body…)
The symbols in the varlist are the variables that are given initial
values by the let
special form. Symbols by themselves are given
the initial value of nil
; and each symbol that is the first
element of a two-element list is bound to the value that is returned
when the Lisp interpreter evaluates the second element.
Thus, a varlist might look like this: (thread (needles 3))
. In
this case, in a let
expression, Emacs binds the symbol
thread
to an initial value of nil
, and binds the symbol
needles
to an initial value of 3.
When you write a let
expression, what you do is put the
appropriate expressions in the slots of the let
expression
template.
If the varlist is composed of two-element lists, as is often the case,
the template for the let
expression looks like this:
(let ((variable value) (variable value) …) body…)
Next: Uninitialized Variables in a let
Statement, Previous: The Parts of a let
Expression, Up: let
[Contents][Index]
3.6.2 Sample let
Expression
The following expression creates and gives initial values
to the two variables zebra
and tiger
. The body of the
let
expression is a list which calls the message
function.
(let ((zebra "stripes") (tiger "fierce")) (message "One kind of animal has %s and another is %s." zebra tiger))
Here, the varlist is ((zebra "stripes") (tiger "fierce"))
.
The two variables are zebra
and tiger
. Each variable is
the first element of a two-element list and each value is the second
element of its two-element list. In the varlist, Emacs binds the
variable zebra
to the value "stripes"
10, and binds the
variable tiger
to the value "fierce"
. In this example,
both values are strings. The values could just as well have been
another list or a symbol. The body of the let
follows after the list holding the variables. In this example, the
body is a list that uses the message
function to print a string
in the echo area.
You may evaluate the example in the usual fashion, by placing the cursor after the last parenthesis and typing C-x C-e. When you do this, the following will appear in the echo area:
"One kind of animal has stripes and another is fierce."
As we have seen before, the message
function prints its first
argument, except for ‘%s’. In this example, the value of the variable
zebra
is printed at the location of the first ‘%s’ and the
value of the variable tiger
is printed at the location of the
second ‘%s’.
Next: How let
Binds Variables, Previous: Sample let
Expression, Up: let
[Contents][Index]
3.6.3 Uninitialized Variables in a let
Statement
If you do not bind the variables in a let
statement to specific
initial values, they will automatically be bound to an initial value of
nil
, as in the following expression:
(let ((birch 3) pine fir (oak 'some)) (message "Here are %d variables with %s, %s, and %s value." birch pine fir oak))
Here, the varlist is ((birch 3) pine fir (oak 'some))
.
If you evaluate this expression in the usual way, the following will appear in your echo area:
"Here are 3 variables with nil, nil, and some value."
In this example, Emacs binds the symbol birch
to the number 3,
binds the symbols pine
and fir
to nil
, and binds
the symbol oak
to the value some
.
Note that in the first part of the let
, the variables pine
and fir
stand alone as atoms that are not surrounded by
parentheses; this is because they are being bound to nil
, the
empty list. But oak
is bound to some
and so is a part of
the list (oak 'some)
. Similarly, birch
is bound to the
number 3 and so is in a list with that number. (Since a number
evaluates to itself, the number does not need to be quoted. Also, the
number is printed in the message using a ‘%d’ rather than a
‘%s’.) The four variables as a group are put into a list to
delimit them from the body of the let
.
Previous: Uninitialized Variables in a let
Statement, Up: let
[Contents][Index]
3.6.4 How let
Binds Variables
Emacs Lisp supports two different ways of binding variable names to their values. These ways affect the parts of your program where a particular binding is valid. For historical reasons, Emacs Lisp uses a form of variable binding called dynamic binding by default. However, in this manual we discuss the preferred form of binding, called lexical binding, unless otherwise noted (in the future, the Emacs maintainers plan to change the default to lexical binding). If you have programmed in other languages before, you’re likely already familiar with how lexical binding behaves.
In order to use lexical binding in a program, you should add this to the first line of your Emacs Lisp file:
;;; -*- lexical-binding: t -*-
For more information about this, see Variable Scoping in The Emacs Lisp Reference Manual.
Differences Between Lexical and Dynamic Binding
As we discussed before (see let
Prevents Confusion), when you create
local variables with let
under lexical binding, those variables
are valid only within the body of the let
expression. In other
parts of your code, they have other meanings, so if you call a
function defined elsewhere within the let
body, that function
would be unable to “see” the local variables you’ve created. (On
the other hand, if you call a function that was defined within a
let
body, that function would be able to see—and
modify—the local variables from that let
expression.)
Under dynamic binding, the rules are different: instead, when you use
let
, the local variables you’ve created are valid during
execution of the let
expression. This means that, if your
let
expression calls a function, that function can see these
local variables, regardless of where the function is defined
(including in another file entirely).
Another way to think about let
when using dynamic binding is
that every variable name has a global “stack” of bindings, and
whenever you use that variable’s name, it refers to the binding on the
top of the stack. (You can imagine this like a stack of papers on
your desk with the values written on them.) When you bind a variable
dynamically with let
, it puts the new binding you’ve specified
on the top of the stack, and then executes the let
body. Once
the let
body finishes, it takes that binding off of the stack,
revealing the one it had (if any) before the let
expression.
Previous: Differences Between Lexical and Dynamic Binding, Up: How let
Binds Variables [Contents][Index]
Example of Lexical vs. Dynamic Binding
In some cases, both lexical and dynamic binding behave identically. However, in other cases, they can change the meaning of your program. For example, see what happens in this code under lexical binding:
;;; -*- lexical-binding: t -*- (setq x 0) (defun getx () x) (setq x 1) (let ((x 2)) (getx)) ⇒ 1
Here, the result of (getx)
is 1
. Under lexical binding,
getx
doesn’t see the value from our let
expression.
That’s because the body of getx
is outside of the body of our
let
expression. Since getx
is defined at the top,
global level of our code (i.e. not inside the body of any let
expression), it looks for and finds x
at the global level as
well. When executing getx
, the current global value of
x
is 1
, so that’s what getx
returns.
If we use dynamic binding instead, the behavior is different:
;;; -*- lexical-binding: nil -*- (setq x 0) (defun getx () x) (setq x 1) (let ((x 2)) (getx)) ⇒ 2
Now, the result of (getx)
is 2
! That’s because under
dynamic binding, when executing getx
, the current binding for
x
at the top of our stack is the one from our let
binding. This time, getx
doesn’t see the global value for
x
, since its binding is below the one from our let
expression in the stack of bindings.
(Some variables are also “special”, and they are always dynamically
bound even when lexical-binding
is t
. See Initializing a Variable with defvar
.)
Next: If–then–else Expressions, Previous: let
, Up: How To Write Function Definitions [Contents][Index]
3.7 The if
Special Form
Another special form is the conditional if
. This form is used
to instruct the computer to make decisions. You can write function
definitions without using if
, but it is used often enough, and
is important enough, to be included here. It is used, for example, in
the code for the function beginning-of-buffer
.
The basic idea behind an if
, is that if a test is true,
then an expression is evaluated. If the test is not true, the
expression is not evaluated. For example, you might make a decision
such as, “if it is warm and sunny, then go to the beach!”
if
in more detail
An if
expression written in Lisp does not use the word “then”;
the test and the action are the second and third elements of the list
whose first element is if
. Nonetheless, the test part of an
if
expression is often called the if-part and the second
argument is often called the then-part.
Also, when an if
expression is written, the true-or-false-test
is usually written on the same line as the symbol if
, but the
action to carry out if the test is true, the then-part, is written
on the second and subsequent lines. This makes the if
expression easier to read.
(if true-or-false-test action-to-carry-out-if-test-is-true)
The true-or-false-test will be an expression that is evaluated by the Lisp interpreter.
Here is an example that you can evaluate in the usual manner. The test is whether the number 5 is greater than the number 4. Since it is, the message ‘5 is greater than 4!’ will be printed.
(if (> 5 4) ; if-part (message "5 is greater than 4!")) ; then-part
(The function >
tests whether its first argument is greater than
its second argument and returns true if it is.)
Of course, in actual use, the test in an if
expression will not
be fixed for all time as it is by the expression (> 5 4)
.
Instead, at least one of the variables used in the test will be bound to
a value that is not known ahead of time. (If the value were known ahead
of time, we would not need to run the test!)
For example, the value may be bound to an argument of a function
definition. In the following function definition, the character of the
animal is a value that is passed to the function. If the value bound to
characteristic
is "fierce"
, then the message, ‘It is a
tiger!’ will be printed; otherwise, nil
will be returned.
(defun type-of-animal (characteristic) "Print message in echo area depending on CHARACTERISTIC. If the CHARACTERISTIC is the string \"fierce\", then warn of a tiger." (if (equal characteristic "fierce") (message "It is a tiger!")))
If you are reading this inside of GNU Emacs, you can evaluate the function definition in the usual way to install it in Emacs, and then you can evaluate the following two expressions to see the results:
(type-of-animal "fierce") (type-of-animal "striped")
When you evaluate (type-of-animal "fierce")
, you will see the
following message printed in the echo area: "It is a tiger!"
; and
when you evaluate (type-of-animal "striped")
you will see nil
printed in the echo area.
Previous: if
in more detail, Up: The if
Special Form [Contents][Index]
3.7.1 The type-of-animal
Function in Detail
Let’s look at the type-of-animal
function in detail.
The function definition for type-of-animal
was written by filling
the slots of two templates, one for a function definition as a whole, and
a second for an if
expression.
The template for every function that is not interactive is:
(defun name-of-function (argument-list) "documentation…" body…)
The parts of the function that match this template look like this:
(defun type-of-animal (characteristic)
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the string \"fierce\",
then warn of a tiger."
body: the if
expression)
The name of function is type-of-animal
; it is passed the value
of one argument. The argument list is followed by a multi-line
documentation string. The documentation string is included in the
example because it is a good habit to write documentation string for
every function definition. The body of the function definition
consists of the if
expression.
The template for an if
expression looks like this:
(if true-or-false-test action-to-carry-out-if-the-test-returns-true)
In the type-of-animal
function, the code for the if
looks like this:
(if (equal characteristic "fierce") (message "It is a tiger!"))
Here, the true-or-false-test is the expression:
(equal characteristic "fierce")
In Lisp, equal
is a function that determines whether its first
argument is equal to its second argument. The second argument is the
string "fierce"
and the first argument is the value of the
symbol characteristic
—in other words, the argument passed to
this function.
In the first exercise of type-of-animal
, the argument
"fierce"
is passed to type-of-animal
. Since "fierce"
is equal to "fierce"
, the expression, (equal characteristic
"fierce")
, returns a value of true. When this happens, the if
evaluates the second argument or then-part of the if
:
(message "It is a tiger!")
.
On the other hand, in the second exercise of type-of-animal
, the
argument "striped"
is passed to type-of-animal
. "striped"
is not equal to "fierce"
, so the then-part is not evaluated and
nil
is returned by the if
expression.
Next: Truth and Falsehood in Emacs Lisp, Previous: The if
Special Form, Up: How To Write Function Definitions [Contents][Index]
3.8 If–then–else Expressions
An if
expression may have an optional third argument, called
the else-part, for the case when the true-or-false-test returns
false. When this happens, the second argument or then-part of the
overall if
expression is not evaluated, but the third or
else-part is evaluated. You might think of this as the cloudy
day alternative for the decision “if it is warm and sunny, then go to
the beach, else read a book!”.
The word “else” is not written in the Lisp code; the else-part of an
if
expression comes after the then-part. In the written Lisp, the
else-part is usually written to start on a line of its own and is
indented less than the then-part:
(if true-or-false-test action-to-carry-out-if-the-test-returns-true action-to-carry-out-if-the-test-returns-false)
For example, the following if
expression prints the message ‘4
is not greater than 5!’ when you evaluate it in the usual way:
(if (> 4 5) ; if-part (message "4 falsely greater than 5!") ; then-part (message "4 is not greater than 5!")) ; else-part
Note that the different levels of indentation make it easy to
distinguish the then-part from the else-part. (GNU Emacs has several
commands that automatically indent if
expressions correctly.
See GNU Emacs Helps You Type Lists.)
We can extend the type-of-animal
function to include an
else-part by simply incorporating an additional part to the if
expression.
You can see the consequences of doing this if you evaluate the following
version of the type-of-animal
function definition to install it
and then evaluate the two subsequent expressions to pass different
arguments to the function.
(defun type-of-animal (characteristic) ; Second version.
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the string \"fierce\",
then warn of a tiger; else say it is not fierce."
(if (equal characteristic "fierce")
(message "It is a tiger!")
(message "It is not fierce!")))
(type-of-animal "fierce") (type-of-animal "striped")
When you evaluate (type-of-animal "fierce")
, you will see the
following message printed in the echo area: "It is a tiger!"
; but
when you evaluate (type-of-animal "striped")
, you will see
"It is not fierce!"
.
(Of course, if the characteristic were "ferocious"
, the
message "It is not fierce!"
would be printed; and it would be
misleading! When you write code, you need to take into account the
possibility that some such argument will be tested by the if
and write your program accordingly.)
Next: save-excursion
, Previous: If–then–else Expressions, Up: How To Write Function Definitions [Contents][Index]
3.9 Truth and Falsehood in Emacs Lisp
There is an important aspect to the truth test in an if
expression. So far, we have spoken of “true” and “false” as values of
predicates as if they were new kinds of Emacs Lisp objects. In fact,
“false” is just our old friend nil
. Anything else—anything
at all—is “true”.
The expression that tests for truth is interpreted as true
if the result of evaluating it is a value that is not nil
. In
other words, the result of the test is considered true if the value
returned is a number such as 47, a string such as "hello"
, or a
symbol (other than nil
) such as flowers
, or a list (so
long as it is not empty), or even a buffer!
An explanation of nil
Before illustrating a test for truth, we need an explanation of nil
.
In Emacs Lisp, the symbol nil
has two meanings. First, it means the
empty list. Second, it means false and is the value returned when a
true-or-false-test tests false. nil
can be written as an empty
list, ()
, or as nil
. As far as the Lisp interpreter is
concerned, ()
and nil
are the same. Humans, however, tend
to use nil
for false and ()
for the empty list.
In Emacs Lisp, any value that is not nil
—is not the empty
list—is considered true. This means that if an evaluation returns
something that is not an empty list, an if
expression will test
true. For example, if a number is put in the slot for the test, it
will be evaluated and will return itself, since that is what numbers
do when evaluated. In this conditional, the if
expression will
test true. The expression tests false only when nil
, an empty
list, is returned by evaluating the expression.
You can see this by evaluating the two expressions in the following examples.
In the first example, the number 4 is evaluated as the test in the
if
expression and returns itself; consequently, the then-part
of the expression is evaluated and returned: ‘true’ appears in
the echo area. In the second example, the nil
indicates false;
consequently, the else-part of the expression is evaluated and
returned: ‘false’ appears in the echo area.
(if 4 'true 'false)
(if nil 'true 'false)
Incidentally, if some other useful value is not available for a test that
returns true, then the Lisp interpreter will return the symbol t
for true. For example, the expression (> 5 4)
returns t
when evaluated, as you can see by evaluating it in the usual way:
(> 5 4)
On the other hand, this function returns nil
if the test is false.
(> 4 5)
Next: Review, Previous: Truth and Falsehood in Emacs Lisp, Up: How To Write Function Definitions [Contents][Index]
3.10 save-excursion
The save-excursion
function is the final special form that we
will discuss in this chapter.
In Emacs Lisp programs used for editing, the save-excursion
function is very common. It saves the location of point,
executes the body of the function, and then restores point to
its previous position if its location was changed. Its primary
purpose is to keep the user from being surprised and disturbed by
unexpected movement of point.
Next: Template for a save-excursion
Expression, Up: save-excursion
[Contents][Index]
Point and Mark
Before discussing save-excursion
, however, it may be useful
first to review what point and mark are in GNU Emacs. Point is
the current location of the cursor. Wherever the cursor
is, that is point. More precisely, on terminals where the cursor
appears to be on top of a character, point is immediately before the
character. In Emacs Lisp, point is an integer. The first character in
a buffer is number one, the second is number two, and so on. The
function point
returns the current position of the cursor as a
number. Each buffer has its own value for point.
The mark is another position in the buffer; its value can be set
with a command such as C-SPC (set-mark-command
). If
a mark has been set, you can use the command C-x C-x
(exchange-point-and-mark
) to cause the cursor to jump to the mark
and set the mark to be the previous position of point. In addition, if
you set another mark, the position of the previous mark is saved in the
mark ring. Many mark positions can be saved this way. You can jump the
cursor to a saved mark by typing C-u C-SPC one or more
times.
The part of the buffer between point and mark is called the
region. Numerous commands work on the region, including
center-region
, count-words-region
, kill-region
, and
print-region
.
The save-excursion
special form saves the location of point and
restores this position after the code within the body of the
special form is evaluated by the Lisp interpreter. Thus, if point were
in the beginning of a piece of text and some code moved point to the end
of the buffer, the save-excursion
would put point back to where
it was before, after the expressions in the body of the function were
evaluated.
In Emacs, a function frequently moves point as part of its internal
workings even though a user would not expect this. For example,
count-words-region
moves point. To prevent the user from being
bothered by jumps that are both unexpected and (from the user’s point of
view) unnecessary, save-excursion
is often used to keep point in
the location expected by the user. The use of
save-excursion
is good housekeeping.
To make sure the house stays clean, save-excursion
restores the
value of point even if something goes wrong in the code inside
of it (or, to be more precise and to use the proper jargon, “in case of
abnormal exit”). This feature is very helpful.
In addition to recording the value of point,
save-excursion
keeps track of the current buffer, and restores
it, too. This means you can write code that will change the buffer and
have save-excursion
switch you back to the original buffer.
This is how save-excursion
is used in append-to-buffer
.
(See The Definition of append-to-buffer
.)
Previous: Point and Mark, Up: save-excursion
[Contents][Index]
3.10.1 Template for a save-excursion
Expression
The template for code using save-excursion
is simple:
(save-excursion body…)
The body of the function is one or more expressions that will be
evaluated in sequence by the Lisp interpreter. If there is more than
one expression in the body, the value of the last one will be returned
as the value of the save-excursion
function. The other
expressions in the body are evaluated only for their side effects; and
save-excursion
itself is used only for its side effect (which
is restoring the position of point).
In more detail, the template for a save-excursion
expression
looks like this:
(save-excursion first-expression-in-body second-expression-in-body third-expression-in-body … last-expression-in-body)
An expression, of course, may be a symbol on its own or a list.
In Emacs Lisp code, a save-excursion
expression often occurs
within the body of a let
expression. It looks like this:
(let varlist (save-excursion body…))
Next: Exercises, Previous: save-excursion
, Up: How To Write Function Definitions [Contents][Index]
3.11 Review
In the last few chapters we have introduced a macro and a fair number of functions and special forms. Here they are described in brief, along with a few similar functions that have not been mentioned yet.
eval-last-sexp
Evaluate the last symbolic expression before the current location of point. The value is printed in the echo area unless the function is invoked with an argument; in that case, the output is printed in the current buffer. This command is normally bound to C-x C-e.
defun
Define function. This macro has up to five parts: the name, a template for the arguments that will be passed to the function, documentation, an optional interactive declaration, and the body of the definition.
For example, in Emacs the function definition of
dired-unmark-all-marks
is as follows.(defun dired-unmark-all-marks () "Remove all marks from all files in the Dired buffer." (interactive) (dired-unmark-all-files ?\r))
interactive
Declare to the interpreter that the function can be used interactively. This special form may be followed by a string with one or more parts that pass the information to the arguments of the function, in sequence. These parts may also tell the interpreter to prompt for information. Parts of the string are separated by newlines, ‘\n’.
Common code characters are:
b
The name of an existing buffer.
f
The name of an existing file.
p
The numeric prefix argument. (Note that this
p
is lower case.)r
Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one.
See Code Characters for ‘interactive’ in The GNU Emacs Lisp Reference Manual, for a complete list of code characters.
let
Declare that a list of variables is for use within the body of the
let
and give them an initial value, eithernil
or a specified value; then evaluate the rest of the expressions in the body of thelet
and return the value of the last one. Inside the body of thelet
, the Lisp interpreter does not see the values of the variables of the same names that are bound outside of thelet
.For example,
(let ((foo (buffer-name)) (bar (buffer-size))) (message "This buffer is %s and has %d characters." foo bar))
save-excursion
Record the values of point and the current buffer before evaluating the body of this special form. Restore the value of point and buffer afterward.
For example,
(message "We are %d characters into this buffer." (- (point) (save-excursion (goto-char (point-min)) (point))))
if
Evaluate the first argument to the function; if it is true, evaluate the second argument; else evaluate the third argument, if there is one.
The
if
special form is called a conditional. There are other conditionals in Emacs Lisp, butif
is perhaps the most commonly used.For example,
(if (= 22 emacs-major-version) (message "This is version 22 Emacs") (message "This is not version 22 Emacs"))
<
>
<=
>=
The
<
function tests whether its first argument is smaller than its second argument. A corresponding function,>
, tests whether the first argument is greater than the second. Likewise,<=
tests whether the first argument is less than or equal to the second and>=
tests whether the first argument is greater than or equal to the second. In all cases, both arguments must be numbers or markers (markers indicate positions in buffers).=
The
=
function tests whether two arguments, both numbers or markers, are equal.equal
eq
Test whether two objects are the same.
equal
uses one meaning of the word “same” andeq
uses another:equal
returns true if the two objects have a similar structure and contents, such as two copies of the same book. On the other hand,eq
, returns true if both arguments are actually the same object.string<
string-lessp
string=
string-equal
The
string-lessp
function tests whether its first argument is smaller than the second argument. A shorter, alternative name for the same function (adefalias
) isstring<
.The arguments to
string-lessp
must be strings or symbols; the ordering is lexicographic, so case is significant. The print names of symbols are used instead of the symbols themselves.An empty string, ‘""’, a string with no characters in it, is smaller than any string of characters.
string-equal
provides the corresponding test for equality. Its shorter, alternative name isstring=
. There are no string test functions that correspond to >,>=
, or<=
.message
Print a message in the echo area. The first argument is a string that can contain ‘%s’, ‘%d’, or ‘%c’ to print the value of arguments that follow the string. The argument used by ‘%s’ must be a string or a symbol; the argument used by ‘%d’ must be a number. The argument used by ‘%c’ must be an ASCII code number; it will be printed as the character with that ASCII code. (Various other %-sequences have not been mentioned.)
setq
set
The
setq
special form sets the value of its first argument to the value of the second argument. The first argument is automatically quoted bysetq
. It does the same for succeeding pairs of arguments.buffer-name
Without an argument, return the name of the buffer, as a string.
buffer-file-name
Without an argument, return the name of the file the buffer is visiting.
current-buffer
Return the buffer in which Emacs is active; it may not be the buffer that is visible on the screen.
other-buffer
Return the most recently selected buffer (other than the buffer passed to
other-buffer
as an argument and other than the current buffer).switch-to-buffer
Select a buffer for Emacs to be active in and display it in the current window so users can look at it. Usually bound to C-x b.
set-buffer
Switch Emacs’s attention to a buffer on which programs will run. Don’t alter what the window is showing.
buffer-size
Return the number of characters in the current buffer.
point
Return the value of the current position of the cursor, as an integer counting the number of characters from the beginning of the buffer.
point-min
Return the minimum permissible value of point in the current buffer. This is 1, unless narrowing is in effect.
point-max
Return the value of the maximum permissible value of point in the current buffer. This is the end of the buffer, unless narrowing is in effect.
Previous: Review, Up: How To Write Function Definitions [Contents][Index]
3.12 Exercises
- Write a non-interactive function that doubles the value of its argument, a number. Make that function interactive.
- Write a function that tests whether the current value of
fill-column
is greater than the argument passed to the function, and if so, prints an appropriate message.
Next: A Few More Complex Functions, Previous: How To Write Function Definitions, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
4 A Few Buffer-Related Functions
In this chapter we study in detail several of the functions used in GNU Emacs. This is called a “walk-through”. These functions are used as examples of Lisp code, but are not imaginary examples; with the exception of the first, simplified function definition, these functions show the actual code used in GNU Emacs. You can learn a great deal from these definitions. The functions described here are all related to buffers. Later, we will study other functions.
- Finding More Information
- A Simplified
beginning-of-buffer
Definition - The Definition of
mark-whole-buffer
- The Definition of
append-to-buffer
- Review
- Exercises
Next: A Simplified beginning-of-buffer
Definition, Up: A Few Buffer-Related Functions [Contents][Index]
4.1 Finding More Information
In this walk-through, I will describe each new function as we come to it, sometimes in detail and sometimes briefly. If you are interested, you can get the full documentation of any Emacs Lisp function at any time by typing C-h f and then the name of the function (and then RET). Similarly, you can get the full documentation for a variable by typing C-h v and then the name of the variable (and then RET).
Also, describe-function
will tell you the location of the
function definition.
Put point into the name of the file that contains the function and
press the RET key. In this case, RET means
push-button
rather than “return” or “enter”. Emacs will take
you directly to the function definition.
More generally, if you want to see a function in its original source
file, you can use the xref-find-definitions
function to jump to
it. xref-find-definitions
works with a wide variety of
languages, not just Lisp, and C, and it works with non-programming
text as well. For example, xref-find-definitions
will jump to
the various nodes in the Texinfo source file of this document
(provided that you’ve run the etags
utility to record all
the nodes in the manuals that come with Emacs; see Create Tags
Table in The GNU Emacs Manual).
To use the xref-find-definitions
command, type M-.
(i.e., press the period key while holding down the META key, or
else type the ESC key and then type the period key), and then,
at the prompt, type in the name of the function whose source code you
want to see, such as mark-whole-buffer
, and then type
RET. (If the command doesn’t prompt, invoke it with an
argument: C-u M-.; see Different Options for interactive
.) Emacs will
switch buffers and display the source code for the function on your
screen11. To switch back to your current buffer, type M-, or
C-x b RET. (On some keyboards, the META key is
labeled ALT.)
Incidentally, the files that contain Lisp code are conventionally called libraries. The metaphor is derived from that of a specialized library, such as a law library or an engineering library, rather than a general library. Each library, or file, contains functions that relate to a particular topic or activity, such as abbrev.el for handling abbreviations and other typing shortcuts, and help.el for help. (Sometimes several libraries provide code for a single activity, as the various rmail… files provide code for reading electronic mail.) In The GNU Emacs Manual, you will see sentences such as “The C-h p command lets you search the standard Emacs Lisp libraries by topic keywords.”
Next: The Definition of mark-whole-buffer
, Previous: Finding More Information, Up: A Few Buffer-Related Functions [Contents][Index]
4.2 A Simplified beginning-of-buffer
Definition
The beginning-of-buffer
command is a good function to start with
since you are likely to be familiar with it and it is easy to
understand. Used as an interactive command, beginning-of-buffer
moves the cursor to the beginning of the buffer, leaving the mark at the
previous position. It is generally bound to M-<.
In this section, we will discuss a shortened version of the function
that shows how it is most frequently used. This shortened function
works as written, but it does not contain the code for a complex option.
In another section, we will describe the entire function.
(See Complete Definition of
beginning-of-buffer
.)
Before looking at the code, let’s consider what the function definition has to contain: it must include an expression that makes the function interactive so it can be called by typing M-x beginning-of-buffer or by typing a keychord such as M-<; it must include code to leave a mark at the original position in the buffer; and it must include code to move the cursor to the beginning of the buffer.
Here is the complete text of the shortened version of the function:
(defun simplified-beginning-of-buffer () "Move point to the beginning of the buffer; leave mark at previous position." (interactive) (push-mark) (goto-char (point-min)))
Like all function definitions, this definition has five parts following
the macro defun
:
- The name: in this example,
simplified-beginning-of-buffer
. - A list of the arguments: in this example, an empty list,
()
, - The documentation string.
- The interactive expression.
- The body.
In this function definition, the argument list is empty; this means that this function does not require any arguments. (When we look at the definition for the complete function, we will see that it may be passed an optional argument.)
The interactive expression tells Emacs that the function is intended to
be used interactively. In this example, interactive
does not have
an argument because simplified-beginning-of-buffer
does not
require one.
The body of the function consists of the two lines:
(push-mark) (goto-char (point-min))
The first of these lines is the expression, (push-mark)
. When
this expression is evaluated by the Lisp interpreter, it sets a mark at
the current position of the cursor, wherever that may be. The position
of this mark is saved in the mark ring.
The next line is (goto-char (point-min))
. This expression
jumps the cursor to the minimum point in the buffer, that is, to the
beginning of the buffer (or to the beginning of the accessible portion
of the buffer if it is narrowed. See Narrowing and Widening.)
The push-mark
command sets a mark at the place where the cursor
was located before it was moved to the beginning of the buffer by the
(goto-char (point-min))
expression. Consequently, you can, if
you wish, go back to where you were originally by typing C-x C-x.
That is all there is to the function definition!
When you are reading code such as this and come upon an unfamiliar
function, such as goto-char
, you can find out what it does by
using the describe-function
command. To use this command, type
C-h f and then type in the name of the function and press
RET. The describe-function
command will print the
function’s documentation string in a *Help* window. For
example, the documentation for goto-char
is:
Set point to POSITION, a number or marker. Beginning of buffer is position (point-min), end is (point-max).
The function’s one argument is the desired position.
(The prompt for describe-function
will offer you the symbol
under or preceding the cursor, so you can save typing by positioning
the cursor right over or after the function and then typing C-h f
RET.)
The end-of-buffer
function definition is written in the same way as
the beginning-of-buffer
definition except that the body of the
function contains the expression (goto-char (point-max))
in place
of (goto-char (point-min))
.
Next: The Definition of append-to-buffer
, Previous: A Simplified beginning-of-buffer
Definition, Up: A Few Buffer-Related Functions [Contents][Index]
4.3 The Definition of mark-whole-buffer
The mark-whole-buffer
function is no harder to understand than the
simplified-beginning-of-buffer
function. In this case, however,
we will look at the complete function, not a shortened version.
The mark-whole-buffer
function is not as commonly used as the
beginning-of-buffer
function, but is useful nonetheless: it
marks a whole buffer as a region by putting point at the beginning and
a mark at the end of the buffer. It is generally bound to C-x
h.
An overview of mark-whole-buffer
In GNU Emacs 22, the code for the complete function looks like this:
(defun mark-whole-buffer () "Put point at beginning and mark at end of buffer. You probably should not use this function in Lisp programs; it is usually a mistake for a Lisp function to use any subroutine that uses or sets the mark." (interactive) (push-mark (point)) (push-mark (point-max) nil t) (goto-char (point-min)))
Like all other functions, the mark-whole-buffer
function fits
into the template for a function definition. The template looks like
this:
(defun name-of-function (argument-list) "documentation…" (interactive-expression…) body…)
Here is how the function works: the name of the function is
mark-whole-buffer
; it is followed by an empty argument list,
‘()’, which means that the function does not require arguments.
The documentation comes next.
The next line is an (interactive)
expression that tells Emacs
that the function will be used interactively. These details are similar
to the simplified-beginning-of-buffer
function described in the
previous section.
Previous: An overview of mark-whole-buffer
, Up: The Definition of mark-whole-buffer
[Contents][Index]
4.3.1 Body of mark-whole-buffer
The body of the mark-whole-buffer
function consists of three
lines of code:
(push-mark (point)) (push-mark (point-max) nil t) (goto-char (point-min))
The first of these lines is the expression, (push-mark (point))
.
This line does exactly the same job as the first line of the body of
the simplified-beginning-of-buffer
function, which is written
(push-mark)
. In both cases, the Lisp interpreter sets a mark
at the current position of the cursor.
I don’t know why the expression in mark-whole-buffer
is written
(push-mark (point))
and the expression in
beginning-of-buffer
is written (push-mark)
. Perhaps
whoever wrote the code did not know that the arguments for
push-mark
are optional and that if push-mark
is not
passed an argument, the function automatically sets mark at the
location of point by default. Or perhaps the expression was written
so as to parallel the structure of the next line. In any case, the
line causes Emacs to determine the position of point and set a mark
there.
In earlier versions of GNU Emacs, the next line of
mark-whole-buffer
was (push-mark (point-max))
. This
expression sets a mark at the point in the buffer that has the highest
number. This will be the end of the buffer (or, if the buffer is
narrowed, the end of the accessible portion of the buffer.
See Narrowing and Widening, for more about
narrowing.) After this mark has been set, the previous mark, the one
set at point, is no longer set, but Emacs remembers its position, just
as all other recent marks are always remembered. This means that you
can, if you wish, go back to that position by typing C-u
C-SPC twice.
In GNU Emacs 22, the (point-max)
is slightly more complicated.
The line reads
(push-mark (point-max) nil t)
The expression works nearly the same as before. It sets a mark at the
highest numbered place in the buffer that it can. However, in this
version, push-mark
has two additional arguments. The second
argument to push-mark
is nil
. This tells the function
it should display a message that says “Mark set” when it pushes
the mark. The third argument is t
. This tells
push-mark
to activate the mark when Transient Mark mode is
turned on. Transient Mark mode highlights the currently active
region. It is often turned off.
Finally, the last line of the function is (goto-char
(point-min)))
. This is written exactly the same way as it is written
in beginning-of-buffer
. The expression moves the cursor to
the minimum point in the buffer, that is, to the beginning of the buffer
(or to the beginning of the accessible portion of the buffer). As a
result of this, point is placed at the beginning of the buffer and mark
is set at the end of the buffer. The whole buffer is, therefore, the
region.
Next: Review, Previous: The Definition of mark-whole-buffer
, Up: A Few Buffer-Related Functions [Contents][Index]
4.4 The Definition of append-to-buffer
The append-to-buffer
command is more complex than the
mark-whole-buffer
command. What it does is copy the region
(that is, the part of the buffer between point and mark) from the
current buffer to a specified buffer.
- An Overview of
append-to-buffer
- The
append-to-buffer
Interactive Expression - The Body of
append-to-buffer
save-excursion
inappend-to-buffer
Next: The append-to-buffer
Interactive Expression, Up: The Definition of append-to-buffer
[Contents][Index]
An Overview of append-to-buffer
The append-to-buffer
command uses the
insert-buffer-substring
function to copy the region.
insert-buffer-substring
is described by its name: it takes a
substring from a buffer, and inserts it into another buffer.
Most of append-to-buffer
is
concerned with setting up the conditions for
insert-buffer-substring
to work: the code must specify both the
buffer to which the text will go, the window it comes from and goes
to, and the region that will be copied.
Here is a possible implementation of the function:
(defun append-to-buffer (buffer start end) "Append to specified buffer the text of the region. It is inserted into that buffer before its point.
When calling from a program, give three arguments: BUFFER (or buffer name), START and END. START and END specify the portion of the current buffer to be copied." (interactive (list (read-buffer "Append to buffer: " (other-buffer (current-buffer) t)) (region-beginning) (region-end)))
(let ((oldbuf (current-buffer))) (save-excursion (let* ((append-to (get-buffer-create buffer)) (windows (get-buffer-window-list append-to t t)) point) (set-buffer append-to) (setq point (point)) (barf-if-buffer-read-only) (insert-buffer-substring oldbuf start end) (dolist (window windows) (when (= (window-point window) point) (set-window-point window (point))))))))
The function can be understood by looking at it as a series of filled-in templates.
The outermost template is for the function definition. In this function, it looks like this (with several slots filled in):
(defun append-to-buffer (buffer start end) "documentation…" (interactive …) body…)
The first line of the function includes its name and three arguments.
The arguments are the buffer
to which the text will be copied, and
the start
and end
of the region in the current buffer that
will be copied.
The next part of the function is the documentation, which is clear and complete. As is conventional, the three arguments are written in upper case so you will notice them easily. Even better, they are described in the same order as in the argument list.
Note that the documentation distinguishes between a buffer and its name. (The function can handle either.)
Next: The Body of append-to-buffer
, Previous: An Overview of append-to-buffer
, Up: The Definition of append-to-buffer
[Contents][Index]
4.4.1 The append-to-buffer
Interactive Expression
Since the append-to-buffer
function will be used interactively,
the function must have an interactive
expression. (For a
review of interactive
, see Making a
Function Interactive.)
The expression reads as follows:
(interactive (list (read-buffer "Append to buffer: " (other-buffer (current-buffer) t)) (region-beginning) (region-end)))
This expression is not one with letters standing for parts, as described earlier. Instead, it starts a list with these parts:
The first part of the list is an expression to read the name of a
buffer and return it as a string. That is read-buffer
. The
function requires a prompt as its first argument, ‘"Append to
buffer: "’. Its second argument tells the command what value to
provide if you don’t specify anything.
In this case that second argument is an expression containing the
function other-buffer
, an exception, and a ‘t’, standing
for true.
The first argument to other-buffer
, the exception, is yet
another function, current-buffer
. That is not going to be
returned. The second argument is the symbol for true, t
. That
tells other-buffer
that it may show visible buffers (except in
this case, it will not show the current buffer, which makes sense).
The expression looks like this:
(other-buffer (current-buffer) t)
The second and third arguments to the list
expression are
(region-beginning)
and (region-end)
. These two
functions specify the beginning and end of the text to be appended.
Originally, the command used the letters ‘B’ and ‘r’.
The whole interactive
expression looked like this:
(interactive "BAppend to buffer: \nr")
But when that was done, the default value of the buffer switched to was invisible. That was not wanted.
(The prompt was separated from the second argument with a newline,
‘\n’. It was followed by an ‘r’ that told Emacs to bind the
two arguments that follow the symbol buffer
in the function’s
argument list (that is, start
and end
) to the values of
point and mark. That argument worked fine.)
Next: save-excursion
in append-to-buffer
, Previous: The append-to-buffer
Interactive Expression, Up: The Definition of append-to-buffer
[Contents][Index]
4.4.2 The Body of append-to-buffer
The body of the append-to-buffer
function begins with let
.
As we have seen before (see let
), the purpose of a
let
expression is to create and give initial values to one or
more variables that will only be used within the body of the
let
. This means that such a variable will not be confused with
any variable of the same name outside the let
expression.
We can see how the let
expression fits into the function as a
whole by showing a template for append-to-buffer
with the
let
expression in outline:
(defun append-to-buffer (buffer start end) "documentation…" (interactive …) (let ((variable value)) body…))
The let
expression has three elements:
- The symbol
let
; - A varlist containing, in this case, a single two-element list,
(variable value)
; - The body of the
let
expression.
In the append-to-buffer
function, the varlist looks like this:
(oldbuf (current-buffer))
In this part of the let
expression, the one variable,
oldbuf
, is bound to the value returned by the
(current-buffer)
expression. The variable, oldbuf
, is
used to keep track of the buffer in which you are working and from
which you will copy.
The element or elements of a varlist are surrounded by a set of
parentheses so the Lisp interpreter can distinguish the varlist from
the body of the let
. As a consequence, the two-element list
within the varlist is surrounded by a circumscribing set of parentheses.
The line looks like this:
(let ((oldbuf (current-buffer))) … )
The two parentheses before oldbuf
might surprise you if you did
not realize that the first parenthesis before oldbuf
marks the
boundary of the varlist and the second parenthesis marks the beginning
of the two-element list, (oldbuf (current-buffer))
.
Previous: The Body of append-to-buffer
, Up: The Definition of append-to-buffer
[Contents][Index]
4.4.3 save-excursion
in append-to-buffer
The body of the let
expression in append-to-buffer
consists of a save-excursion
expression.
The save-excursion
function saves the location of point, and restores it
to that position after the expressions in the
body of the save-excursion
complete execution. In addition,
save-excursion
keeps track of the original buffer, and
restores it. This is how save-excursion
is used in
append-to-buffer
.
Incidentally, it is worth noting here that a Lisp function is normally
formatted so that everything that is enclosed in a multi-line spread is
indented more to the right than the first symbol. In this function
definition, the let
is indented more than the defun
, and
the save-excursion
is indented more than the let
, like
this:
(defun … … … (let… (save-excursion …
This formatting convention makes it easy to see that the lines in
the body of the save-excursion
are enclosed by the parentheses
associated with save-excursion
, just as the
save-excursion
itself is enclosed by the parentheses associated
with the let
:
(let ((oldbuf (current-buffer))) (save-excursion … (set-buffer …) (insert-buffer-substring oldbuf start end) …))
The use of the save-excursion
function can be viewed as a process
of filling in the slots of a template:
(save-excursion first-expression-in-body second-expression-in-body … last-expression-in-body)
In this function, the body of the save-excursion
contains only
one expression, the let*
expression. You know about a
let
function. The let*
function is different. It
enables Emacs to set each variable in its varlist in sequence, one
after another; such that variables in the latter part of the varlist
can make use of the values to which Emacs set variables earlier in the
varlist.
Looking at the let*
expression in append-to-buffer
:
(let* ((append-to (get-buffer-create buffer)) (windows (get-buffer-window-list append-to t t)) point) BODY...)
we see that append-to
is bound to the value returned by the
(get-buffer-create buffer)
. On the next line,
append-to
is used as an argument to
get-buffer-window-list
; this would not be possible with the
let
expression. Note that point
is automatically bound
to nil
, the same way as it would be done in the let
statement.
Now let’s focus on the functions set-buffer
and
insert-buffer-substring
in the body of the let*
expression.
In the old days, the set-buffer
expression was simply
(set-buffer (get-buffer-create buffer))
but now it is
(set-buffer append-to)
This is because append-to
was bound to (get-buffer-create
buffer)
earlier on in the let*
expression.
The append-to-buffer
function definition inserts text from the
buffer in which you are currently to a named buffer. It happens that
insert-buffer-substring
does just the reverse—it copies text
from another buffer to the current buffer—that is why the
append-to-buffer
definition starts out with a let
that
binds the local symbol oldbuf
to the value returned by
current-buffer
.
The insert-buffer-substring
expression looks like this:
(insert-buffer-substring oldbuf start end)
The insert-buffer-substring
function copies a string
from the buffer specified as its first argument and inserts the
string into the present buffer. In this case, the argument to
insert-buffer-substring
is the value of the variable created
and bound by the let
, namely the value of oldbuf
, which
was the current buffer when you gave the append-to-buffer
command.
After insert-buffer-substring
has done its work,
save-excursion
will restore the action to the original buffer
and append-to-buffer
will have done its job.
Written in skeletal form, the workings of the body look like this:
(let (bind-oldbuf
-to-value-of-current-buffer
) (save-excursion ; Keep track of buffer. change-buffer insert-substring-from-oldbuf
-into-buffer) change-back-to-original-buffer-when-finished let-the-local-meaning-of-oldbuf
-disappear-when-finished
In summary, append-to-buffer
works as follows: it saves the
value of the current buffer in the variable called oldbuf
. It
gets the new buffer (creating one if need be) and switches Emacs’s
attention to it. Using the value of oldbuf
, it inserts the
region of text from the old buffer into the new buffer; and then using
save-excursion
, it brings you back to your original buffer.
In looking at append-to-buffer
, you have explored a fairly
complex function. It shows how to use let
and
save-excursion
, and how to change to and come back from another
buffer. Many function definitions use let
,
save-excursion
, and set-buffer
this way.
Next: Exercises, Previous: The Definition of append-to-buffer
, Up: A Few Buffer-Related Functions [Contents][Index]
4.5 Review
Here is a brief summary of the various functions discussed in this chapter.
describe-function
describe-variable
Print the documentation for a function or variable. Conventionally bound to C-h f and C-h v.
xref-find-definitions
Find the file containing the source for a function or variable and switch buffers to it, positioning point at the beginning of the item. Conventionally bound to M-. (that’s a period following the META key).
save-excursion
Save the location of point and restore its value after the arguments to
save-excursion
have been evaluated. Also, remember the current buffer and return to it.push-mark
Set mark at a location and record the value of the previous mark on the mark ring. The mark is a location in the buffer that will keep its relative position even if text is added to or removed from the buffer.
goto-char
Set point to the location specified by the value of the argument, which can be a number, a marker, or an expression that returns the number of a position, such as
(point-min)
.insert-buffer-substring
Copy a region of text from a buffer that is passed to the function as an argument and insert the region into the current buffer.
mark-whole-buffer
Mark the whole buffer as a region. Normally bound to C-x h.
let*
Declare a list of variables and give them an initial value; then evaluate the rest of the expressions in the body of
let*
. The values of the variables can be used to bind ensuing variables in the list.set-buffer
Switch the attention of Emacs to another buffer, but do not change the window being displayed. Used when the program rather than a human is to work on a different buffer.
get-buffer-create
get-buffer
Find a named buffer or create one if a buffer of that name does not exist. The
get-buffer
function returnsnil
if the named buffer does not exist.
Previous: Review, Up: A Few Buffer-Related Functions [Contents][Index]
4.6 Exercises
- Write your own
simplified-end-of-buffer
function definition; then test it to see whether it works. - Use
if
andget-buffer
to write a function that prints a message telling you whether a buffer exists. - Using
xref-find-definitions
, find the source for thecopy-to-buffer
function.
Next: Narrowing and Widening, Previous: A Few Buffer-Related Functions, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
5 A Few More Complex Functions
In this chapter, we build on what we have learned in previous chapters
by looking at more complex functions. The copy-to-buffer
function illustrates use of two save-excursion
expressions in
one definition, while the insert-buffer
function illustrates
use of an asterisk in an interactive
expression, use of
or
, and the important distinction between a name and the object
to which the name refers.
- The Definition of
copy-to-buffer
- The Definition of
insert-buffer
- Complete Definition of
beginning-of-buffer
- Review
optional
Argument Exercise
5.1 The Definition of copy-to-buffer
After understanding how append-to-buffer
works, it is easy to
understand copy-to-buffer
. This function copies text into a
buffer, but instead of adding to the second buffer, it replaces all the
previous text in the second buffer.
The body of copy-to-buffer
looks like this,
… (interactive "BCopy to buffer: \nr") (let ((oldbuf (current-buffer))) (with-current-buffer (get-buffer-create buffer) (barf-if-buffer-read-only) (erase-buffer) (save-excursion (insert-buffer-substring oldbuf start end)))))
The copy-to-buffer
function has a simpler interactive
expression than append-to-buffer
.
The definition then says
(with-current-buffer (get-buffer-create buffer) …
First, look at the earliest inner expression; that is evaluated first.
That expression starts with get-buffer-create buffer
. The
function tells the computer to use the buffer with the name specified
as the one to which you are copying, or if such a buffer does not
exist, to create it. Then, the with-current-buffer
function
evaluates its body with that buffer temporarily current, after which
it will switch back to the buffer we are at now12.
(This demonstrates another way to shift the computer’s attention but
not the user’s. The append-to-buffer
function showed how to do
the same with save-excursion
and set-buffer
.
with-current-buffer
is a newer, and arguably easier,
mechanism.)
The barf-if-buffer-read-only
function sends you an error
message saying the buffer is read-only if you cannot modify it.
The next line has the erase-buffer
function as its sole
contents. That function erases the buffer.
Finally, the last two lines contain the save-excursion
expression with insert-buffer-substring
as its body.
The insert-buffer-substring
expression copies the text from
the buffer you are in (and you have not seen the computer shift its
attention, so you don’t know that that buffer is now called
oldbuf
).
Incidentally, this is what is meant by “replacement”. To replace text, Emacs erases the previous text and then inserts new text.
In outline, the body of copy-to-buffer
looks like this:
(let (bind-oldbuf
-to-value-of-current-buffer
) (with-the-buffer-you-are-copying-to (but-do-not-erase-or-copy-to-a-read-only-buffer) (erase-buffer) (save-excursion insert-substring-from-oldbuf
-into-buffer)))
Next: Complete Definition of beginning-of-buffer
, Previous: The Definition of copy-to-buffer
, Up: A Few More Complex Functions [Contents][Index]
5.2 The Definition of insert-buffer
insert-buffer
is yet another buffer-related function. This
command copies another buffer into the current buffer. It is the
reverse of append-to-buffer
or copy-to-buffer
, since they
copy a region of text from the current buffer to another buffer.
Here is a discussion based on the original code. The code was simplified in 2003 and is harder to understand.
(See New Body for insert-buffer
, to see
a discussion of the new body.)
In addition, this code illustrates the use of interactive
with a
buffer that might be read-only and the important distinction
between the name of an object and the object actually referred to.
- The Code for
insert-buffer
- The Interactive Expression in
insert-buffer
- The Body of the
insert-buffer
Function insert-buffer
With anif
Instead of anor
- The
or
in the Body - The
let
Expression ininsert-buffer
- New Body for
insert-buffer
Next: The Interactive Expression in insert-buffer
, Up: The Definition of insert-buffer
[Contents][Index]
The Code for insert-buffer
Here is the earlier code:
(defun insert-buffer (buffer) "Insert after point the contents of BUFFER. Puts mark after the inserted text. BUFFER may be a buffer or a buffer name." (interactive "*bInsert buffer: ")
(or (bufferp buffer) (setq buffer (get-buffer buffer))) (let (start end newmark) (save-excursion (save-excursion (set-buffer buffer) (setq start (point-min) end (point-max)))
(insert-buffer-substring buffer start end) (setq newmark (point))) (push-mark newmark)))
As with other function definitions, you can use a template to see an outline of the function:
(defun insert-buffer (buffer) "documentation…" (interactive "*bInsert buffer: ") body…)
Next: The Body of the insert-buffer
Function, Previous: The Code for insert-buffer
, Up: The Definition of insert-buffer
[Contents][Index]
5.2.1 The Interactive Expression in insert-buffer
In insert-buffer
, the argument to the interactive
declaration has two parts, an asterisk, ‘*’, and ‘bInsert
buffer: ’.
Next: ‘b’ in an Interactive Expression, Up: The Interactive Expression in insert-buffer
[Contents][Index]
A Read-only Buffer
The asterisk is for the situation when the current buffer is a
read-only buffer—a buffer that cannot be modified. If
insert-buffer
is called when the current buffer is read-only, a
message to this effect is printed in the echo area and the terminal
may beep or blink at you; you will not be permitted to insert anything
into current buffer. The asterisk does not need to be followed by a
newline to separate it from the next argument.
Previous: A Read-only Buffer, Up: The Interactive Expression in insert-buffer
[Contents][Index]
‘b’ in an Interactive Expression
The next argument in the interactive expression starts with a lower
case ‘b’. (This is different from the code for
append-to-buffer
, which uses an upper-case ‘B’.
See The Definition of append-to-buffer
.)
The lower-case ‘b’ tells the Lisp interpreter that the argument
for insert-buffer
should be an existing buffer or else its
name. (The upper-case ‘B’ option provides for the possibility
that the buffer does not exist.) Emacs will prompt you for the name
of the buffer, offering you a default buffer, with name completion
enabled. If the buffer does not exist, you receive a message that
says “No match”; your terminal may beep at you as well.
The new and simplified code generates a list for interactive
.
It uses the barf-if-buffer-read-only
and read-buffer
functions with which we are already familiar and the progn
special form with which we are not. (It will be described later.)
Next: insert-buffer
With an if
Instead of an or
, Previous: The Interactive Expression in insert-buffer
, Up: The Definition of insert-buffer
[Contents][Index]
5.2.2 The Body of the insert-buffer
Function
The body of the insert-buffer
function has two major parts: an
or
expression and a let
expression. The purpose of the
or
expression is to ensure that the argument buffer
is
bound to a buffer and not just the name of a buffer. The body of the
let
expression contains the code which copies the other buffer
into the current buffer.
In outline, the two expressions fit into the insert-buffer
function like this:
(defun insert-buffer (buffer) "documentation…" (interactive "*bInsert buffer: ") (or … …
(let (varlist)
body-of-let
… )
To understand how the or
expression ensures that the argument
buffer
is bound to a buffer and not to the name of a buffer, it
is first necessary to understand the or
function.
Before doing this, let me rewrite this part of the function using
if
so that you can see what is done in a manner that will be familiar.
Next: The or
in the Body, Previous: The Body of the insert-buffer
Function, Up: The Definition of insert-buffer
[Contents][Index]
5.2.3 insert-buffer
With an if
Instead of an or
The job to be done is to make sure the value of buffer
is a
buffer itself and not the name of a buffer. If the value is the name,
then the buffer itself must be got.
You can imagine yourself at a conference where an usher is wandering around holding a list with your name on it and looking for you: the usher is bound to your name, not to you; but when the usher finds you and takes your arm, the usher becomes bound to you.
In Lisp, you might describe this situation like this:
(if (not (holding-on-to-guest)) (find-and-take-arm-of-guest))
We want to do the same thing with a buffer—if we do not have the buffer itself, we want to get it.
Using a predicate called bufferp
that tells us whether we have a
buffer (rather than its name), we can write the code like this:
(if (not (bufferp buffer)) ; if-part (setq buffer (get-buffer buffer))) ; then-part
Here, the true-or-false-test of the if
expression is
(not (bufferp buffer))
; and the then-part is the expression
(setq buffer (get-buffer buffer))
.
In the test, the function bufferp
returns true if its argument is
a buffer—but false if its argument is the name of the buffer. (The
last character of the function name bufferp
is the character
‘p’; as we saw earlier, such use of ‘p’ is a convention that
indicates that the function is a predicate, which is a term that means
that the function will determine whether some property is true or false.
See Using the Wrong Type Object as an
Argument.)
The function not
precedes the expression (bufferp buffer)
,
so the true-or-false-test looks like this:
(not (bufferp buffer))
not
is a function that returns true if its argument is false
and false if its argument is true. So if (bufferp buffer)
returns true, the not
expression returns false and vice versa.
Using this test, the if
expression works as follows: when the
value of the variable buffer
is actually a buffer rather than
its name, the true-or-false-test returns false and the if
expression does not evaluate the then-part. This is fine, since we do
not need to do anything to the variable buffer
if it really is
a buffer.
On the other hand, when the value of buffer
is not a buffer
itself, but the name of a buffer, the true-or-false-test returns true
and the then-part of the expression is evaluated. In this case, the
then-part is (setq buffer (get-buffer buffer))
. This
expression uses the get-buffer
function to return an actual
buffer itself, given its name. The setq
then sets the variable
buffer
to the value of the buffer itself, replacing its previous
value (which was the name of the buffer).
Next: The let
Expression in insert-buffer
, Previous: insert-buffer
With an if
Instead of an or
, Up: The Definition of insert-buffer
[Contents][Index]
5.2.4 The or
in the Body
The purpose of the or
expression in the insert-buffer
function is to ensure that the argument buffer
is bound to a
buffer and not just to the name of a buffer. The previous section shows
how the job could have been done using an if
expression.
However, the insert-buffer
function actually uses or
.
To understand this, it is necessary to understand how or
works.
An or
function can have any number of arguments. It evaluates
each argument in turn and returns the value of the first of its
arguments that is not nil
. Also, and this is a crucial feature
of or
, it does not evaluate any subsequent arguments after
returning the first non-nil
value.
The or
expression looks like this:
(or (bufferp buffer) (setq buffer (get-buffer buffer)))
The first argument to or
is the expression (bufferp buffer)
.
This expression returns true (a non-nil
value) if the buffer is
actually a buffer, and not just the name of a buffer. In the or
expression, if this is the case, the or
expression returns this
true value and does not evaluate the next expression—and this is fine
with us, since we do not want to do anything to the value of
buffer
if it really is a buffer.
On the other hand, if the value of (bufferp buffer)
is nil
,
which it will be if the value of buffer
is the name of a buffer,
the Lisp interpreter evaluates the next element of the or
expression. This is the expression (setq buffer (get-buffer
buffer))
. This expression returns a non-nil
value, which
is the value to which it sets the variable buffer
—and this
value is a buffer itself, not the name of a buffer.
The result of all this is that the symbol buffer
is always
bound to a buffer itself rather than to the name of a buffer. All
this is necessary because the set-buffer
function in a
following line only works with a buffer itself, not with the name to a
buffer.
Incidentally, using or
, the situation with the usher would be
written like this:
(or (holding-on-to-guest) (find-and-take-arm-of-guest))
Next: New Body for insert-buffer
, Previous: The or
in the Body, Up: The Definition of insert-buffer
[Contents][Index]
5.2.5 The let
Expression in insert-buffer
After ensuring that the variable buffer
refers to a buffer itself
and not just to the name of a buffer, the insert-buffer
function
continues with a let
expression. This specifies three local
variables, start
, end
, and newmark
and binds them
to the initial value nil
. These variables are used inside the
remainder of the let
and temporarily hide any other occurrence of
variables of the same name in Emacs until the end of the let
.
The body of the let
contains two save-excursion
expressions. First, we will look at the inner save-excursion
expression in detail. The expression looks like this:
(save-excursion (set-buffer buffer) (setq start (point-min) end (point-max)))
The expression (set-buffer buffer)
changes Emacs’s attention
from the current buffer to the one from which the text will copied.
In that buffer, the variables start
and end
are set to
the beginning and end of the buffer, using the commands
point-min
and point-max
. Note that we have here an
illustration of how setq
is able to set two variables in the
same expression. The first argument of setq
is set to the
value of its second, and its third argument is set to the value of its
fourth.
After the body of the inner save-excursion
is evaluated, the
save-excursion
restores the original buffer, but start
and
end
remain set to the values of the beginning and end of the
buffer from which the text will be copied.
The outer save-excursion
expression looks like this:
(save-excursion (inner-save-excursion
-expression (go-to-new-buffer-and-set-start
-and-end
) (insert-buffer-substring buffer start end) (setq newmark (point)))
The insert-buffer-substring
function copies the text
into the current buffer from the region indicated by
start
and end
in buffer
. Since the whole of the
second buffer lies between start
and end
, the whole of
the second buffer is copied into the buffer you are editing. Next,
the value of point, which will be at the end of the inserted text, is
recorded in the variable newmark
.
After the body of the outer save-excursion
is evaluated, point
is relocated to its original place.
However, it is convenient to locate a mark at the end of the newly
inserted text and locate point at its beginning. The newmark
variable records the end of the inserted text. In the last line of
the let
expression, the (push-mark newmark)
expression
function sets a mark to this location. (The previous location of the
mark is still accessible; it is recorded on the mark ring and you can
go back to it with C-u C-SPC.) Meanwhile, point is
located at the beginning of the inserted text, which is where it was
before you called the insert function, the position of which was saved
by the first save-excursion
.
The whole let
expression looks like this:
(let (start end newmark) (save-excursion (save-excursion (set-buffer buffer) (setq start (point-min) end (point-max))) (insert-buffer-substring buffer start end) (setq newmark (point))) (push-mark newmark))
Like the append-to-buffer
function, the insert-buffer
function uses let
, save-excursion
, and
set-buffer
. In addition, the function illustrates one way to
use or
. All these functions are building blocks that we will
find and use again and again.
Previous: The let
Expression in insert-buffer
, Up: The Definition of insert-buffer
[Contents][Index]
5.2.6 New Body for insert-buffer
The body in the GNU Emacs 22 version is more confusing than the original.
It consists of two expressions,
(push-mark (save-excursion (insert-buffer-substring (get-buffer buffer)) (point))) nil
except, and this is what confuses novices, very important work is done
inside the push-mark
expression.
The get-buffer
function returns a buffer with the name
provided. You will note that the function is not called
get-buffer-create
; it does not create a buffer if one does not
already exist. The buffer returned by get-buffer
, an existing
buffer, is passed to insert-buffer-substring
, which inserts the
whole of the buffer (since you did not specify anything else).
The location into which the buffer is inserted is recorded by
push-mark
. Then the function returns nil
, the value of
its last command. Put another way, the insert-buffer
function
exists only to produce a side effect, inserting another buffer, not to
return any value.
Next: Review, Previous: The Definition of insert-buffer
, Up: A Few More Complex Functions [Contents][Index]
5.3 Complete Definition of beginning-of-buffer
The basic structure of the beginning-of-buffer
function has
already been discussed. (See A
Simplified beginning-of-buffer
Definition.)
This section describes the complex part of the definition.
As previously described, when invoked without an argument,
beginning-of-buffer
moves the cursor to the beginning of the
buffer (in truth, the beginning of the accessible portion of the
buffer), leaving the mark at the previous position. However, when the
command is invoked with a number between one and ten, the function
considers that number to be a fraction of the length of the buffer,
measured in tenths, and Emacs moves the cursor that fraction of the
way from the beginning of the buffer. Thus, you can either call this
function with the key command M-<, which will move the cursor to
the beginning of the buffer, or with a key command such as C-u 7
M-< which will move the cursor to a point 70% of the way through the
buffer. If a number bigger than ten is used for the argument, it
moves to the end of the buffer.
The beginning-of-buffer
function can be called with or without an
argument. The use of the argument is optional.
Next: beginning-of-buffer
with an Argument, Up: Complete Definition of beginning-of-buffer
[Contents][Index]
5.3.1 Optional Arguments
Unless told otherwise, Lisp expects that a function with an argument in its function definition will be called with a value for that argument. If that does not happen, you get an error and a message that says ‘Wrong number of arguments’.
However, optional arguments are a feature of Lisp: a particular
keyword is used to tell the Lisp interpreter that an argument is
optional. The keyword is &optional
. (The ‘&’ in front of
‘optional’ is part of the keyword.) In a function definition, if
an argument follows the keyword &optional
, no value need be
passed to that argument when the function is called.
The first line of the function definition of beginning-of-buffer
therefore looks like this:
(defun beginning-of-buffer (&optional arg)
In outline, the whole function looks like this:
(defun beginning-of-buffer (&optional arg) "documentation…" (interactive "P") (or (is-the-argument-a-cons-cell arg) (and are-both-transient-mark-mode-and-mark-active-true) (push-mark)) (let (determine-size-and-set-it) (goto-char (if-there-is-an-argument figure-out-where-to-go else-go-to (point-min)))) do-nicety
The function is similar to the simplified-beginning-of-buffer
function except that the interactive
expression has "P"
as an argument and the goto-char
function is followed by an
if-then-else expression that figures out where to put the cursor if
there is an argument that is not a cons cell.
(Since I do not explain a cons cell for many more chapters, please
consider ignoring the function consp
. See How Lists are Implemented, and Cons Cell and List Types in The GNU Emacs Lisp Reference
Manual.)
The "P"
in the interactive
expression tells Emacs to
pass a prefix argument, if there is one, to the function in raw form.
A prefix argument is made by typing the META key followed by a
number, or by typing C-u and then a number. (If you don’t type
a number, C-u defaults to a cons cell with a 4. A lowercase
"p"
in the interactive
expression causes the function to
convert a prefix arg to a number.)
The true-or-false-test of the if
expression looks complex, but
it is not: it checks whether arg
has a value that is not
nil
and whether it is a cons cell. (That is what consp
does; it checks whether its argument is a cons cell.) If arg
has a value that is not nil
(and is not a cons cell), which
will be the case if beginning-of-buffer
is called with a
numeric argument, then this true-or-false-test will return true and
the then-part of the if
expression will be evaluated. On the
other hand, if beginning-of-buffer
is not called with an
argument, the value of arg
will be nil
and the else-part
of the if
expression will be evaluated. The else-part is
simply point-min
, and when this is the outcome, the whole
goto-char
expression is (goto-char (point-min))
, which
is how we saw the beginning-of-buffer
function in its
simplified form.
Next: The Complete beginning-of-buffer
, Previous: Optional Arguments, Up: Complete Definition of beginning-of-buffer
[Contents][Index]
5.3.2 beginning-of-buffer
with an Argument
When beginning-of-buffer
is called with an argument, an
expression is evaluated which calculates what value to pass to
goto-char
. This expression is rather complicated at first sight.
It includes an inner if
expression and much arithmetic. It looks
like this:
(if (> (buffer-size) 10000)
;; Avoid overflow for large buffer sizes!
(* (prefix-numeric-value arg)
(/ size 10))
(/
(+ 10
(* size
(prefix-numeric-value arg)))
10))
Disentangle beginning-of-buffer
Like other complex-looking expressions, the conditional expression
within beginning-of-buffer
can be disentangled by looking at it
as parts of a template, in this case, the template for an if-then-else
expression. In skeletal form, the expression looks like this:
(if (buffer-is-large divide-buffer-size-by-10-and-multiply-by-arg else-use-alternate-calculation
The true-or-false-test of this inner if
expression checks the
size of the buffer. The reason for this is that the old version 18
Emacs used numbers that are no bigger than eight million or so and in
the computation that followed, the programmer feared that Emacs might
try to use over-large numbers if the buffer were large. The term
“overflow”, mentioned in the comment, means numbers that are over
large. More recent versions of Emacs use larger numbers, but this
code has not been touched, if only because people now look at buffers
that are far, far larger than ever before.
There are two cases: if the buffer is large and if it is not.
Next: What happens in a small buffer, Previous: Disentangle beginning-of-buffer
, Up: beginning-of-buffer
with an Argument [Contents][Index]
What happens in a large buffer
In beginning-of-buffer
, the inner if
expression tests
whether the size of the buffer is greater than 10,000 characters. To do
this, it uses the >
function and the computation of size
that comes from the let expression.
In the old days, the function buffer-size
was used. Not only
was that function called several times, it gave the size of the whole
buffer, not the accessible part. The computation makes much more
sense when it handles just the accessible part. (See Narrowing and Widening, for more information on focusing
attention to an accessible part.)
The line looks like this:
(if (> size 10000)
When the buffer is large, the then-part of the if
expression is
evaluated. It reads like this (after formatting for easy reading):
(* (prefix-numeric-value arg) (/ size 10))
This expression is a multiplication, with two arguments to the function
*
.
The first argument is (prefix-numeric-value arg)
. When
"P"
is used as the argument for interactive
, the value
passed to the function as its argument is passed a raw prefix
argument, and not a number. (It is a number in a list.) To perform
the arithmetic, a conversion is necessary, and
prefix-numeric-value
does the job.
The second argument is (/ size 10)
. This expression divides
the numeric value by ten—the numeric value of the size of the
accessible portion of the buffer. This produces a number that tells
how many characters make up one tenth of the buffer size. (In Lisp,
/
is used for division, just as *
is used for
multiplication.)
In the multiplication expression as a whole, this amount is multiplied by the value of the prefix argument—the multiplication looks like this:
(* numeric-value-of-prefix-arg number-of-characters-in-one-tenth-of-the-accessible-buffer)
If, for example, the prefix argument is ‘7’, the one-tenth value will be multiplied by 7 to give a position 70% of the way through.
The result of all this is that if the accessible portion of the buffer
is large, the goto-char
expression reads like this:
(goto-char (* (prefix-numeric-value arg) (/ size 10)))
This puts the cursor where we want it.
Previous: What happens in a large buffer, Up: beginning-of-buffer
with an Argument [Contents][Index]
What happens in a small buffer
If the buffer contains fewer than 10,000 characters, a slightly different computation is performed. You might think this is not necessary, since the first computation could do the job. However, in a small buffer, the first method may not put the cursor on exactly the desired line; the second method does a better job.
The code looks like this:
(/ (+ 10 (* size (prefix-numeric-value arg))) 10)
This is code in which you figure out what happens by discovering how the functions are embedded in parentheses. It is easier to read if you reformat it with each expression indented more deeply than its enclosing expression:
(/ (+ 10 (* size (prefix-numeric-value arg))) 10)
Looking at parentheses, we see that the innermost operation is
(prefix-numeric-value arg)
, which converts the raw argument to
a number. In the following expression, this number is multiplied by
the size of the accessible portion of the buffer:
(* size (prefix-numeric-value arg))
This multiplication creates a number that may be larger than the size of the buffer—seven times larger if the argument is 7, for example. Ten is then added to this number and finally the large number is divided by ten to provide a value that is one character larger than the percentage position in the buffer.
The number that results from all this is passed to goto-char
and
the cursor is moved to that point.
Previous: beginning-of-buffer
with an Argument, Up: Complete Definition of beginning-of-buffer
[Contents][Index]
5.3.3 The Complete beginning-of-buffer
Here is the complete text of the beginning-of-buffer
function:
(defun beginning-of-buffer (&optional arg) "Move point to the beginning of the buffer; leave mark at previous position. With \\[universal-argument] prefix, do not set mark at previous position. With numeric arg N, put point N/10 of the way from the beginning. If the buffer is narrowed, this command uses the beginning and size of the accessible part of the buffer.
Don't use this command in Lisp programs! \(goto-char (point-min)) is faster and avoids clobbering the mark." (interactive "P") (or (consp arg) (and transient-mark-mode mark-active) (push-mark))
(let ((size (- (point-max) (point-min)))) (goto-char (if (and arg (not (consp arg))) (+ (point-min) (if (> size 10000) ;; Avoid overflow for large buffer sizes! (* (prefix-numeric-value arg) (/ size 10)) (/ (+ 10 (* size (prefix-numeric-value arg))) 10))) (point-min)))) (if (and arg (not (consp arg))) (forward-line 1)))
Except for two small points, the previous discussion shows how this function works. The first point deals with a detail in the documentation string, and the second point concerns the last line of the function.
In the documentation string, there is reference to an expression:
\\[universal-argument]
A ‘\\’ is used before the first square bracket of this
expression. This ‘\\’ tells the Lisp interpreter to substitute
whatever key is currently bound to the ‘[…]’. In the case
of universal-argument
, that is usually C-u, but it might
be different. (See Tips for Documentation
Strings in The GNU Emacs Lisp Reference Manual, for more
information.)
Finally, the last line of the beginning-of-buffer
command says
to move point to the beginning of the next line if the command is
invoked with an argument:
(if (and arg (not (consp arg))) (forward-line 1))
This puts the cursor at the beginning of the first line after the
appropriate tenths position in the buffer. This is a flourish that
means that the cursor is always located at least the requested
tenths of the way through the buffer, which is a nicety that is,
perhaps, not necessary, but which, if it did not occur, would be sure
to draw complaints. (The (not (consp arg))
portion is so that
if you specify the command with a C-u, but without a number,
that is to say, if the raw prefix argument is simply a cons cell,
the command does not put you at the beginning of the second line.)
Next: optional
Argument Exercise, Previous: Complete Definition of beginning-of-buffer
, Up: A Few More Complex Functions [Contents][Index]
5.4 Review
Here is a brief summary of some of the topics covered in this chapter.
or
Evaluate each argument in sequence, and return the value of the first argument that is not
nil
; if none return a value that is notnil
, returnnil
. In brief, return the first true value of the arguments; return a true value if one or any of the others are true.and
Evaluate each argument in sequence, and if any are
nil
, returnnil
; if none arenil
, return the value of the last argument. In brief, return a true value only if all the arguments are true; return a true value if one and each of the others is true.&optional
A keyword used to indicate that an argument to a function definition is optional; this means that the function can be evaluated without the argument, if desired.
prefix-numeric-value
Convert the raw prefix argument produced by
(interactive "P")
to a numeric value.forward-line
Move point forward to the beginning of the next line, or if the argument is greater than one, forward that many lines. If it can’t move as far forward as it is supposed to,
forward-line
goes forward as far as it can and then returns a count of the number of additional lines it was supposed to move but couldn’t.erase-buffer
Delete the entire contents of the current buffer.
bufferp
Return
t
if its argument is a buffer; otherwise returnnil
.
Previous: Review, Up: A Few More Complex Functions [Contents][Index]
5.5 optional
Argument Exercise
Write an interactive function with an optional argument that tests
whether its argument, a number, is greater than or equal to, or else,
less than the value of fill-column
, and tells you which, in a
message. However, if you do not pass an argument to the function, use
56 as a default value.
Next: car
, cdr
, cons
: Fundamental Functions, Previous: A Few More Complex Functions, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
6 Narrowing and Widening
Narrowing is a feature of Emacs that makes it possible for you to focus on a specific part of a buffer, and work without accidentally changing other parts. Narrowing is normally disabled since it can confuse novices.
Next: The save-restriction
Special Form, Up: Narrowing and Widening [Contents][Index]
The Advantages of Narrowing
With narrowing, the rest of a buffer is made invisible, as if it weren’t
there. This is an advantage if, for example, you want to replace a word
in one part of a buffer but not in another: you narrow to the part you want
and the replacement is carried out only in that section, not in the rest
of the buffer. Searches will only work within a narrowed region, not
outside of one, so if you are fixing a part of a document, you can keep
yourself from accidentally finding parts you do not need to fix by
narrowing just to the region you want.
(The key binding for narrow-to-region
is C-x n n.)
However, narrowing does make the rest of the buffer invisible, which
can scare people who inadvertently invoke narrowing and think they
have deleted a part of their file. Moreover, the undo
command
(which is usually bound to C-x u) does not turn off narrowing
(nor should it), so people can become quite desperate if they do not
know that they can return the rest of a buffer to visibility with the
widen
command.
(The key binding for widen
is C-x n w.)
Narrowing is just as useful to the Lisp interpreter as to a human.
Often, an Emacs Lisp function is designed to work on just part of a
buffer; or conversely, an Emacs Lisp function needs to work on all of a
buffer that has been narrowed. The what-line
function, for
example, removes the narrowing from a buffer, if it has any narrowing
and when it has finished its job, restores the narrowing to what it was.
On the other hand, the count-lines
function
uses narrowing to restrict itself to just that portion
of the buffer in which it is interested and then restores the previous
situation.
Next: what-line
, Previous: The Advantages of Narrowing, Up: Narrowing and Widening [Contents][Index]
6.1 The save-restriction
Special Form
In Emacs Lisp, you can use the save-restriction
special form to
keep track of whatever narrowing is in effect, if any. When the Lisp
interpreter meets with save-restriction
, it executes the code
in the body of the save-restriction
expression, and then undoes
any changes to narrowing that the code caused. If, for example, the
buffer is narrowed and the code that follows save-restriction
gets rid of the narrowing, save-restriction
returns the buffer
to its narrowed region afterwards. In the what-line
command,
any narrowing the buffer may have is undone by the widen
command that immediately follows the save-restriction
command.
Any original narrowing is restored just before the completion of the
function.
The template for a save-restriction
expression is simple:
(save-restriction body… )
The body of the save-restriction
is one or more expressions that
will be evaluated in sequence by the Lisp interpreter.
Finally, a point to note: when you use both save-excursion
and
save-restriction
, one right after the other, you should use
save-excursion
outermost. If you write them in reverse order,
you may fail to record narrowing in the buffer to which Emacs switches
after calling save-excursion
. Thus, when written together,
save-excursion
and save-restriction
should be written
like this:
(save-excursion (save-restriction body…))
In other circumstances, when not written together, the
save-excursion
and save-restriction
special forms must
be written in the order appropriate to the function.
For example,
(save-restriction (widen) (save-excursion body…))
Next: Exercise with Narrowing, Previous: The save-restriction
Special Form, Up: Narrowing and Widening [Contents][Index]
6.2 what-line
The what-line
command tells you the number of the line in which
the cursor is located. The function illustrates the use of the
save-restriction
and save-excursion
commands. Here is the
original text of the function:
(defun what-line () "Print the current line number (in the buffer) of point." (interactive) (save-restriction (widen) (save-excursion (beginning-of-line) (message "Line %d" (1+ (count-lines 1 (point)))))))
(In modern versions of GNU Emacs, the what-line
function has
been expanded to tell you your line number in a narrowed buffer as
well as your line number in a widened buffer. The modern version is
more complex than the version shown here. If you feel adventurous,
you might want to look at it after figuring out how this version
works. You will probably need to use C-h f
(describe-function
). The newer version uses a conditional to
determine whether the buffer has been narrowed.
Also, the modern version of what-line
uses
line-number-at-pos
, which among other simple expressions, such
as (goto-char (point-min))
, moves point to the beginning of the
current line with (forward-line 0)
rather than
beginning-of-line
.)
The what-line
function as shown here has a documentation line
and is interactive, as you would expect. The next two lines use the
functions save-restriction
and widen
.
The save-restriction
special form notes whatever narrowing is in
effect, if any, in the current buffer and restores that narrowing after
the code in the body of the save-restriction
has been evaluated.
The save-restriction
special form is followed by widen
.
This function undoes any narrowing the current buffer may have had
when what-line
was called. (The narrowing that was there is
the narrowing that save-restriction
remembers.) This widening
makes it possible for the line counting commands to count from the
beginning of the buffer. Otherwise, they would have been limited to
counting within the accessible region. Any original narrowing is
restored just before the completion of the function by the
save-restriction
special form.
The call to widen
is followed by save-excursion
, which
saves the location of the cursor (i.e., of point), and
restores it after the code in the body of the save-excursion
uses the beginning-of-line
function to move point.
(Note that the (widen)
expression comes between the
save-restriction
and save-excursion
special forms. When
you write the two save- …
expressions in sequence, write
save-excursion
outermost.)
The last two lines of the what-line
function are functions to
count the number of lines in the buffer and then print the number in the
echo area.
(message "Line %d" (1+ (count-lines 1 (point)))))))
The message
function prints a one-line message at the bottom of
the Emacs screen. The first argument is inside of quotation marks and
is printed as a string of characters. However, it may contain a
‘%d’ expression to print a following argument. ‘%d’ prints
the argument as a decimal, so the message will say something such as
‘Line 243’.
The number that is printed in place of the ‘%d’ is computed by the last line of the function:
(1+ (count-lines 1 (point)))
What this does is count the lines from the first position of the
buffer, indicated by the 1
, up to (point)
, and then add
one to that number. (The 1+
function adds one to its
argument.) We add one to it because line 2 has only one line before
it, and count-lines
counts only the lines before the
current line.
After count-lines
has done its job, and the message has been
printed in the echo area, the save-excursion
restores point to
its original position; and save-restriction
restores
the original narrowing, if any.
Previous: what-line
, Up: Narrowing and Widening [Contents][Index]
6.3 Exercise with Narrowing
Write a function that will display the first 60 characters of the
current buffer, even if you have narrowed the buffer to its latter
half so that the first line is inaccessible. Restore point, mark, and
narrowing. For this exercise, you need to use a whole potpourri of
functions, including save-restriction
, widen
,
goto-char
, point-min
, message
, and
buffer-substring
.
(buffer-substring
is a previously unmentioned function you will
have to investigate yourself; or perhaps you will have to use
buffer-substring-no-properties
or
filter-buffer-substring
…, yet other functions. Text
properties are a feature otherwise not discussed here. See Text Properties in The GNU Emacs Lisp Reference
Manual.)
Additionally, do you really need goto-char
or point-min
?
Or can you write the function without them?
Next: Cutting and Storing Text, Previous: Narrowing and Widening, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
7 car
, cdr
, cons
: Fundamental Functions
In Lisp, car
, cdr
, and cons
are fundamental
functions. The cons
function is used to construct lists, and
the car
and cdr
functions are used to take them apart.
In the walk through of the copy-region-as-kill
function, we
will see cons
as well as two variants on cdr
,
namely, setcdr
and nthcdr
. (See copy-region-as-kill
.)
Next: car
and cdr
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
Strange Names
The name of the cons
function is not unreasonable: it is an
abbreviation of the word “construct”. The origins of the names for
car
and cdr
, on the other hand, are esoteric: car
is an acronym from the phrase “Contents of the Address part of the
Register”; and cdr
(pronounced “could-er”) is an acronym
from the phrase “Contents of the Decrement part of the Register”.
These phrases refer to the IBM 704 computer on which the original Lisp
was developed.
The IBM 704 is a footnote in history, but these names are now beloved traditions of Lisp.
Next: cons
, Previous: Strange Names, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.1 car
and cdr
The CAR of a list is, quite simply, the first item in the list.
Thus the CAR of the list (rose violet daisy buttercup)
is
rose
.
If you are reading this in Info in GNU Emacs, you can see this by evaluating the following:
(car '(rose violet daisy buttercup))
After evaluating the expression, rose
will appear in the echo
area.
car
does not remove the first item from the list; it only reports
what it is. After car
has been applied to a list, the list is
still the same as it was. In the jargon, car
is
“non-destructive”. This feature turns out to be important.
The CDR of a list is the rest of the list, that is, the
cdr
function returns the part of the list that follows the
first item. Thus, while the CAR of the list '(rose violet
daisy buttercup)
is rose
, the rest of the list, the value
returned by the cdr
function, is (violet daisy
buttercup)
.
You can see this by evaluating the following in the usual way:
(cdr '(rose violet daisy buttercup))
When you evaluate this, (violet daisy buttercup)
will appear in
the echo area.
Like car
, cdr
does not remove any elements from the
list—it just returns a report of what the second and subsequent
elements are.
Incidentally, in the example, the list of flowers is quoted. If it were
not, the Lisp interpreter would try to evaluate the list by calling
rose
as a function. In this example, we do not want to do that.
For operating on lists, the names first
and rest
would
make more sense than the names car
and cdr
. Indeed,
some programmers define first
and rest
as aliases for
car
and cdr
, then write first
and rest
in
their code.
However, lists in Lisp are built using a lower-level structure known
as “cons cells” (see How Lists are Implemented), in which there is no
such thing as “first” or “rest”, and the CAR and the CDR
are symmetrical. Lisp does not try to hide the existence of cons
cells, and programs do use them for things other than lists. For this
reason, the names are helpful for reminding programmers that
car
and cdr
are in fact symmetrical, despite the
asymmetrical way they are used in lists.
When car
and cdr
are applied to a list made up of symbols,
such as the list (pine fir oak maple)
, the element of the list
returned by the function car
is the symbol pine
without
any parentheses around it. pine
is the first element in the
list. However, the CDR of the list is a list itself, (fir
oak maple)
, as you can see by evaluating the following expressions in
the usual way:
(car '(pine fir oak maple)) (cdr '(pine fir oak maple))
On the other hand, in a list of lists, the first element is itself a
list. car
returns this first element as a list. For example,
the following list contains three sub-lists, a list of carnivores, a
list of herbivores and a list of sea mammals:
(car '((lion tiger cheetah) (gazelle antelope zebra) (whale dolphin seal)))
In this example, the first element or CAR of the list is the list of
carnivores, (lion tiger cheetah)
, and the rest of the list is
((gazelle antelope zebra) (whale dolphin seal))
.
(cdr '((lion tiger cheetah) (gazelle antelope zebra) (whale dolphin seal)))
It is worth saying again that car
and cdr
are
non-destructive—that is, they do not modify or change lists to which
they are applied. This is very important for how they are used.
Also, in the first chapter, in the discussion about atoms, I said that
in Lisp, certain kinds of atom, such as an array, can be separated
into parts; but the mechanism for doing this is different from the
mechanism for splitting a list. As far as Lisp is concerned, the
atoms of a list are unsplittable. (See Lisp Atoms.) The
car
and cdr
functions are used for splitting lists and
are considered fundamental to Lisp. Since they cannot split or gain
access to the parts of an array, an array is considered an atom.
Conversely, the other fundamental function, cons
, can put
together or construct a list, but not an array. (Arrays are handled
by array-specific functions. See Arrays in The GNU
Emacs Lisp Reference Manual.)
Next: nthcdr
, Previous: car
and cdr
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.2 cons
The cons
function constructs lists; it is the inverse of
car
and cdr
. For example, cons
can be used to make
a four element list from the three element list, (fir oak maple)
:
(cons 'pine '(fir oak maple))
After evaluating this list, you will see
(pine fir oak maple)
appear in the echo area. cons
causes the creation of a new
list in which the element is followed by the elements of the original
list.
We often say that cons
puts a new element at the beginning of
a list, or that it attaches or pushes elements onto the list, but this
phrasing can be misleading, since cons
does not change an
existing list, but creates a new one.
Like car
and cdr
, cons
is non-destructive.
Next: Find the Length of a List: length
, Up: cons
[Contents][Index]
Build a list
cons
must have a list to attach to.13 You
cannot start from absolutely nothing. If you are building a list, you
need to provide at least an empty list at the beginning. Here is a
series of cons
expressions that build up a list of flowers. If
you are reading this in Info in GNU Emacs, you can evaluate each of
the expressions in the usual way; the value is printed in this text
after ‘⇒’, which you may read as “evaluates to”.
(cons 'buttercup ()) ⇒ (buttercup)
(cons 'daisy '(buttercup)) ⇒ (daisy buttercup)
(cons 'violet '(daisy buttercup)) ⇒ (violet daisy buttercup)
(cons 'rose '(violet daisy buttercup)) ⇒ (rose violet daisy buttercup)
In the first example, the empty list is shown as ()
and a list
made up of buttercup
followed by the empty list is constructed.
As you can see, the empty list is not shown in the list that was
constructed. All that you see is (buttercup)
. The empty list is
not counted as an element of a list because there is nothing in an empty
list. Generally speaking, an empty list is invisible.
The second example, (cons 'daisy '(buttercup))
constructs a new,
two element list by putting daisy
in front of buttercup
;
and the third example constructs a three element list by putting
violet
in front of daisy
and buttercup
.
Previous: Build a list, Up: cons
[Contents][Index]
7.2.1 Find the Length of a List: length
You can find out how many elements there are in a list by using the Lisp
function length
, as in the following examples:
(length '(buttercup)) ⇒ 1
(length '(daisy buttercup)) ⇒ 2
(length (cons 'violet '(daisy buttercup))) ⇒ 3
In the third example, the cons
function is used to construct a
three element list which is then passed to the length
function as
its argument.
We can also use length
to count the number of elements in an
empty list:
(length ()) ⇒ 0
As you would expect, the number of elements in an empty list is zero.
An interesting experiment is to find out what happens if you try to find
the length of no list at all; that is, if you try to call length
without giving it an argument, not even an empty list:
(length )
What you see, if you evaluate this, is the error message
Lisp error: (wrong-number-of-arguments length 0)
This means that the function receives the wrong number of arguments, zero, when it expects some other number of arguments. In this case, one argument is expected, the argument being a list whose length the function is measuring. (Note that one list is one argument, even if the list has many elements inside it.)
The part of the error message that says ‘length’ is the name of the function.
Next: nth
, Previous: cons
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.3 nthcdr
The nthcdr
function is associated with the cdr
function.
What it does is take the CDR of a list repeatedly.
If you take the CDR of the list (pine fir
oak maple)
, you will be returned the list (fir oak maple)
. If you
repeat this on what was returned, you will be returned the list
(oak maple)
. (Of course, repeated CDRing on the original
list will just give you the original CDR since the function does
not change the list. You need to evaluate the CDR of the
CDR and so on.) If you continue this, eventually you will be
returned an empty list, which in this case, instead of being shown as
()
is shown as nil
.
For review, here is a series of repeated CDRs, the text following the ‘⇒’ shows what is returned.
(cdr '(pine fir oak maple)) ⇒ (fir oak maple)
(cdr '(fir oak maple)) ⇒ (oak maple)
(cdr '(oak maple)) ⇒ (maple)
(cdr '(maple)) ⇒ nil
(cdr 'nil) ⇒ nil
(cdr ()) ⇒ nil
You can also do several CDRs without printing the values in between, like this:
(cdr (cdr '(pine fir oak maple))) ⇒ (oak maple)
In this example, the Lisp interpreter evaluates the innermost list first.
The innermost list is quoted, so it just passes the list as it is to the
innermost cdr
. This cdr
passes a list made up of the
second and subsequent elements of the list to the outermost cdr
,
which produces a list composed of the third and subsequent elements of
the original list. In this example, the cdr
function is repeated
and returns a list that consists of the original list without its
first two elements.
The nthcdr
function does the same as repeating the call to
cdr
. In the following example, the argument 2 is passed to the
function nthcdr
, along with the list, and the value returned is
the list without its first two items, which is exactly the same
as repeating cdr
twice on the list:
(nthcdr 2 '(pine fir oak maple)) ⇒ (oak maple)
Using the original four element list, we can see what happens when
various numeric arguments are passed to nthcdr
, including 0, 1,
and 5:
;; Leave the list as it was.
(nthcdr 0 '(pine fir oak maple))
⇒ (pine fir oak maple)
;; Return a copy without the first element.
(nthcdr 1 '(pine fir oak maple))
⇒ (fir oak maple)
;; Return a copy of the list without three elements.
(nthcdr 3 '(pine fir oak maple))
⇒ (maple)
;; Return a copy lacking all four elements.
(nthcdr 4 '(pine fir oak maple))
⇒ nil
;; Return a copy lacking all elements.
(nthcdr 5 '(pine fir oak maple))
⇒ nil
Next: setcar
, Previous: nthcdr
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.4 nth
The nthcdr
function takes the CDR of a list repeatedly.
The nth
function takes the CAR of the result returned by
nthcdr
. It returns the Nth element of the list.
Thus, if it were not defined in C for speed, the definition of
nth
would be:
(defun nth (n list) "Returns the Nth element of LIST. N counts from zero. If LIST is not that long, nil is returned." (car (nthcdr n list)))
(Originally, nth
was defined in Emacs Lisp in subr.el,
but its definition was redone in C in the 1980s.)
The nth
function returns a single element of a list.
This can be very convenient.
Note that the elements are numbered from zero, not one. That is to say, the first element of a list, its CAR is the zeroth element. This zero-based counting often bothers people who are accustomed to the first element in a list being number one, which is one-based.
For example:
(nth 0 '("one" "two" "three")) ⇒ "one" (nth 1 '("one" "two" "three")) ⇒ "two"
It is worth mentioning that nth
, like nthcdr
and
cdr
, does not change the original list—the function is
non-destructive. This is in sharp contrast to the setcar
and
setcdr
functions.
Next: setcdr
, Previous: nth
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.5 setcar
As you might guess from their names, the setcar
and setcdr
functions set the CAR or the CDR of a list to a new value.
They actually change the original list, unlike car
and cdr
which leave the original list as it was. One way to find out how this
works is to experiment. We will start with the setcar
function.
First, we can make a list and then set the value of a variable to the
list, using the setq
special form. Because we intend to use
setcar
to change the list, this setq
should not use the
quoted form '(antelope giraffe lion tiger)
, as that would yield
a list that is part of the program and bad things could happen if we
tried to change part of the program while running it. Generally
speaking an Emacs Lisp program’s components should be constant (or
unchanged) while the program is running. So we instead construct an
animal list by using the list
function, as follows:
(setq animals (list 'antelope 'giraffe 'lion 'tiger))
If you are reading this in Info inside of GNU Emacs, you can evaluate this expression in the usual fashion, by positioning the cursor after the expression and typing C-x C-e. (I’m doing this right here as I write this. This is one of the advantages of having the interpreter built into the computing environment. Incidentally, when there is nothing on the line after the final parentheses, such as a comment, point can be on the next line. Thus, if your cursor is in the first column of the next line, you do not need to move it. Indeed, Emacs permits any amount of white space after the final parenthesis.)
When we evaluate the variable animals
, we see that it is bound to
the list (antelope giraffe lion tiger)
:
animals ⇒ (antelope giraffe lion tiger)
Put another way, the variable animals
points to the list
(antelope giraffe lion tiger)
.
Next, evaluate the function setcar
while passing it two
arguments, the variable animals
and the quoted symbol
hippopotamus
; this is done by writing the three element list
(setcar animals 'hippopotamus)
and then evaluating it in the
usual fashion:
(setcar animals 'hippopotamus)
After evaluating this expression, evaluate the variable animals
again. You will see that the list of animals has changed:
animals ⇒ (hippopotamus giraffe lion tiger)
The first element on the list, antelope
is replaced by
hippopotamus
.
So we can see that setcar
did not add a new element to the list
as cons
would have; it replaced antelope
with
hippopotamus
; it changed the list.
Next: Exercise, Previous: setcar
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.6 setcdr
The setcdr
function is similar to the setcar
function,
except that the function replaces the second and subsequent elements of
a list rather than the first element.
(To see how to change the last element of a list, look ahead to
The kill-new
function, which uses
the nthcdr
and setcdr
functions.)
To see how this works, set the value of the variable to a list of domesticated animals by evaluating the following expression:
(setq domesticated-animals (list 'horse 'cow 'sheep 'goat))
If you now evaluate the list, you will be returned the list
(horse cow sheep goat)
:
domesticated-animals ⇒ (horse cow sheep goat)
Next, evaluate setcdr
with two arguments, the name of the
variable which has a list as its value, and the list to which the
CDR of the first list will be set;
(setcdr domesticated-animals '(cat dog))
If you evaluate this expression, the list (cat dog)
will appear
in the echo area. This is the value returned by the function. The
result we are interested in is the side effect, which we can see by
evaluating the variable domesticated-animals
:
domesticated-animals ⇒ (horse cat dog)
Indeed, the list is changed from (horse cow sheep goat)
to
(horse cat dog)
. The CDR of the list is changed from
(cow sheep goat)
to (cat dog)
.
Previous: setcdr
, Up: car
, cdr
, cons
: Fundamental Functions [Contents][Index]
7.7 Exercise
Construct a list of four birds by evaluating several expressions with
cons
. Find out what happens when you cons
a list onto
itself. Replace the first element of the list of four birds with a
fish. Replace the rest of that list with a list of other fish.
Next: How Lists are Implemented, Previous: car
, cdr
, cons
: Fundamental Functions, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
8 Cutting and Storing Text
Whenever you cut or clip text out of a buffer with a kill command in GNU Emacs, it is stored in a list and you can bring it back with a yank command.
(The use of the word “kill” in Emacs for processes which specifically do not destroy the values of the entities is an unfortunate historical accident. A much more appropriate word would be “clip” since that is what the kill commands do; they clip text out of a buffer and put it into storage from which it can be brought back. I have often been tempted to replace globally all occurrences of “kill” in the Emacs sources with “clip” and all occurrences of “killed” with “clipped”.)
- Storing Text in a List
zap-to-char
kill-region
copy-region-as-kill
- Digression into C
- Initializing a Variable with
defvar
- Review
- Searching Exercises
Next: zap-to-char
, Up: Cutting and Storing Text [Contents][Index]
Storing Text in a List
When text is cut out of a buffer, it is stored on a list. Successive pieces of text are stored on the list successively, so the list might look like this:
("a piece of text" "previous piece")
The function cons
can be used to create a new list from a piece
of text (an “atom”, to use the jargon) and an existing list, like
this:
(cons "another piece" '("a piece of text" "previous piece"))
If you evaluate this expression, a list of three elements will appear in the echo area:
("another piece" "a piece of text" "previous piece")
With the car
and nthcdr
functions, you can retrieve
whichever piece of text you want. For example, in the following code,
nthcdr 1 …
returns the list with the first item removed;
and the car
returns the first element of that remainder—the
second element of the original list:
(car (nthcdr 1 '("another piece" "a piece of text" "previous piece"))) ⇒ "a piece of text"
The actual functions in Emacs are more complex than this, of course. The code for cutting and retrieving text has to be written so that Emacs can figure out which element in the list you want—the first, second, third, or whatever. In addition, when you get to the end of the list, Emacs should give you the first element of the list, rather than nothing at all.
The list that holds the pieces of text is called the kill ring.
This chapter leads up to a description of the kill ring and how it is
used by first tracing how the zap-to-char
function works. This
function calls a function that invokes a function that
manipulates the kill ring. Thus, before reaching the mountains, we
climb the foothills.
A subsequent chapter describes how text that is cut from the buffer is retrieved. See Yanking Text Back.
Next: kill-region
, Previous: Storing Text in a List, Up: Cutting and Storing Text [Contents][Index]
8.1 zap-to-char
Let us look at the interactive zap-to-char
function.
- The Complete
zap-to-char
Implementation - The
interactive
Expression - The Body of
zap-to-char
- The
search-forward
Function - The
progn
Special Form - Summing up
zap-to-char
Next: The interactive
Expression, Up: zap-to-char
[Contents][Index]
The Complete zap-to-char
Implementation
The zap-to-char
function removes the text in the region between
the location of the cursor (i.e., of point) up to and including the
next occurrence of a specified character. The text that
zap-to-char
removes is put in the kill ring; and it can be
retrieved from the kill ring by typing C-y (yank
). If
the command is given an argument, it removes text through that number
of occurrences. Thus, if the cursor were at the beginning of this
sentence and the character were ‘s’, ‘Thus’ would be
removed. If the argument were two, ‘Thus, if the curs’ would be
removed, up to and including the ‘s’ in ‘cursor’.
If the specified character is not found, zap-to-char
will say
“Search failed”, tell you the character you typed, and not remove
any text.
In order to determine how much text to remove, zap-to-char
uses
a search function. Searches are used extensively in code that
manipulates text, and we will focus attention on them as well as on the
deletion command.
Here is the complete text of the version 22 implementation of the function:
(defun zap-to-char (arg char) "Kill up to and including ARG'th occurrence of CHAR. Case is ignored if `case-fold-search' is non-nil in the current buffer. Goes backward if ARG is negative; error if CHAR not found." (interactive "p\ncZap to char: ") (if (char-table-p translation-table-for-input) (setq char (or (aref translation-table-for-input char) char))) (kill-region (point) (progn (search-forward (char-to-string char) nil nil arg) (point))))
The documentation is thorough. You do need to know the jargon meaning of the word “kill”.
The version 22 documentation string for zap-to-char
uses ASCII
grave accent and apostrophe to quote a symbol, so it appears as
`case-fold-search'. This quoting style was inspired by 1970s-era
displays in which grave accent and apostrophe were often mirror images
suitable for use as quotes. On most modern displays this is no longer
true, and when these two ASCII characters appear in documentation
strings or diagnostic message formats, Emacs typically transliterates
them to curved quotes (left and right single quotation marks),
so that the abovequoted symbol appears
as ‘case-fold-search’. Source-code strings can also simply use
curved quotes directly.
Next: The Body of zap-to-char
, Previous: The Complete zap-to-char
Implementation, Up: zap-to-char
[Contents][Index]
8.1.1 The interactive
Expression
The interactive expression in the zap-to-char
command looks like
this:
(interactive "p\ncZap to char: ")
The part within quotation marks, "p\ncZap to char: "
, specifies
two different things. First, and most simply, is the ‘p’.
This part is separated from the next part by a newline, ‘\n’.
The ‘p’ means that the first argument to the function will be
passed the value of a processed prefix. The prefix argument is
passed by typing C-u and a number, or M- and a number. If
the function is called interactively without a prefix, 1 is passed to
this argument.
The second part of "p\ncZap to char: "
is
‘cZap to char: ’. In this part, the lower case ‘c’
indicates that interactive
expects a prompt and that the
argument will be a character. The prompt follows the ‘c’ and is
the string ‘Zap to char: ’ (with a space after the colon to
make it look good).
What all this does is prepare the arguments to zap-to-char
so they
are of the right type, and give the user a prompt.
In a read-only buffer, the zap-to-char
function copies the text
to the kill ring, but does not remove it. The echo area displays a
message saying that the buffer is read-only. Also, the terminal may
beep or blink at you.
Next: The search-forward
Function, Previous: The interactive
Expression, Up: zap-to-char
[Contents][Index]
8.1.2 The Body of zap-to-char
The body of the zap-to-char
function contains the code that
kills (that is, removes) the text in the region from the current
position of the cursor up to and including the specified character.
The first part of the code looks like this:
(if (char-table-p translation-table-for-input) (setq char (or (aref translation-table-for-input char) char))) (kill-region (point) (progn (search-forward (char-to-string char) nil nil arg) (point)))
char-table-p
is a hitherto unseen function. It determines
whether its argument is a character table. When it is, it sets the
character passed to zap-to-char
to one of them, if that
character exists, or to the character itself. (This becomes important
for certain characters in non-European languages. The aref
function extracts an element from an array. It is an array-specific
function that is not described in this document. See Arrays in The GNU Emacs Lisp Reference Manual.)
(point)
is the current position of the cursor.
The next part of the code is an expression using progn
. The body
of the progn
consists of calls to search-forward
and
point
.
It is easier to understand how progn
works after learning about
search-forward
, so we will look at search-forward
and
then at progn
.
Next: The progn
Special Form, Previous: The Body of zap-to-char
, Up: zap-to-char
[Contents][Index]
8.1.3 The search-forward
Function
The search-forward
function is used to locate the
zapped-for-character in zap-to-char
. If the search is
successful, search-forward
leaves point immediately after the
last character in the target string. (In zap-to-char
, the
target string is just one character long. zap-to-char
uses the
function char-to-string
to ensure that the computer treats that
character as a string.) If the search is backwards,
search-forward
leaves point just before the first character in
the target. Also, search-forward
returns t
for true.
(Moving point is therefore a side effect.)
In zap-to-char
, the search-forward
function looks like this:
(search-forward (char-to-string char) nil nil arg)
The search-forward
function takes four arguments:
- The first argument is the target, what is searched for. This must be a
string, such as ‘"z"’.
As it happens, the argument passed to
zap-to-char
is a single character. Because of the way computers are built, the Lisp interpreter may treat a single character as being different from a string of characters. Inside the computer, a single character has a different electronic format than a string of one character. (A single character can often be recorded in the computer using exactly one byte; but a string may be longer, and the computer needs to be ready for this.) Since thesearch-forward
function searches for a string, the character that thezap-to-char
function receives as its argument must be converted inside the computer from one format to the other; otherwise thesearch-forward
function will fail. Thechar-to-string
function is used to make this conversion. - The second argument bounds the search; it is specified as a position in
the buffer. In this case, the search can go to the end of the buffer,
so no bound is set and the second argument is
nil
. - The third argument tells the function what it should do if the search
fails—it can signal an error (and print a message) or it can return
nil
. Anil
as the third argument causes the function to signal an error when the search fails. - The fourth argument to
search-forward
is the repeat count—how many occurrences of the string to look for. This argument is optional and if the function is called without a repeat count, this argument is passed the value 1. If this argument is negative, the search goes backwards.
In template form, a search-forward
expression looks like this:
(search-forward "target-string" limit-of-search what-to-do-if-search-fails repeat-count)
We will look at progn
next.
Next: Summing up zap-to-char
, Previous: The search-forward
Function, Up: zap-to-char
[Contents][Index]
8.1.4 The progn
Special Form
progn
is a special form that causes each of its arguments to be
evaluated in sequence and then returns the value of the last one. The
preceding expressions are evaluated only for the side effects they
perform. The values produced by them are discarded.
The template for a progn
expression is very simple:
(progn body…)
In zap-to-char
, the progn
expression has to do two things:
put point in exactly the right position; and return the location of
point so that kill-region
will know how far to kill to.
The first argument to the progn
is search-forward
. When
search-forward
finds the string, the function leaves point
immediately after the last character in the target string. (In this
case the target string is just one character long.) If the search is
backwards, search-forward
leaves point just before the first
character in the target. The movement of point is a side effect.
The second and last argument to progn
is the expression
(point)
. This expression returns the value of point, which in
this case will be the location to which it has been moved by
search-forward
. (In the source, a line that tells the function
to go to the previous character, if it is going forward, was commented
out in 1999; I don’t remember whether that feature or mis-feature was
ever a part of the distributed source.) The value of point
is
returned by the progn
expression and is passed to
kill-region
as kill-region
’s second argument.
Previous: The progn
Special Form, Up: zap-to-char
[Contents][Index]
8.1.5 Summing up zap-to-char
Now that we have seen how search-forward
and progn
work,
we can see how the zap-to-char
function works as a whole.
The first argument to kill-region
is the position of the cursor
when the zap-to-char
command is given—the value of point at
that time. Within the progn
, the search function then moves
point to just after the zapped-to-character and point
returns the
value of this location. The kill-region
function puts together
these two values of point, the first one as the beginning of the region
and the second one as the end of the region, and removes the region.
The progn
special form is necessary because the
kill-region
command takes two arguments; and it would fail if
search-forward
and point
expressions were written in
sequence as two additional arguments. The progn
expression is
a single argument to kill-region
and returns the one value that
kill-region
needs for its second argument.
Next: copy-region-as-kill
, Previous: zap-to-char
, Up: Cutting and Storing Text [Contents][Index]
8.2 kill-region
The zap-to-char
function uses the kill-region
function.
This function clips text from a region and copies that text to
the kill ring, from which it may be retrieved.
The Emacs 22 version of that function uses condition-case
and
copy-region-as-kill
, both of which we will explain.
condition-case
is an important special form.
In essence, the kill-region
function calls
condition-case
, which takes three arguments. In this function,
the first argument does nothing. The second argument contains the
code that does the work when all goes well. The third argument
contains the code that is called in the event of an error.
Next: condition-case
, Up: kill-region
[Contents][Index]
The Complete kill-region
Definition
We will go through the condition-case
code in a moment. First,
let us look at the definition of kill-region
, with comments
added:
(defun kill-region (beg end) "Kill (\"cut\") text between point and mark. This deletes the text from the buffer and saves it in the kill ring. The command \\[yank] can retrieve it from there. … "
;; • Since order matters, pass point first. (interactive (list (point) (mark))) ;; • And tell us if we cannot cut the text. ;; 'unless' is an 'if' without a then-part. (unless (and beg end) (error "The mark is not set now, so there is no region"))
;; • 'condition-case' takes three arguments. ;; If the first argument is nil, as it is here, ;; information about the error signal is not ;; stored for use by another function. (condition-case nil
;; • The second argument to 'condition-case' tells the ;; Lisp interpreter what to do when all goes well.
;; It starts with a 'let' function that extracts the string ;; and tests whether it exists. If so (that is what the ;; 'when' checks), it calls an 'if' function that determines ;; whether the previous command was another call to ;; 'kill-region'; if it was, then the new text is appended to ;; the previous text; if not, then a different function, ;; 'kill-new', is called.
;; The 'kill-append' function concatenates the new string and ;; the old. The 'kill-new' function inserts text into a new ;; item in the kill ring.
;; 'when' is an 'if' without an else-part. The second 'when' ;; again checks whether the current string exists; in ;; addition, it checks whether the previous command was ;; another call to 'kill-region'. If one or the other ;; condition is true, then it sets the current command to ;; be 'kill-region'.
(let ((string (filter-buffer-substring beg end t))) (when string ;STRING is nil if BEG = END ;; Add that string to the kill ring, one way or another. (if (eq last-command 'kill-region)
;; - 'yank-handler' is an optional argument to ;; 'kill-region' that tells the 'kill-append' and ;; 'kill-new' functions how deal with properties ;; added to the text, such as 'bold' or 'italics'. (kill-append string (< end beg) yank-handler) (kill-new string nil yank-handler))) (when (or string (eq last-command 'kill-region)) (setq this-command 'kill-region)) nil)
;; • The third argument to 'condition-case' tells the interpreter ;; what to do with an error.
;; The third argument has a conditions part and a body part. ;; If the conditions are met (in this case, ;; if text or buffer are read-only) ;; then the body is executed.
;; The first part of the third argument is the following: ((buffer-read-only text-read-only) ;; the if-part ;; … the then-part (copy-region-as-kill beg end)
;; Next, also as part of the then-part, set this-command, so ;; it will be set in an error (setq this-command 'kill-region) ;; Finally, in the then-part, send a message if you may copy ;; the text to the kill ring without signaling an error, but ;; don't if you may not.
(if kill-read-only-ok (progn (message "Read only text copied to kill ring") nil) (barf-if-buffer-read-only) ;; If the buffer isn't read-only, the text is. (signal 'text-read-only (list (current-buffer)))))))
Next: Lisp macro, Previous: The Complete kill-region
Definition, Up: kill-region
[Contents][Index]
8.2.1 condition-case
As we have seen earlier (see Generate an Error Message), when the Emacs Lisp interpreter has trouble evaluating an expression, it provides you with help; in the jargon, this is called “signaling an error”. Usually, the computer stops the program and shows you a message.
However, some programs undertake complicated actions. They should not
simply stop on an error. In the kill-region
function, the most
likely error is that you will try to kill text that is read-only and
cannot be removed. So the kill-region
function contains code
to handle this circumstance. This code, which makes up the body of
the kill-region
function, is inside of a condition-case
special form.
The template for condition-case
looks like this:
(condition-case var bodyform error-handler…)
The second argument, bodyform, is straightforward. The
condition-case
special form causes the Lisp interpreter to
evaluate the code in bodyform. If no error occurs, the special
form returns the code’s value and produces the side-effects, if any.
In short, the bodyform part of a condition-case
expression determines what should happen when everything works
correctly.
However, if an error occurs, among its other actions, the function generating the error signal will define one or more error condition names.
An error handler is the third argument to condition-case
.
An error handler has two parts, a condition-name and a
body. If the condition-name part of an error handler
matches a condition name generated by an error, then the body
part of the error handler is run.
As you will expect, the condition-name part of an error handler may be either a single condition name or a list of condition names.
Also, a complete condition-case
expression may contain more
than one error handler. When an error occurs, the first applicable
handler is run.
Lastly, the first argument to the condition-case
expression,
the var argument, is sometimes bound to a variable that contains
information about the error. However, if that argument is nil
,
as is the case in kill-region
, that information is discarded.
In brief, in the kill-region
function, the code
condition-case
works like this:
If no errors, run only this code but, if errors, run this other code.
Previous: condition-case
, Up: kill-region
[Contents][Index]
8.2.2 Lisp macro
The part of the condition-case
expression that is evaluated in
the expectation that all goes well has a when
. The code uses
when
to determine whether the string
variable points to
text that exists.
A when
expression is simply a programmers’ convenience. It is
like an if
without the possibility of an else clause. In your
mind, you can replace when
with if
and understand what
goes on. That is what the Lisp interpreter does.
Technically speaking, when
is a Lisp macro. A Lisp macro
enables you to define new control constructs and other language
features. It tells the interpreter how to compute another Lisp
expression which will in turn compute the value. In this case, the
other expression is an if
expression.
The kill-region
function definition also has an unless
macro; it is the opposite of when
. The unless
macro is
like an if
except that it has no then-clause, and it supplies
an implicit nil
for that.
For more about Lisp macros, see Macros in The GNU Emacs Lisp Reference Manual. The C programming language also provides macros. These are different, but also useful.
Regarding the when
macro, in the condition-case
expression, when the string has content, then another conditional
expression is executed. This is an if
with both a then-part
and an else-part.
(if (eq last-command 'kill-region) (kill-append string (< end beg) yank-handler) (kill-new string nil yank-handler))
The then-part is evaluated if the previous command was another call to
kill-region
; if not, the else-part is evaluated.
yank-handler
is an optional argument to kill-region
that
tells the kill-append
and kill-new
functions how deal
with properties added to the text, such as bold or italics.
last-command
is a variable that comes with Emacs that we have
not seen before. Normally, whenever a function is executed, Emacs
sets the value of last-command
to the previous command.
In this segment of the definition, the if
expression checks
whether the previous command was kill-region
. If it was,
(kill-append string (< end beg) yank-handler)
concatenates a copy of the newly clipped text to the just previously clipped text in the kill ring.
Next: Digression into C, Previous: kill-region
, Up: Cutting and Storing Text [Contents][Index]
8.3 copy-region-as-kill
The copy-region-as-kill
function copies a region of text from a
buffer and (via either kill-append
or kill-new
) saves it
in the kill-ring
.
If you call copy-region-as-kill
immediately after a
kill-region
command, Emacs appends the newly copied text to the
previously copied text. This means that if you yank back the text, you
get it all, from both this and the previous operation. On the other
hand, if some other command precedes the copy-region-as-kill
,
the function copies the text into a separate entry in the kill ring.
Next: The Body of copy-region-as-kill
, Up: copy-region-as-kill
[Contents][Index]
The complete copy-region-as-kill
function definition
Here is the complete text of the version 22 copy-region-as-kill
function:
(defun copy-region-as-kill (beg end) "Save the region as if killed, but don't kill it. In Transient Mark mode, deactivate the mark. If `interprogram-cut-function' is non-nil, also save the text for a window system cut and paste." (interactive "r")
(if (eq last-command 'kill-region) (kill-append (filter-buffer-substring beg end) (< end beg)) (kill-new (filter-buffer-substring beg end)))
(if transient-mark-mode (setq deactivate-mark t)) nil)
As usual, this function can be divided into its component parts:
(defun copy-region-as-kill (argument-list) "documentation…" (interactive "r") body…)
The arguments are beg
and end
and the function is
interactive with "r"
, so the two arguments must refer to the
beginning and end of the region. If you have been reading through this
document from the beginning, understanding these parts of a function is
almost becoming routine.
The documentation is somewhat confusing unless you remember that the
word “kill” has a meaning different from usual. The Transient Mark
and interprogram-cut-function
comments explain certain
side-effects.
After you once set a mark, a buffer always contains a region. If you wish, you can use Transient Mark mode to highlight the region temporarily. (No one wants to highlight the region all the time, so Transient Mark mode highlights it only at appropriate times. Many people turn off Transient Mark mode, so the region is never highlighted.)
Also, a windowing system allows you to copy, cut, and paste among
different programs. In the X windowing system, for example, the
interprogram-cut-function
function is x-select-text
,
which works with the windowing system’s equivalent of the Emacs kill
ring.
The body of the copy-region-as-kill
function starts with an
if
clause. What this clause does is distinguish between two
different situations: whether or not this command is executed
immediately after a previous kill-region
command. In the first
case, the new region is appended to the previously copied text.
Otherwise, it is inserted into the beginning of the kill ring as a
separate piece of text from the previous piece.
The last two lines of the function prevent the region from lighting up if Transient Mark mode is turned on.
The body of copy-region-as-kill
merits discussion in detail.
Previous: The complete copy-region-as-kill
function definition, Up: copy-region-as-kill
[Contents][Index]
8.3.1 The Body of copy-region-as-kill
The copy-region-as-kill
function works in much the same way as
the kill-region
function. Both are written so that two or more
kills in a row combine their text into a single entry. If you yank
back the text from the kill ring, you get it all in one piece.
Moreover, kills that kill forward from the current position of the
cursor are added to the end of the previously copied text and commands
that copy text backwards add it to the beginning of the previously
copied text. This way, the words in the text stay in the proper
order.
Like kill-region
, the copy-region-as-kill
function makes
use of the last-command
variable that keeps track of the
previous Emacs command.
Next: The kill-append
function, Up: The Body of copy-region-as-kill
[Contents][Index]
last-command
and this-command
Normally, whenever a function is executed, Emacs sets the value of
this-command
to the function being executed (which in this case
would be copy-region-as-kill
). At the same time, Emacs sets
the value of last-command
to the previous value of
this-command
.
In the first part of the body of the copy-region-as-kill
function, an if
expression determines whether the value of
last-command
is kill-region
. If so, the then-part of
the if
expression is evaluated; it uses the kill-append
function to concatenate the text copied at this call to the function
with the text already in the first element (the CAR) of the kill
ring. On the other hand, if the value of last-command
is not
kill-region
, then the copy-region-as-kill
function
attaches a new element to the kill ring using the kill-new
function.
The if
expression reads as follows; it uses eq
:
(if (eq last-command 'kill-region) ;; then-part (kill-append (filter-buffer-substring beg end) (< end beg)) ;; else-part (kill-new (filter-buffer-substring beg end)))
(The filter-buffer-substring
function returns a filtered
substring of the buffer, if any. Optionally—the arguments are not
here, so neither is done—the function may delete the initial text or
return the text without its properties; this function is a replacement
for the older buffer-substring
function, which came before text
properties were implemented.)
The eq
function tests whether its first argument is the same Lisp
object as its second argument. The eq
function is similar to the
equal
function in that it is used to test for equality, but
differs in that it determines whether two representations are actually
the same object inside the computer, but with different names.
equal
determines whether the structure and contents of two
expressions are the same.
If the previous command was kill-region
, then the Emacs Lisp
interpreter calls the kill-append
function
Next: The kill-new
function, Previous: last-command
and this-command
, Up: The Body of copy-region-as-kill
[Contents][Index]
The kill-append
function
The kill-append
function looks like this:
(defun kill-append (string before-p &optional yank-handler) "Append STRING to the end of the latest kill in the kill ring. If BEFORE-P is non-nil, prepend STRING to the kill. … " (let* ((cur (car kill-ring))) (kill-new (if before-p (concat string cur) (concat cur string)) (or (= (length cur) 0) (equal yank-handler (get-text-property 0 'yank-handler cur))) yank-handler)))
The kill-append
function is fairly straightforward. It uses
the kill-new
function, which we will discuss in more detail in
a moment.
(Also, the function provides an optional argument called
yank-handler
; when invoked, this argument tells the function
how to deal with properties added to the text, such as bold or
italics.)
It has a let*
function to set the value of the first element of
the kill ring to cur
. (I do not know why the function does not
use let
instead; only one value is set in the expression.
Perhaps this is a bug that produces no problems?)
Consider the conditional that is one of the two arguments to
kill-new
. It uses concat
to concatenate the new text to
the CAR of the kill ring. Whether it prepends or appends the
text depends on the results of an if
expression:
(if before-p ; if-part (concat string cur) ; then-part (concat cur string)) ; else-part
If the region being killed is before the region that was killed in the
last command, then it should be prepended before the material that was
saved in the previous kill; and conversely, if the killed text follows
what was just killed, it should be appended after the previous text.
The if
expression depends on the predicate before-p
to
decide whether the newly saved text should be put before or after the
previously saved text.
The symbol before-p
is the name of one of the arguments to
kill-append
. When the kill-append
function is
evaluated, it is bound to the value returned by evaluating the actual
argument. In this case, this is the expression (< end beg)
.
This expression does not directly determine whether the killed text in
this command is located before or after the kill text of the last
command; what it does is determine whether the value of the variable
end
is less than the value of the variable beg
. If it
is, it means that the user is most likely heading towards the
beginning of the buffer. Also, the result of evaluating the predicate
expression, (< end beg)
, will be true and the text will be
prepended before the previous text. On the other hand, if the value of
the variable end
is greater than the value of the variable
beg
, the text will be appended after the previous text.
When the newly saved text will be prepended, then the string with the new text will be concatenated before the old text:
(concat string cur)
But if the text will be appended, it will be concatenated after the old text:
(concat cur string))
To understand how this works, we first need to review the
concat
function. The concat
function links together or
unites two strings of text. The result is a string. For example:
(concat "abc" "def") ⇒ "abcdef"
(concat "new " (car '("first element" "second element"))) ⇒ "new first element" (concat (car '("first element" "second element")) " modified") ⇒ "first element modified"
We can now make sense of kill-append
: it modifies the contents
of the kill ring. The kill ring is a list, each element of which is
saved text. The kill-append
function uses the kill-new
function which in turn uses the setcar
function.
Previous: The kill-append
function, Up: The Body of copy-region-as-kill
[Contents][Index]
The kill-new
function
In version 22 the kill-new
function looks like this:
(defun kill-new (string &optional replace yank-handler) "Make STRING the latest kill in the kill ring. Set `kill-ring-yank-pointer' to point to it. If `interprogram-cut-function' is non-nil, apply it to STRING. Optional second argument REPLACE non-nil means that STRING will replace the front of the kill ring, rather than being added to the list. …"
(if (> (length string) 0) (if yank-handler (put-text-property 0 (length string) 'yank-handler yank-handler string)) (if yank-handler (signal 'args-out-of-range (list string "yank-handler specified for empty string"))))
(if (fboundp 'menu-bar-update-yank-menu) (menu-bar-update-yank-menu string (and replace (car kill-ring))))
(if (and replace kill-ring) (setcar kill-ring string) (push string kill-ring) (if (> (length kill-ring) kill-ring-max) (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
(setq kill-ring-yank-pointer kill-ring) (if interprogram-cut-function (funcall interprogram-cut-function string (not replace))))
(Notice that the function is not interactive.)
As usual, we can look at this function in parts.
The function definition has an optional yank-handler
argument,
which when invoked tells the function how to deal with properties
added to the text, such as bold or italics. We will skip that.
The first line of the documentation makes sense:
Make STRING the latest kill in the kill ring.
Let’s skip over the rest of the documentation for the moment.
Also, let’s skip over the initial if
expression and those lines
of code involving menu-bar-update-yank-menu
. We will explain
them below.
The critical lines are these:
(if (and replace kill-ring)
;; then
(setcar kill-ring string)
;; else
(push string kill-ring)
(if (> (length kill-ring) kill-ring-max)
;; avoid overly long kill ring
(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
(setq kill-ring-yank-pointer kill-ring) (if interprogram-cut-function (funcall interprogram-cut-function string (not replace))))
The conditional test is (and replace kill-ring)
.
This will be true when two conditions are met: the kill ring has
something in it, and the replace
variable is true.
When the kill-append
function sets replace
to be true
and when the kill ring has at least one item in it, the setcar
expression is executed:
(setcar kill-ring string)
The setcar
function actually changes the first element of the
kill-ring
list to the value of string
. It replaces the
first element.
On the other hand, if the kill ring is empty, or replace is false, the else-part of the condition is executed:
(push string kill-ring)
push
puts its first argument onto the second. It is similar to
the older
(setq kill-ring (cons string kill-ring))
or the newer
(add-to-list kill-ring string)
When it is false, the expression first constructs a new version of the
kill ring by prepending string
to the existing kill ring as a
new element (that is what the push
does). Then it executes a
second if
clause. This second if
clause keeps the kill
ring from growing too long.
Let’s look at these two expressions in order.
The push
line of the else-part sets the new value of the kill
ring to what results from adding the string being killed to the old
kill ring.
We can see how this works with an example.
First,
(setq example-list '("here is a clause" "another clause"))
After evaluating this expression with C-x C-e, you can evaluate
example-list
and see what it returns:
example-list ⇒ ("here is a clause" "another clause")
Now, we can add a new element on to this list by evaluating the following expression:
(push "a third clause" example-list)
When we evaluate example-list
, we find its value is:
example-list ⇒ ("a third clause" "here is a clause" "another clause")
Thus, the third clause is added to the list by push
.
Now for the second part of the if
clause. This expression
keeps the kill ring from growing too long. It looks like this:
(if (> (length kill-ring) kill-ring-max) (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))
The code checks whether the length of the kill ring is greater than
the maximum permitted length. This is the value of
kill-ring-max
(which is 120, by default). If the length of the
kill ring is too long, then this code sets the last element of the
kill ring to nil
. It does this by using two functions,
nthcdr
and setcdr
.
We looked at setcdr
earlier (see setcdr
).
It sets the CDR of a list, just as setcar
sets the
CAR of a list. In this case, however, setcdr
will not be
setting the CDR of the whole kill ring; the nthcdr
function is used to cause it to set the CDR of the next to last
element of the kill ring—this means that since the CDR of the
next to last element is the last element of the kill ring, it will set
the last element of the kill ring.
The nthcdr
function works by repeatedly taking the CDR of a
list—it takes the CDR of the CDR of the CDR
… It does this N times and returns the results.
(See nthcdr
.)
Thus, if we had a four element list that was supposed to be three
elements long, we could set the CDR of the next to last element
to nil
, and thereby shorten the list. (If you set the last
element to some other value than nil
, which you could do, then
you would not have shortened the list. See setcdr
.)
You can see shortening by evaluating the following three expressions
in turn. First set the value of trees
to (maple oak pine
birch)
, then set the CDR of its second CDR to nil
and then find the value of trees
:
(setq trees (list 'maple 'oak 'pine 'birch)) ⇒ (maple oak pine birch)
(setcdr (nthcdr 2 trees) nil) ⇒ nil trees ⇒ (maple oak pine)
(The value returned by the setcdr
expression is nil
since
that is what the CDR is set to.)
To repeat, in kill-new
, the nthcdr
function takes the
CDR a number of times that is one less than the maximum permitted
size of the kill ring and setcdr
sets the CDR of that
element (which will be the rest of the elements in the kill ring) to
nil
. This prevents the kill ring from growing too long.
The next to last expression in the kill-new
function is
(setq kill-ring-yank-pointer kill-ring)
The kill-ring-yank-pointer
is a global variable that is set to be
the kill-ring
.
Even though the kill-ring-yank-pointer
is called a
‘pointer’, it is a variable just like the kill ring. However, the
name has been chosen to help humans understand how the variable is used.
Now, to return to an early expression in the body of the function:
(if (fboundp 'menu-bar-update-yank-menu) (menu-bar-update-yank-menu string (and replace (car kill-ring))))
It starts with an if
expression
In this case, the expression tests first to see whether
menu-bar-update-yank-menu
exists as a function, and if so,
calls it. The fboundp
function returns true if the symbol it
is testing has a function definition that is not void. If the
symbol’s function definition were void, we would receive an error
message, as we did when we created errors intentionally (see Generate an Error Message).
The then-part contains an expression whose first element is the
function and
.
The and
special form evaluates each of its arguments until one
of the arguments returns a value of nil
, in which case the
and
expression returns nil
; however, if none of the
arguments returns a value of nil
, the value resulting from
evaluating the last argument is returned. (Since such a value is not
nil
, it is considered true in Emacs Lisp.) In other words, an
and
expression returns a true value only if all its arguments
are true. (See Review.)
The expression determines whether the second argument to
menu-bar-update-yank-menu
is true or not.
menu-bar-update-yank-menu
is one of the functions that make it
possible to use the “Select and Paste” menu in the Edit item of a menu
bar; using a mouse, you can look at the various pieces of text you
have saved and select one piece to paste.
The last expression in the kill-new
function adds the newly
copied string to whatever facility exists for copying and pasting
among different programs running in a windowing system. In the X
Windowing system, for example, the x-select-text
function takes
the string and stores it in memory operated by X. You can paste the
string in another program, such as an Xterm.
The expression looks like this:
(if interprogram-cut-function (funcall interprogram-cut-function string (not replace))))
If an interprogram-cut-function
exists, then Emacs executes
funcall
, which in turn calls its first argument as a function
and passes the remaining arguments to it. (Incidentally, as far as I
can see, this if
expression could be replaced by an and
expression similar to the one in the first part of the function.)
We are not going to discuss windowing systems and other programs further, but merely note that this is a mechanism that enables GNU Emacs to work easily and well with other programs.
This code for placing text in the kill ring, either concatenated with an existing element or as a new element, leads us to the code for bringing back text that has been cut out of the buffer—the yank commands. However, before discussing the yank commands, it is better to learn how lists are implemented in a computer. This will make clear such mysteries as the use of the term “pointer”. But before that, we will digress into C.
Next: Initializing a Variable with defvar
, Previous: copy-region-as-kill
, Up: Cutting and Storing Text [Contents][Index]
8.4 Digression into C
The copy-region-as-kill
function (see copy-region-as-kill
) uses the filter-buffer-substring
function, which in turn uses the delete-and-extract-region
function. It removes the contents of a region and you cannot get them
back.
Unlike the other code discussed here, the
delete-and-extract-region
function is not written in Emacs
Lisp; it is written in C and is one of the primitives of the GNU Emacs
system. Since it is very simple, I will digress briefly from Lisp and
describe it here.
Like many of the other Emacs primitives,
delete-and-extract-region
is written as an instance of a C
macro, a macro being a template for code. The complete macro looks
like this:
DEFUN ("delete-and-extract-region", Fdelete_and_extract_region, Sdelete_and_extract_region, 2, 2, 0, doc: /* Delete the text between START and END and return it. */) (Lisp_Object start, Lisp_Object end) { validate_region (&start, &end); if (XFIXNUM (start) == XFIXNUM (end)) return empty_unibyte_string; return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1); }
Without going into the details of the macro writing process, let me
point out that this macro starts with the word DEFUN
. The word
DEFUN
was chosen since the code serves the same purpose as
defun
does in Lisp. (The DEFUN
C macro is defined in
emacs/src/lisp.h.)
The word DEFUN
is followed by seven parts inside of
parentheses:
- The first part is the name given to the function in Lisp,
delete-and-extract-region
. - The second part is the name of the function in C,
Fdelete_and_extract_region
. By convention, it starts with ‘F’. Since C does not use hyphens in names, underscores are used instead. - The third part is the name for the C constant structure that records information on this function for internal use. It is the name of the function in C but begins with an ‘S’ instead of an ‘F’.
- The fourth and fifth parts specify the minimum and maximum number of arguments the function can have. This function demands exactly 2 arguments.
- The sixth part is nearly like the argument that follows the
interactive
declaration in a function written in Lisp: a letter followed, perhaps, by a prompt. The only difference from Lisp is when the macro is called with no arguments. Then you write a0
(which is a null string), as in this macro.If you were to specify arguments, you would place them between quotation marks. The C macro for
goto-char
includes"NGoto char: "
in this position to indicate that the function expects a raw prefix, in this case, a numerical location in a buffer, and provides a prompt. - The seventh part is a documentation string, just like the one for a
function written in Emacs Lisp. This is written as a C comment. (When
you build Emacs, the program
lib-src/make-docfile
extracts these comments and uses them to make the documentation.)
In a C macro, the formal parameters come next, with a statement of
what kind of object they are, followed by the body
of the macro. For delete-and-extract-region
the body
consists of the following four lines:
validate_region (&start, &end); if (XFIXNUM (start) == XFIXNUM (end)) return empty_unibyte_string; return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
The validate_region
function checks whether the values
passed as the beginning and end of the region are the proper type and
are within range. If the beginning and end positions are the same,
then return an empty string.
The del_range_1
function actually deletes the text. It is a
complex function we will not look into. It updates the buffer and
does other things. However, it is worth looking at the two arguments
passed to del_range_1
. These are XFIXNUM (start)
and
XFIXNUM (end)
.
As far as the C language is concerned, start
and end
are
two opaque values that mark the beginning and end of the region to be
deleted. More precisely, and requiring more expert knowledge
to understand, the two values are of type Lisp_Object
, which
might be a C pointer, a C integer, or a C struct
; C code
ordinarily should not care how Lisp_Object
is implemented.
Lisp_Object
widths depend on the machine, and are typically 32
or 64 bits. A few of the bits are used to specify the type of
information; the remaining bits are used as content.
‘XFIXNUM’ is a C macro that extracts the relevant integer from the longer collection of bits; the type bits are discarded.
The command in delete-and-extract-region
looks like this:
del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
It deletes the region between the beginning position, start
,
and the ending position, end
.
From the point of view of the person writing Lisp, Emacs is all very simple; but hidden underneath is a great deal of complexity to make it all work.
Next: Review, Previous: Digression into C, Up: Cutting and Storing Text [Contents][Index]
8.5 Initializing a Variable with defvar
The copy-region-as-kill
function is written in Emacs Lisp. Two
functions within it, kill-append
and kill-new
, copy a
region in a buffer and save it in a variable called the
kill-ring
. This section describes how the kill-ring
variable is created and initialized using the defvar
special
form.
(Again we note that the term kill-ring
is a misnomer. The text
that is clipped out of the buffer can be brought back; it is not a ring
of corpses, but a ring of resurrectable text.)
In Emacs Lisp, a variable such as the kill-ring
is created and
given an initial value by using the defvar
special form. The
name comes from “define variable”.
The defvar
special form is similar to setq
in that it
sets the value of a variable. It is unlike setq
in three ways:
first, it marks the variable as “special” so that it is always
dynamically bound, even when lexical-binding
is t
(see How let
Binds Variables). Second, it only sets the value of
the variable if the variable does not already have a value. If the
variable already has a value, defvar
does not override the
existing value. Third, defvar
has a documentation string.
(There is a related macro, defcustom
, designed for variables
that people customize. It has more features than defvar
.
(See Setting Variables with defcustom
.)
Seeing the Current Value of a Variable
You can see the current value of a variable, any variable, by using
the describe-variable
function, which is usually invoked by
typing C-h v. If you type C-h v and then kill-ring
(followed by RET) when prompted, you will see what is in your
current kill ring—this may be quite a lot! Conversely, if you have
been doing nothing this Emacs session except read this document, you
may have nothing in it. Also, you will see the documentation for
kill-ring
:
Documentation: List of killed text sequences. Since the kill ring is supposed to interact nicely with cut-and-paste facilities offered by window systems, use of this variable should
interact nicely with `interprogram-cut-function' and `interprogram-paste-function'. The functions `kill-new', `kill-append', and `current-kill' are supposed to implement this interaction; you may want to use them instead of manipulating the kill ring directly.
The kill ring is defined by a defvar
in the following way:
(defvar kill-ring nil "List of killed text sequences. …")
In this variable definition, the variable is given an initial value of
nil
, which makes sense, since if you have saved nothing, you want
nothing back if you give a yank
command. The documentation
string is written just like the documentation string of a defun
.
As with the documentation string of the defun
, the first line of
the documentation should be a complete sentence, since some commands,
like apropos
, print only the first line of documentation.
Succeeding lines should not be indented; otherwise they look odd when
you use C-h v (describe-variable
).
Previous: Seeing the Current Value of a Variable, Up: Initializing a Variable with defvar
[Contents][Index]
8.5.1 defvar
and an asterisk
In the past, Emacs used the defvar
special form both for
internal variables that you would not expect a user to change and for
variables that you do expect a user to change. Although you can still
use defvar
for user customizable variables, please use
defcustom
instead, since it provides a path into
the Customization commands. (See Specifying Variables
using defcustom
.)
When you specified a variable using the defvar
special form,
you could distinguish a variable that a user might want to change from
others by typing an asterisk, ‘*’, in the first column of its
documentation string. For example:
(defvar shell-command-default-error-buffer nil "*Buffer name for `shell-command' … error output. … ")
You could (and still can) use the set-variable
command to
change the value of shell-command-default-error-buffer
temporarily. However, options set using set-variable
are set
only for the duration of your editing session. The new values are not
saved between sessions. Each time Emacs starts, it reads the original
value, unless you change the value within your .emacs file,
either by setting it manually or by using customize
.
See Your .emacs File.
For me, the major use of the set-variable
command is to suggest
variables that I might want to set in my .emacs file. There
are now more than 700 such variables, far too many to remember
readily. Fortunately, you can press TAB after calling the
M-x set-variable
command to see the list of variables.
(See Examining and Setting Variables in The GNU Emacs Manual.)
Next: Searching Exercises, Previous: Initializing a Variable with defvar
, Up: Cutting and Storing Text [Contents][Index]
8.6 Review
Here is a brief summary of some recently introduced functions.
car
cdr
car
returns the first element of a list;cdr
returns the second and subsequent elements of a list.For example:
(car '(1 2 3 4 5 6 7)) ⇒ 1 (cdr '(1 2 3 4 5 6 7)) ⇒ (2 3 4 5 6 7)
cons
cons
constructs a list by prepending its first argument to its second argument.For example:
(cons 1 '(2 3 4)) ⇒ (1 2 3 4)
funcall
funcall
evaluates its first argument as a function. It passes its remaining arguments to its first argument.nthcdr
Return the result of taking CDR n times on a list. The “rest of the rest”, as it were.
For example:
(nthcdr 3 '(1 2 3 4 5 6 7)) ⇒ (4 5 6 7)
setcar
setcdr
setcar
changes the first element of a list;setcdr
changes the second and subsequent elements of a list.For example:
(setq triple (list 1 2 3)) (setcar triple '37) triple ⇒ (37 2 3) (setcdr triple '("foo" "bar")) triple ⇒ (37 "foo" "bar")
progn
Evaluate each argument in sequence and then return the value of the last.
For example:
(progn 1 2 3 4) ⇒ 4
save-restriction
Record whatever narrowing is in effect in the current buffer, if any, and restore that narrowing after evaluating the arguments.
search-forward
Search for a string, and if the string is found, move point. With a regular expression, use the similar
re-search-forward
. (See Regular Expression Searches, for an explanation of regular expression patterns and searches.)search-forward
andre-search-forward
take four arguments:- The string or regular expression to search for.
- Optionally, the limit of the search.
- Optionally, what to do if the search fails, return
nil
or an error message. - Optionally, how many times to repeat the search; if negative, the search goes backwards.
kill-region
delete-and-extract-region
copy-region-as-kill
-
kill-region
cuts the text between point and mark from the buffer and stores that text in the kill ring, so you can get it back by yanking.copy-region-as-kill
copies the text between point and mark into the kill ring, from which you can get it by yanking. The function does not cut or remove the text from the buffer.
delete-and-extract-region
removes the text between point and
mark from the buffer and throws it away. You cannot get it back.
(This is not an interactive command.)
Previous: Review, Up: Cutting and Storing Text [Contents][Index]
8.7 Searching Exercises
- Write an interactive function that searches for a string. If the
search finds the string, leave point after it and display a message
that says “Found!”. (Do not use
search-forward
for the name of this function; if you do, you will overwrite the existing version ofsearch-forward
that comes with Emacs. Use a name such astest-search
instead.) - Write a function that prints the third element of the kill ring in the echo area, if any; if the kill ring does not contain a third element, print an appropriate message.
Next: Yanking Text Back, Previous: Cutting and Storing Text, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
9 How Lists are Implemented
In Lisp, atoms are recorded in a straightforward fashion; if the
implementation is not straightforward in practice, it is, nonetheless,
straightforward in theory. The atom ‘rose’, for example, is
recorded as the four contiguous letters ‘r’, ‘o’, ‘s’,
‘e’. A list, on the other hand, is kept differently. The mechanism
is equally simple, but it takes a moment to get used to the idea. A
list is kept using a series of pairs of pointers. In the series, the
first pointer in each pair points to an atom or to another list, and the
second pointer in each pair points to the next pair, or to the symbol
nil
, which marks the end of the list.
A pointer itself is quite simply the electronic address of what is pointed to. Hence, a list is kept as a series of electronic addresses.
Next: Symbols as a Chest of Drawers, Up: How Lists are Implemented [Contents][Index]
Lists diagrammed
For example, the list (rose violet buttercup)
has three
elements, ‘rose’, ‘violet’, and ‘buttercup’. In the
computer, the electronic address of ‘rose’ is recorded in a
segment of computer memory called a cons cell (because it’s what
the function cons
actually creates). That cons cell also holds
the address of a second cons cell, whose CAR is the atom
‘violet’; and that address (the one that tells where to find
‘violet’) is kept along with the address of a third cons cell
which holds the address for the atom ‘buttercup’.
This sounds more complicated than it is and is easier seen in a diagram:
___ ___ ___ ___ ___ ___ |___|___|--> |___|___|--> |___|___|--> nil | | | | | | --> rose --> violet --> buttercup
In the diagram, each box represents a word of computer memory that
holds a Lisp object, usually in the form of a memory address. The boxes,
i.e., the addresses, are in pairs. Each arrow points to what the address
is the address of, either an atom or another pair of addresses. The
first box is the electronic address of ‘rose’ and the arrow points
to ‘rose’; the second box is the address of the next pair of boxes,
the first part of which is the address of ‘violet’ and the second
part of which is the address of the next pair. The very last box
points to the symbol nil
, which marks the end of the list.
When a variable is set to a list with an operation such as setq
,
it stores the address of the first box in the variable. Thus,
evaluation of the expression
(setq bouquet '(rose violet buttercup))
creates a situation like this:
bouquet | | ___ ___ ___ ___ ___ ___ --> |___|___|--> |___|___|--> |___|___|--> nil | | | | | | --> rose --> violet --> buttercup
In this example, the symbol bouquet
holds the address of the first
pair of boxes.
This same list can be illustrated in a different sort of box notation like this:
bouquet | | -------------- --------------- ---------------- | | car | cdr | | car | cdr | | car | cdr | -->| rose | o------->| violet | o------->| butter- | nil | | | | | | | | cup | | -------------- --------------- ----------------
(Symbols consist of more than pairs of addresses, but the structure of
a symbol is made up of addresses. Indeed, the symbol bouquet
consists of a group of address-boxes, one of which is the address of
the printed word ‘bouquet’, a second of which is the address of a
function definition attached to the symbol, if any, a third of which
is the address of the first pair of address-boxes for the list
(rose violet buttercup)
, and so on. Here we are showing that
the symbol’s third address-box points to the first pair of
address-boxes for the list.)
If a symbol is set to the CDR of a list, the list itself is not changed; the symbol simply has an address further down the list. (In the jargon, CAR and CDR are “non-destructive”.) Thus, evaluation of the following expression
(setq flowers (cdr bouquet))
produces this:
bouquet flowers | | | ___ ___ | ___ ___ ___ ___ --> | | | --> | | | | | | |___|___|----> |___|___|--> |___|___|--> nil | | | | | | --> rose --> violet --> buttercup
The value of flowers
is (violet buttercup)
, which is
to say, the symbol flowers
holds the address of the pair of
address-boxes, the first of which holds the address of violet
,
and the second of which holds the address of buttercup
.
A pair of address-boxes is called a cons cell or dotted pair. See Cons Cell and List Types in The GNU Emacs Lisp Reference Manual, and Dotted Pair Notation in The GNU Emacs Lisp Reference Manual, for more information about cons cells and dotted pairs.
The function cons
adds a new pair of addresses to the front of
a series of addresses like that shown above. For example, evaluating
the expression
(setq bouquet (cons 'lily bouquet))
produces:
bouquet flowers | | | ___ ___ ___ ___ | ___ ___ ___ ___ --> | | | | | | --> | | | | | | |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil | | | | | | | | --> lily --> rose --> violet --> buttercup
However, this does not change the value of the symbol
flowers
, as you can see by evaluating the following,
(eq (cdr (cdr bouquet)) flowers)
which returns t
for true.
Until it is reset, flowers
still has the value
(violet buttercup)
; that is, it has the address of the cons
cell whose first address is of violet
. Also, this does not
alter any of the pre-existing cons cells; they are all still there.
Thus, in Lisp, to get the CDR of a list, you just get the address
of the next cons cell in the series; to get the CAR of a list,
you get the address of the first element of the list; to cons
a
new element on a list, you add a new cons cell to the front of the list.
That is all there is to it! The underlying structure of Lisp is
brilliantly simple!
And what does the last address in a series of cons cells refer to? It
is the address of the empty list, of nil
.
In summary, when a Lisp variable is set to a value, it is provided with the address of the list to which the variable refers.
Next: Exercise, Previous: Lists diagrammed, Up: How Lists are Implemented [Contents][Index]
9.1 Symbols as a Chest of Drawers
In an earlier section, I suggested that you might imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice versa.
Actually, what is put in each drawer is the address of the value or function definition. It is as if you found an old chest in the attic, and in one of its drawers you found a map giving you directions to where the buried treasure lies.
(In addition to its name, symbol definition, and variable value, a symbol has a drawer for a property list which can be used to record other information. Property lists are not discussed here; see Property Lists in The GNU Emacs Lisp Reference Manual.)
Here is a fanciful representation:
Chest of Drawers Contents of Drawers __ o0O0o __ / \ --------------------- | directions to | [map to] | symbol name | bouquet | | +---------------------+ | directions to | | symbol definition | [none] | | +---------------------+ | directions to | [map to] | variable value | (rose violet buttercup) | | +---------------------+ | directions to | | property list | [not described here] | | +---------------------+ |/ \|
Previous: Symbols as a Chest of Drawers, Up: How Lists are Implemented [Contents][Index]
9.2 Exercise
Set flowers
to violet
and buttercup
. Cons two
more flowers on to this list and set this new list to
more-flowers
. Set the CAR of flowers
to a fish.
What does the more-flowers
list now contain?
Next: Loops and Recursion, Previous: How Lists are Implemented, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
10 Yanking Text Back
Whenever you cut text out of a buffer with a kill command in GNU Emacs, you can bring it back with a yank command. The text that is cut out of the buffer is put in the kill ring and the yank commands insert the appropriate contents of the kill ring back into a buffer (not necessarily the original buffer).
A simple C-y (yank
) command inserts the first item from
the kill ring into the current buffer. If the C-y command is
followed immediately by M-y, the first element is replaced by
the second element. Successive M-y commands replace the second
element with the third, fourth, or fifth element, and so on. When the
last element in the kill ring is reached, it is replaced by the first
element and the cycle is repeated. (Thus the kill ring is called a
“ring” rather than just a “list”. However, the actual data structure
that holds the text is a list.
See Handling the Kill Ring, for the details of how the
list is handled as a ring.)
Next: The kill-ring-yank-pointer
Variable, Up: Yanking Text Back [Contents][Index]
10.1 Kill Ring Overview
The kill ring is a list of textual strings. This is what it looks like:
("some text" "a different piece of text" "yet more text")
If this were the contents of my kill ring and I pressed C-y, the string of characters saying ‘some text’ would be inserted in this buffer where my cursor is located.
The yank
command is also used for duplicating text by copying it.
The copied text is not cut from the buffer, but a copy of it is put on the
kill ring and is inserted by yanking it back.
Three functions are used for bringing text back from the kill ring:
yank
, which is usually bound to C-y; yank-pop
,
which is usually bound to M-y; and rotate-yank-pointer
,
which is used by the two other functions.
These functions refer to the kill ring through a variable called the
kill-ring-yank-pointer
. Indeed, the insertion code for both the
yank
and yank-pop
functions is:
(insert (car kill-ring-yank-pointer))
(Well, no more. In GNU Emacs 22, the function has been replaced by
insert-for-yank
which calls insert-for-yank-1
repetitively for each yank-handler
segment. In turn,
insert-for-yank-1
strips text properties from the inserted text
according to yank-excluded-properties
. Otherwise, it is just
like insert
. We will stick with plain insert
since it
is easier to understand.)
To begin to understand how yank
and yank-pop
work, it is
first necessary to look at the kill-ring-yank-pointer
variable.
Next: Exercises with yank
and nthcdr
, Previous: Kill Ring Overview, Up: Yanking Text Back [Contents][Index]
10.2 The kill-ring-yank-pointer
Variable
kill-ring-yank-pointer
is a variable, just as kill-ring
is
a variable. It points to something by being bound to the value of what
it points to, like any other Lisp variable.
Thus, if the value of the kill ring is:
("some text" "a different piece of text" "yet more text")
and the kill-ring-yank-pointer
points to the second clause, the
value of kill-ring-yank-pointer
is:
("a different piece of text" "yet more text")
As explained in the previous chapter (see How Lists are Implemented), the
computer does not keep two different copies of the text being pointed to
by both the kill-ring
and the kill-ring-yank-pointer
. The
words “a different piece of text” and “yet more text” are not
duplicated. Instead, the two Lisp variables point to the same pieces of
text. Here is a diagram:
kill-ring kill-ring-yank-pointer | | | ___ ___ | ___ ___ ___ ___ ---> | | | --> | | | | | | |___|___|----> |___|___|--> |___|___|--> nil | | | | | | | | --> "yet more text" | | | --> "a different piece of text" | --> "some text"
Both the variable kill-ring
and the variable
kill-ring-yank-pointer
are pointers. But the kill ring itself is
usually described as if it were actually what it is composed of. The
kill-ring
is spoken of as if it were the list rather than that it
points to the list. Conversely, the kill-ring-yank-pointer
is
spoken of as pointing to a list.
These two ways of talking about the same thing sound confusing at first but
make sense on reflection. The kill ring is generally thought of as the
complete structure of data that holds the information of what has recently
been cut out of the Emacs buffers. The kill-ring-yank-pointer
on the other hand, serves to indicate—that is, to point to—that part
of the kill ring of which the first element (the CAR) will be
inserted.
Previous: The kill-ring-yank-pointer
Variable, Up: Yanking Text Back [Contents][Index]
10.3 Exercises with yank
and nthcdr
- Using C-h v (
describe-variable
), look at the value of your kill ring. Add several items to your kill ring; look at its value again. Using M-y (yank-pop)
, move all the way around the kill ring. How many items were in your kill ring? Find the value ofkill-ring-max
. Was your kill ring full, or could you have kept more blocks of text within it? - Using
nthcdr
andcar
, construct a series of expressions to return the first, second, third, and fourth elements of a list.
Next: Regular Expression Searches, Previous: Yanking Text Back, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
11 Loops and Recursion
Emacs Lisp has two primary ways to cause an expression, or a series of
expressions, to be evaluated repeatedly: one uses a while
loop, and the other uses recursion.
Repetition can be very valuable. For example, to move forward four sentences, you need only write a program that will move forward one sentence and then repeat the process four times. Since a computer does not get bored or tired, such repetitive action does not have the deleterious effects that excessive or the wrong kinds of repetition can have on humans.
People mostly write Emacs Lisp functions using while
loops and
their kin; but you can use recursion, which provides a very powerful
way to think about and then to solve problems14.
Next: Save your time: dolist
and dotimes
, Up: Loops and Recursion [Contents][Index]
11.1 while
The while
special form tests whether the value returned by
evaluating its first argument is true or false. This is similar to what
the Lisp interpreter does with an if
; what the interpreter does
next, however, is different.
In a while
expression, if the value returned by evaluating the
first argument is false, the Lisp interpreter skips the rest of the
expression (the body of the expression) and does not evaluate it.
However, if the value is true, the Lisp interpreter evaluates the body
of the expression and then again tests whether the first argument to
while
is true or false. If the value returned by evaluating the
first argument is again true, the Lisp interpreter again evaluates the
body of the expression.
The template for a while
expression looks like this:
(while true-or-false-test body…)
- Looping with
while
- A
while
Loop and a List - An Example:
print-elements-of-list
- A Loop with an Incrementing Counter
- Details of an Incrementing Loop
- Loop with a Decrementing Counter
Next: A while
Loop and a List, Up: while
[Contents][Index]
Looping with while
So long as the true-or-false-test of the while
expression
returns a true value when it is evaluated, the body is repeatedly
evaluated. This process is called a loop since the Lisp interpreter
repeats the same thing again and again, like an airplane doing a loop.
When the result of evaluating the true-or-false-test is false, the
Lisp interpreter does not evaluate the rest of the while
expression and exits the loop.
Clearly, if the value returned by evaluating the first argument to
while
is always true, the body following will be evaluated
again and again … and again … forever. Conversely, if the
value returned is never true, the expressions in the body will never
be evaluated. The craft of writing a while
loop consists of
choosing a mechanism such that the true-or-false-test returns true
just the number of times that you want the subsequent expressions to
be evaluated, and then have the test return false.
The value returned by evaluating a while
is the value of the
true-or-false-test. An interesting consequence of this is that a
while
loop that evaluates without error will return nil
or false regardless of whether it has looped 1 or 100 times or none at
all. A while
expression that evaluates successfully never
returns a true value! What this means is that while
is always
evaluated for its side effects, which is to say, the consequences of
evaluating the expressions within the body of the while
loop.
This makes sense. It is not the mere act of looping that is desired,
but the consequences of what happens when the expressions in the loop
are repeatedly evaluated.
Next: An Example: print-elements-of-list
, Previous: Looping with while
, Up: while
[Contents][Index]
11.1.1 A while
Loop and a List
A common way to control a while
loop is to test whether a list
has any elements. If it does, the loop is repeated; but if it does not,
the repetition is ended. Since this is an important technique, we will
create a short example to illustrate it.
A simple way to test whether a list has elements is to evaluate the
list: if it has no elements, it is an empty list and will return the
empty list, ()
, which is a synonym for nil
or false. On
the other hand, a list with elements will return those elements when it
is evaluated. Since Emacs Lisp considers as true any value that is not
nil
, a list that returns elements will test true in a
while
loop.
For example, you can set the variable empty-list
to nil
by
evaluating the following setq
expression:
(setq empty-list ())
After evaluating the setq
expression, you can evaluate the
variable empty-list
in the usual way, by placing the cursor after
the symbol and typing C-x C-e; nil
will appear in your
echo area:
empty-list
On the other hand, if you set a variable to be a list with elements, the list will appear when you evaluate the variable, as you can see by evaluating the following two expressions:
(setq animals '(gazelle giraffe lion tiger)) animals
Thus, to create a while
loop that tests whether there are any
items in the list animals
, the first part of the loop will be
written like this:
(while animals …
When the while
tests its first argument, the variable
animals
is evaluated. It returns a list. So long as the list
has elements, the while
considers the results of the test to be
true; but when the list is empty, it considers the results of the test
to be false.
To prevent the while
loop from running forever, some mechanism
needs to be provided to empty the list eventually. An oft-used
technique is to have one of the subsequent forms in the while
expression set the value of the list to be the CDR of the list.
Each time the cdr
function is evaluated, the list will be made
shorter, until eventually only the empty list will be left. At this
point, the test of the while
loop will return false, and the
arguments to the while
will no longer be evaluated.
For example, the list of animals bound to the variable animals
can be set to be the CDR of the original list with the
following expression:
(setq animals (cdr animals))
If you have evaluated the previous expressions and then evaluate this
expression, you will see (giraffe lion tiger)
appear in the echo
area. If you evaluate the expression again, (lion tiger)
will
appear in the echo area. If you evaluate it again and yet again,
(tiger)
appears and then the empty list, shown by nil
.
A template for a while
loop that uses the cdr
function
repeatedly to cause the true-or-false-test eventually to test false
looks like this:
(while test-whether-list-is-empty body… set-list-to-cdr-of-list)
This test and use of cdr
can be put together in a function that
goes through a list and prints each element of the list on a line of its
own.
Next: A Loop with an Incrementing Counter, Previous: A while
Loop and a List, Up: while
[Contents][Index]
11.1.2 An Example: print-elements-of-list
The print-elements-of-list
function illustrates a while
loop with a list.
The function requires several lines for its output. If you are reading this in a recent instance of GNU Emacs, you can evaluate the following expression inside of Info, as usual.
If you are using an earlier version of Emacs, you need to copy the necessary expressions to your *scratch* buffer and evaluate them there. This is because the echo area had only one line in the earlier versions.
You can copy the expressions by marking the beginning of the region
with C-SPC (set-mark-command
), moving the cursor to
the end of the region and then copying the region using M-w
(kill-ring-save
, which calls copy-region-as-kill
and
then provides visual feedback). In the *scratch*
buffer, you can yank the expressions back by typing C-y
(yank
).
After you have copied the expressions to the *scratch* buffer,
evaluate each expression in turn. Be sure to evaluate the last
expression, (print-elements-of-list animals)
, by typing
C-u C-x C-e, that is, by giving an argument to
eval-last-sexp
. This will cause the result of the evaluation
to be printed in the *scratch* buffer instead of being printed
in the echo area. (Otherwise you will see something like this in your
echo area: ^Jgazelle^J^Jgiraffe^J^Jlion^J^Jtiger^Jnil
, in which
each ‘^J’ stands for a newline.)
You can evaluate these expressions directly in the Info buffer, and the echo area will grow to show the results.
(setq animals '(gazelle giraffe lion tiger)) (defun print-elements-of-list (list) "Print each element of LIST on a line of its own." (while list (print (car list)) (setq list (cdr list)))) (print-elements-of-list animals)
When you evaluate the three expressions in sequence, you will see this:
gazelle giraffe lion tiger nil
Each element of the list is printed on a line of its own (that is what
the function print
does) and then the value returned by the
function is printed. Since the last expression in the function is the
while
loop, and since while
loops always return
nil
, a nil
is printed after the last element of the list.
Next: Details of an Incrementing Loop, Previous: An Example: print-elements-of-list
, Up: while
[Contents][Index]
11.1.3 A Loop with an Incrementing Counter
A loop is not useful unless it stops when it ought. Besides controlling a loop with a list, a common way of stopping a loop is to write the first argument as a test that returns false when the correct number of repetitions are complete. This means that the loop must have a counter—an expression that counts how many times the loop repeats itself.
Next: Loop with a Decrementing Counter, Previous: A Loop with an Incrementing Counter, Up: while
[Contents][Index]
Details of an Incrementing Loop
The test for a loop with an incrementing counter can be an expression
such as (< count desired-number)
which returns t
for
true if the value of count
is less than the
desired-number
of repetitions and nil
for false if the
value of count
is equal to or is greater than the
desired-number
. The expression that increments the count can
be a simple setq
such as (setq count (1+ count))
, where
1+
is a built-in function in Emacs Lisp that adds 1 to its
argument. (The expression (1+ count)
has the same result
as (+ count 1)
, but is easier for a human to read.)
The template for a while
loop controlled by an incrementing
counter looks like this:
set-count-to-initial-value (while (< count desired-number) ; true-or-false-test body… (setq count (1+ count))) ; incrementer
Note that you need to set the initial value of count
; usually it
is set to 1.
- Example with incrementing counter
- The parts of the function definition
- Putting the function definition together
Example with incrementing counter
Suppose you are playing on the beach and decide to make a triangle of pebbles, putting one pebble in the first row, two in the second row, three in the third row and so on, like this:
* * * * * * * * * *
(About 2500 years ago, Pythagoras and others developed the beginnings of number theory by considering questions such as this.)
Suppose you want to know how many pebbles you will need to make a triangle with 7 rows?
Clearly, what you need to do is add up the numbers from 1 to 7. There
are two ways to do this; start with the smallest number, one, and add up
the list in sequence, 1, 2, 3, 4 and so on; or start with the largest
number and add the list going down: 7, 6, 5, 4 and so on. Because both
mechanisms illustrate common ways of writing while
loops, we will
create two examples, one counting up and the other counting down. In
this first example, we will start with 1 and add 2, 3, 4 and so on.
If you are just adding up a short list of numbers, the easiest way to do it is to add up all the numbers at once. However, if you do not know ahead of time how many numbers your list will have, or if you want to be prepared for a very long list, then you need to design your addition so that what you do is repeat a simple process many times instead of doing a more complex process once.
For example, instead of adding up all the pebbles all at once, what you can do is add the number of pebbles in the first row, 1, to the number in the second row, 2, and then add the total of those two rows to the third row, 3. Then you can add the number in the fourth row, 4, to the total of the first three rows; and so on.
The critical characteristic of the process is that each repetitive action is simple. In this case, at each step we add only two numbers, the number of pebbles in the row and the total already found. This process of adding two numbers is repeated again and again until the last row has been added to the total of all the preceding rows. In a more complex loop the repetitive action might not be so simple, but it will be simpler than doing everything all at once.
Next: Putting the function definition together, Previous: Example with incrementing counter, Up: Details of an Incrementing Loop [Contents][Index]
The parts of the function definition
The preceding analysis gives us the bones of our function definition:
first, we will need a variable that we can call total
that will
be the total number of pebbles. This will be the value returned by
the function.
Second, we know that the function will require an argument: this
argument will be the total number of rows in the triangle. It can be
called number-of-rows
.
Finally, we need a variable to use as a counter. We could call this
variable counter
, but a better name is row-number
. That
is because what the counter does in this function is count rows, and a
program should be written to be as understandable as possible.
When the Lisp interpreter first starts evaluating the expressions in the
function, the value of total
should be set to zero, since we have
not added anything to it. Then the function should add the number of
pebbles in the first row to the total, and then add the number of
pebbles in the second to the total, and then add the number of
pebbles in the third row to the total, and so on, until there are no
more rows left to add.
Both total
and row-number
are used only inside the
function, so they can be declared as local variables with let
and given initial values. Clearly, the initial value for total
should be 0. The initial value of row-number
should be 1,
since we start with the first row. This means that the let
statement will look like this:
(let ((total 0) (row-number 1)) body…)
After the internal variables are declared and bound to their initial
values, we can begin the while
loop. The expression that serves
as the test should return a value of t
for true so long as the
row-number
is less than or equal to the number-of-rows
.
(If the expression tests true only so long as the row number is less
than the number of rows in the triangle, the last row will never be
added to the total; hence the row number has to be either less than or
equal to the number of rows.)
Lisp provides the <=
function that returns true if the value of
its first argument is less than or equal to the value of its second
argument and false otherwise. So the expression that the while
will evaluate as its test should look like this:
(<= row-number number-of-rows)
The total number of pebbles can be found by repeatedly adding the number of pebbles in a row to the total already found. Since the number of pebbles in the row is equal to the row number, the total can be found by adding the row number to the total. (Clearly, in a more complex situation, the number of pebbles in the row might be related to the row number in a more complicated way; if this were the case, the row number would be replaced by the appropriate expression.)
(setq total (+ total row-number))
What this does is set the new value of total
to be equal to the
sum of adding the number of pebbles in the row to the previous total.
After setting the value of total
, the conditions need to be
established for the next repetition of the loop, if there is one. This
is done by incrementing the value of the row-number
variable,
which serves as a counter. After the row-number
variable has
been incremented, the true-or-false-test at the beginning of the
while
loop tests whether its value is still less than or equal to
the value of the number-of-rows
and if it is, adds the new value
of the row-number
variable to the total
of the previous
repetition of the loop.
The built-in Emacs Lisp function 1+
adds 1 to a number, so the
row-number
variable can be incremented with this expression:
(setq row-number (1+ row-number))
Previous: The parts of the function definition, Up: Details of an Incrementing Loop [Contents][Index]
Putting the function definition together
We have created the parts for the function definition; now we need to put them together.
First, the contents of the while
expression:
(while (<= row-number number-of-rows) ; true-or-false-test (setq total (+ total row-number)) (setq row-number (1+ row-number))) ; incrementer
Along with the let
expression varlist, this very nearly
completes the body of the function definition. However, it requires
one final element, the need for which is somewhat subtle.
The final touch is to place the variable total
on a line by
itself after the while
expression. Otherwise, the value returned
by the whole function is the value of the last expression that is
evaluated in the body of the let
, and this is the value
returned by the while
, which is always nil
.
This may not be evident at first sight. It almost looks as if the
incrementing expression is the last expression of the whole function.
But that expression is part of the body of the while
; it is the
last element of the list that starts with the symbol while
.
Moreover, the whole of the while
loop is a list within the body
of the let
.
In outline, the function will look like this:
(defun name-of-function (argument-list)
"documentation…"
(let (varlist)
(while (true-or-false-test)
body-of-while… )
… )) ; Need final expression here.
The result of evaluating the let
is what is going to be returned
by the defun
since the let
is not embedded within any
containing list, except for the defun
as a whole. However, if
the while
is the last element of the let
expression, the
function will always return nil
. This is not what we want!
Instead, what we want is the value of the variable total
. This
is returned by simply placing the symbol as the last element of the list
starting with let
. It gets evaluated after the preceding
elements of the list are evaluated, which means it gets evaluated after
it has been assigned the correct value for the total.
It may be easier to see this by printing the list starting with
let
all on one line. This format makes it evident that the
varlist and while
expressions are the second and third
elements of the list starting with let
, and the total
is
the last element:
(let (varlist) (while (true-or-false-test) body-of-while… ) total)
Putting everything together, the triangle
function definition
looks like this:
(defun triangle (number-of-rows) ; Version with ; incrementing counter. "Add up the number of pebbles in a triangle. The first row has one pebble, the second row two pebbles, the third row three pebbles, and so on. The argument is NUMBER-OF-ROWS."
(let ((total 0) (row-number 1)) (while (<= row-number number-of-rows) (setq total (+ total row-number)) (setq row-number (1+ row-number))) total))
After you have installed triangle
by evaluating the function, you
can try it out. Here are two examples:
(triangle 4) (triangle 7)
The sum of the first four numbers is 10 and the sum of the first seven numbers is 28.
Previous: Details of an Incrementing Loop, Up: while
[Contents][Index]
11.1.4 Loop with a Decrementing Counter
Another common way to write a while
loop is to write the test
so that it determines whether a counter is greater than zero. So long
as the counter is greater than zero, the loop is repeated. But when
the counter is equal to or less than zero, the loop is stopped. For
this to work, the counter has to start out greater than zero and then
be made smaller and smaller by a form that is evaluated
repeatedly.
The test will be an expression such as (> counter 0)
which
returns t
for true if the value of counter
is greater
than zero, and nil
for false if the value of counter
is
equal to or less than zero. The expression that makes the number
smaller and smaller can be a simple setq
such as (setq
counter (1- counter))
, where 1-
is a built-in function in
Emacs Lisp that subtracts 1 from its argument.
The template for a decrementing while
loop looks like this:
(while (> counter 0) ; true-or-false-test body… (setq counter (1- counter))) ; decrementer
- Example with decrementing counter
- The parts of the function definition
- Putting the function definition together
Example with decrementing counter
To illustrate a loop with a decrementing counter, we will rewrite the
triangle
function so the counter decreases to zero.
This is the reverse of the earlier version of the function. In this case, to find out how many pebbles are needed to make a triangle with 3 rows, add the number of pebbles in the third row, 3, to the number in the preceding row, 2, and then add the total of those two rows to the row that precedes them, which is 1.
Likewise, to find the number of pebbles in a triangle with 7 rows, add the number of pebbles in the seventh row, 7, to the number in the preceding row, which is 6, and then add the total of those two rows to the row that precedes them, which is 5, and so on. As in the previous example, each addition only involves adding two numbers, the total of the rows already added up and the number of pebbles in the row that is being added to the total. This process of adding two numbers is repeated again and again until there are no more pebbles to add.
We know how many pebbles to start with: the number of pebbles in the last row is equal to the number of rows. If the triangle has seven rows, the number of pebbles in the last row is 7. Likewise, we know how many pebbles are in the preceding row: it is one less than the number in the row.
Next: Putting the function definition together, Previous: Example with decrementing counter, Up: Loop with a Decrementing Counter [Contents][Index]
The parts of the function definition
We start with three variables: the total number of rows in the
triangle; the number of pebbles in a row; and the total number of
pebbles, which is what we want to calculate. These variables can be
named number-of-rows
, number-of-pebbles-in-row
, and
total
, respectively.
Both total
and number-of-pebbles-in-row
are used only
inside the function and are declared with let
. The initial
value of total
should, of course, be zero. However, the
initial value of number-of-pebbles-in-row
should be equal to
the number of rows in the triangle, since the addition will start with
the longest row.
This means that the beginning of the let
expression will look
like this:
(let ((total 0) (number-of-pebbles-in-row number-of-rows)) body…)
The total number of pebbles can be found by repeatedly adding the number of pebbles in a row to the total already found, that is, by repeatedly evaluating the following expression:
(setq total (+ total number-of-pebbles-in-row))
After the number-of-pebbles-in-row
is added to the total
,
the number-of-pebbles-in-row
should be decremented by one, since
the next time the loop repeats, the preceding row will be
added to the total.
The number of pebbles in a preceding row is one less than the number of
pebbles in a row, so the built-in Emacs Lisp function 1-
can be
used to compute the number of pebbles in the preceding row. This can be
done with the following expression:
(setq number-of-pebbles-in-row (1- number-of-pebbles-in-row))
Finally, we know that the while
loop should stop making repeated
additions when there are no pebbles in a row. So the test for
the while
loop is simply:
(while (> number-of-pebbles-in-row 0)
Previous: The parts of the function definition, Up: Loop with a Decrementing Counter [Contents][Index]
Putting the function definition together
We can put these expressions together to create a function definition that works. However, on examination, we find that one of the local variables is unneeded!
The function definition looks like this:
;;; First subtractive version.
(defun triangle (number-of-rows)
"Add up the number of pebbles in a triangle."
(let ((total 0)
(number-of-pebbles-in-row number-of-rows))
(while (> number-of-pebbles-in-row 0)
(setq total (+ total number-of-pebbles-in-row))
(setq number-of-pebbles-in-row
(1- number-of-pebbles-in-row)))
total))
As written, this function works.
However, we do not need number-of-pebbles-in-row
.
When the triangle
function is evaluated, the symbol
number-of-rows
will be bound to a number, giving it an initial
value. That number can be changed in the body of the function as if
it were a local variable, without any fear that such a change will
effect the value of the variable outside of the function. This is a
very useful characteristic of Lisp; it means that the variable
number-of-rows
can be used anywhere in the function where
number-of-pebbles-in-row
is used.
Here is a second version of the function written a bit more cleanly:
(defun triangle (number) ; Second version.
"Return sum of numbers 1 through NUMBER inclusive."
(let ((total 0))
(while (> number 0)
(setq total (+ total number))
(setq number (1- number)))
total))
In brief, a properly written while
loop will consist of three parts:
- A test that will return false after the loop has repeated itself the correct number of times.
- An expression the evaluation of which will return the value desired after being repeatedly evaluated.
- An expression to change the value passed to the true-or-false-test so that the test returns false after the loop has repeated itself the right number of times.
Next: Recursion, Previous: while
, Up: Loops and Recursion [Contents][Index]
11.2 Save your time: dolist
and dotimes
In addition to while
, both dolist
and dotimes
provide for looping. Sometimes these are quicker to write than the
equivalent while
loop. Both are Lisp macros. (See Macros in The GNU Emacs Lisp Reference Manual. )
dolist
works like a while
loop that CDRs down a
list: dolist
automatically shortens the list each time it
loops—takes the CDR of the list—and binds the CAR of
each shorter version of the list to the first of its arguments.
dotimes
loops a specific number of times: you specify the number.
Next: The dotimes
Macro, Up: Save your time: dolist
and dotimes
[Contents][Index]
The dolist
Macro
Suppose, for example, you want to reverse a list, so that “first” “second” “third” becomes “third” “second” “first”.
In practice, you would use the reverse
function, like this:
(setq animals '(gazelle giraffe lion tiger)) (reverse animals)
Here is how you could reverse the list using a while
loop:
(setq animals '(gazelle giraffe lion tiger)) (defun reverse-list-with-while (list) "Using while, reverse the order of LIST." (let (value) ; make sure list starts empty (while list (setq value (cons (car list) value)) (setq list (cdr list))) value)) (reverse-list-with-while animals)
And here is how you could use the dolist
macro:
(setq animals '(gazelle giraffe lion tiger)) (defun reverse-list-with-dolist (list) "Using dolist, reverse the order of LIST." (let (value) ; make sure list starts empty (dolist (element list value) (setq value (cons element value))))) (reverse-list-with-dolist animals)
In Info, you can place your cursor after the closing parenthesis of each expression and type C-x C-e; in each case, you should see
(tiger lion giraffe gazelle)
in the echo area.
For this example, the existing reverse
function is obviously best.
The while
loop is just like our first example (see A while
Loop and a List). The while
first
checks whether the list has elements; if so, it constructs a new list
by adding the first element of the list to the existing list (which in
the first iteration of the loop is nil
). Since the second
element is prepended in front of the first element, and the third
element is prepended in front of the second element, the list is reversed.
In the expression using a while
loop,
the (setq list (cdr list))
expression shortens the list, so the while
loop eventually
stops. In addition, it provides the cons
expression with a new
first element by creating a new and shorter list at each repetition of
the loop.
The dolist
expression does very much the same as the
while
expression, except that the dolist
macro does some
of the work you have to do when writing a while
expression.
Like a while
loop, a dolist
loops. What is different is
that it automatically shortens the list each time it loops—it
CDRs down the list on its own—and it automatically binds
the CAR of each shorter version of the list to the first of its
arguments.
In the example, the CAR of each shorter version of the list is
referred to using the symbol ‘element’, the list itself is called
‘list’, and the value returned is called ‘value’. The
remainder of the dolist
expression is the body.
The dolist
expression binds the CAR of each shorter
version of the list to element
and then evaluates the body of
the expression; and repeats the loop. The result is returned in
value
.
Previous: The dolist
Macro, Up: Save your time: dolist
and dotimes
[Contents][Index]
The dotimes
Macro
The dotimes
macro is similar to dolist
, except that it
loops a specific number of times.
The first argument to dotimes
is assigned the numbers 0, 1, 2
and so forth each time around the loop. You need to provide the value
of the second argument, which is how many times the macro loops.
For example, the following binds the numbers from 0 up to, but not including, the number 3 to the first argument, number, and then constructs a list of the three numbers. (The first number is 0, the second number is 1, and the third number is 2; this makes a total of three numbers in all, starting with zero as the first number.)
(let (value) ; otherwise a value is a void variable (dotimes (number 3) (setq value (cons number value))) value) ⇒ (2 1 0)
The way to use dotimes
is to operate on some expression
number number of times and then return the result, either as
a list or an atom.
Here is an example of a defun
that uses dotimes
to add
up the number of pebbles in a triangle.
(defun triangle-using-dotimes (number-of-rows) "Using `dotimes', add up the number of pebbles in a triangle." (let ((total 0)) ; otherwise a total is a void variable (dotimes (number number-of-rows) (setq total (+ total (1+ number)))) total)) (triangle-using-dotimes 4)
Next: Looping Exercise, Previous: Save your time: dolist
and dotimes
, Up: Loops and Recursion [Contents][Index]
11.3 Recursion
A recursive function contains code that tells the Lisp interpreter to call a program that runs exactly like itself, but with slightly different arguments. The code runs exactly the same because it has the same name. However, even though the program has the same name, it is not the same entity. It is different. In the jargon, it is a different “instance”.
Eventually, if the program is written correctly, the slightly different arguments will become sufficiently different from the first arguments that the final instance will stop.
- Building Robots: Extending the Metaphor
- The Parts of a Recursive Definition
- Recursion with a List
- Recursion in Place of a Counter
- Recursion Example Using
cond
- Recursive Patterns
- Recursion without Deferments
- No Deferment Solution
Next: The Parts of a Recursive Definition, Up: Recursion [Contents][Index]
11.3.1 Building Robots: Extending the Metaphor
It is sometimes helpful to think of a running program as a robot that does a job. In doing its job, a recursive function calls on a second robot to help it. The second robot is identical to the first in every way, except that the second robot helps the first and has been passed different arguments than the first.
In a recursive function, the second robot may call a third; and the third may call a fourth, and so on. Each of these is a different entity; but all are clones.
Since each robot has slightly different instructions—the arguments will differ from one robot to the next—the last robot should know when to stop.
Let’s expand on the metaphor in which a computer program is a robot.
A function definition provides the blueprints for a robot. When you
install a function definition, that is, when you evaluate a
defun
macro, you install the necessary equipment to build
robots. It is as if you were in a factory, setting up an assembly
line. Robots with the same name are built according to the same
blueprints. So they have the same model number, but a
different serial number.
We often say that a recursive function “calls itself”. What we mean is that the instructions in a recursive function cause the Lisp interpreter to run a different function that has the same name and does the same job as the first, but with different arguments.
It is important that the arguments differ from one instance to the next; otherwise, the process will never stop.
Next: Recursion with a List, Previous: Building Robots: Extending the Metaphor, Up: Recursion [Contents][Index]
11.3.2 The Parts of a Recursive Definition
A recursive function typically contains a conditional expression which has three parts:
- A true-or-false-test that determines whether the function is called again, here called the do-again-test.
- The name of the function. When this name is called, a new instance of the function—a new robot, as it were—is created and told what to do.
- An expression that returns a different value each time the function is called, here called the next-step-expression. Consequently, the argument (or arguments) passed to the new instance of the function will be different from that passed to the previous instance. This causes the conditional expression, the do-again-test, to test false after the correct number of repetitions.
Recursive functions can be much simpler than any other kind of function. Indeed, when people first start to use them, they often look so mysteriously simple as to be incomprehensible. Like riding a bicycle, reading a recursive function definition takes a certain knack which is hard at first but then seems simple.
There are several different common recursive patterns. A very simple pattern looks like this:
(defun name-of-recursive-function (argument-list) "documentation…" (if do-again-test body… (name-of-recursive-function next-step-expression)))
Each time a recursive function is evaluated, a new instance of it is created and told what to do. The arguments tell the instance what to do.
An argument is bound to the value of the next-step-expression. Each instance runs with a different value of the next-step-expression.
The value in the next-step-expression is used in the do-again-test.
The value returned by the next-step-expression is passed to the new instance of the function, which evaluates it (or some transmogrification of it) to determine whether to continue or stop. The next-step-expression is designed so that the do-again-test returns false when the function should no longer be repeated.
The do-again-test is sometimes called the stop condition, since it stops the repetitions when it tests false.
Next: Recursion in Place of a Counter, Previous: The Parts of a Recursive Definition, Up: Recursion [Contents][Index]
11.3.3 Recursion with a List
The example of a while
loop that printed the elements of a list
of numbers can be written recursively. Here is the code, including
an expression to set the value of the variable animals
to a list.
If you are reading this in Info in Emacs, you can evaluate this
expression directly in Info. Otherwise, you must copy the example
to the *scratch* buffer and evaluate each expression there.
Use C-u C-x C-e to evaluate the
(print-elements-recursively animals)
expression so that the
results are printed in the buffer; otherwise the Lisp interpreter will
try to squeeze the results into the one line of the echo area.
Also, place your cursor immediately after the last closing parenthesis
of the print-elements-recursively
function, before the comment.
Otherwise, the Lisp interpreter will try to evaluate the comment.
(setq animals '(gazelle giraffe lion tiger)) (defun print-elements-recursively (list) "Print each element of LIST on a line of its own. Uses recursion." (when list ; do-again-test (print (car list)) ; body (print-elements-recursively ; recursive call (cdr list)))) ; next-step-expression (print-elements-recursively animals)
The print-elements-recursively
function first tests whether
there is any content in the list; if there is, the function prints the
first element of the list, the CAR of the list. Then the
function invokes itself, but gives itself as its argument, not the
whole list, but the second and subsequent elements of the list, the
CDR of the list.
Put another way, if the list is not empty, the function invokes another instance of code that is similar to the initial code, but is a different thread of execution, with different arguments than the first instance.
Put in yet another way, if the list is not empty, the first robot assembles a second robot and tells it what to do; the second robot is a different individual from the first, but is the same model.
When the second evaluation occurs, the when
expression is
evaluated and if true, prints the first element of the list it
receives as its argument (which is the second element of the original
list). Then the function calls itself with the CDR of the list
it is invoked with, which (the second time around) is the CDR of
the CDR of the original list.
Note that although we say that the function “calls itself”, what we mean is that the Lisp interpreter assembles and instructs a new instance of the program. The new instance is a clone of the first, but is a separate individual.
Each time the function invokes itself, it does so on a shorter version of the original list. It creates a new instance that works on a shorter list.
Eventually, the function invokes itself on an empty list. It creates
a new instance whose argument is nil
. The conditional expression
tests the value of list
. Since the value of list
is
nil
, the when
expression tests false so the then-part is
not evaluated. The function as a whole then returns nil
.
When you evaluate the expression (print-elements-recursively
animals)
in the *scratch* buffer, you see this result:
gazelle giraffe lion tiger nil
Next: Recursion Example Using cond
, Previous: Recursion with a List, Up: Recursion [Contents][Index]
11.3.4 Recursion in Place of a Counter
The triangle
function described in a previous section can also
be written recursively. It looks like this:
(defun triangle-recursively (number) "Return the sum of the numbers 1 through NUMBER inclusive. Uses recursion." (if (= number 1) ; do-again-test 1 ; then-part (+ number ; else-part (triangle-recursively ; recursive call (1- number))))) ; next-step-expression (triangle-recursively 7)
You can install this function by evaluating it and then try it by
evaluating (triangle-recursively 7)
. (Remember to put your
cursor immediately after the last parenthesis of the function
definition, before the comment.) The function evaluates to 28.
To understand how this function works, let’s consider what happens in the various cases when the function is passed 1, 2, 3, or 4 as the value of its argument.
Next: An argument of 3 or 4, Up: Recursion in Place of a Counter [Contents][Index]
An argument of 1 or 2
First, what happens if the value of the argument is 1?
The function has an if
expression after the documentation
string. It tests whether the value of number
is equal to 1; if
so, Emacs evaluates the then-part of the if
expression, which
returns the number 1 as the value of the function. (A triangle with
one row has one pebble in it.)
Suppose, however, that the value of the argument is 2. In this case,
Emacs evaluates the else-part of the if
expression.
The else-part consists of an addition, the recursive call to
triangle-recursively
and a decrementing action; and it looks like
this:
(+ number (triangle-recursively (1- number)))
When Emacs evaluates this expression, the innermost expression is evaluated first; then the other parts in sequence. Here are the steps in detail:
- Step 1 Evaluate the innermost expression.
-
The innermost expression is
(1- number)
so Emacs decrements the value ofnumber
from 2 to 1. - Step 2 Evaluate the
triangle-recursively
function. -
The Lisp interpreter creates an individual instance of
triangle-recursively
. It does not matter that this function is contained within itself. Emacs passes the result Step 1 as the argument used by this instance of thetriangle-recursively
functionIn this case, Emacs evaluates
triangle-recursively
with an argument of 1. This means that this evaluation oftriangle-recursively
returns 1. - Step 3 Evaluate the value of
number
. -
The variable
number
is the second element of the list that starts with+
; its value is 2. - Step 4 Evaluate the
+
expression. -
The
+
expression receives two arguments, the first from the evaluation ofnumber
(Step 3) and the second from the evaluation oftriangle-recursively
(Step 2).The result of the addition is the sum of 2 plus 1, and the number 3 is returned, which is correct. A triangle with two rows has three pebbles in it.
Previous: An argument of 1 or 2, Up: Recursion in Place of a Counter [Contents][Index]
An argument of 3 or 4
Suppose that triangle-recursively
is called with an argument of
3.
- Step 1 Evaluate the do-again-test.
-
The
if
expression is evaluated first. This is the do-again test and returns false, so the else-part of theif
expression is evaluated. (Note that in this example, the do-again-test causes the function to call itself when it tests false, not when it tests true.) - Step 2 Evaluate the innermost expression of the else-part.
-
The innermost expression of the else-part is evaluated, which decrements 3 to 2. This is the next-step-expression.
- Step 3 Evaluate the
triangle-recursively
function. -
The number 2 is passed to the
triangle-recursively
function.We already know what happens when Emacs evaluates
triangle-recursively
with an argument of 2. After going through the sequence of actions described earlier, it returns a value of 3. So that is what will happen here. - Step 4 Evaluate the addition.
-
3 will be passed as an argument to the addition and will be added to the number with which the function was called, which is 3.
The value returned by the function as a whole will be 6.
Now that we know what will happen when triangle-recursively
is
called with an argument of 3, it is evident what will happen if it is
called with an argument of 4:
In the recursive call, the evaluation of
(triangle-recursively (1- 4))will return the value of evaluating
(triangle-recursively 3)which is 6 and this value will be added to 4 by the addition in the third line.
The value returned by the function as a whole will be 10.
Each time triangle-recursively
is evaluated, it evaluates a
version of itself—a different instance of itself—with a smaller
argument, until the argument is small enough so that it does not
evaluate itself.
Note that this particular design for a recursive function requires that operations be deferred.
Before (triangle-recursively 7)
can calculate its answer, it
must call (triangle-recursively 6)
; and before
(triangle-recursively 6)
can calculate its answer, it must call
(triangle-recursively 5)
; and so on. That is to say, the
calculation that (triangle-recursively 7)
makes must be
deferred until (triangle-recursively 6)
makes its calculation;
and (triangle-recursively 6)
must defer until
(triangle-recursively 5)
completes; and so on.
If each of these instances of triangle-recursively
are thought
of as different robots, the first robot must wait for the second to
complete its job, which must wait until the third completes, and so
on.
There is a way around this kind of waiting, which we will discuss in Recursion without Deferments.
Next: Recursive Patterns, Previous: Recursion in Place of a Counter, Up: Recursion [Contents][Index]
11.3.5 Recursion Example Using cond
The version of triangle-recursively
described earlier is written
with the if
special form. It can also be written using another
special form called cond
. The name of the special form
cond
is an abbreviation of the word ‘conditional’.
Although the cond
special form is not used as often in the
Emacs Lisp sources as if
, it is used often enough to justify
explaining it.
The template for a cond
expression looks like this:
(cond body…)
where the body is a series of lists.
Written out more fully, the template looks like this:
(cond (first-true-or-false-test first-consequent) (second-true-or-false-test second-consequent) (third-true-or-false-test third-consequent) …)
When the Lisp interpreter evaluates the cond
expression, it
evaluates the first element (the CAR or true-or-false-test) of
the first expression in a series of expressions within the body of the
cond
.
If the true-or-false-test returns nil
the rest of that
expression, the consequent, is skipped and the true-or-false-test of the
next expression is evaluated. When an expression is found whose
true-or-false-test returns a value that is not nil
, the
consequent of that expression is evaluated. The consequent can be one
or more expressions. If the consequent consists of more than one
expression, the expressions are evaluated in sequence and the value of
the last one is returned. If the expression does not have a consequent,
the value of the true-or-false-test is returned.
If none of the true-or-false-tests test true, the cond
expression
returns nil
.
Written using cond
, the triangle
function looks like this:
(defun triangle-using-cond (number) (cond ((<= number 0) 0) ((= number 1) 1) ((> number 1) (+ number (triangle-using-cond (1- number))))))
In this example, the cond
returns 0 if the number is less than or
equal to 0, it returns 1 if the number is 1 and it evaluates (+
number (triangle-using-cond (1- number)))
if the number is greater than
1.
Next: Recursion without Deferments, Previous: Recursion Example Using cond
, Up: Recursion [Contents][Index]
11.3.6 Recursive Patterns
Here are three common recursive patterns. Each involves a list. Recursion does not need to involve lists, but Lisp is designed for lists and this provides a sense of its primal capabilities.
Next: Recursive Pattern: accumulate, Up: Recursive Patterns [Contents][Index]
Recursive Pattern: every
In the every
recursive pattern, an action is performed on every
element of a list.
The basic pattern is:
- If a list be empty, return
nil
. - Else, act on the beginning of the list (the CAR of the list)
- - through a recursive call by the function on the rest (the CDR) of the list,
- - and, optionally, combine the acted-on element, using
cons
, with the results of acting on the rest.
Here is an example:
(defun square-each (numbers-list) "Square each of a NUMBERS LIST, recursively." (if (not numbers-list) ; do-again-test nil (cons (* (car numbers-list) (car numbers-list)) (square-each (cdr numbers-list))))) ; next-step-expression
(square-each '(1 2 3)) ⇒ (1 4 9)
If numbers-list
is empty, do nothing. But if it has content,
construct a list combining the square of the first number in the list
with the result of the recursive call.
(The example follows the pattern exactly: nil
is returned if
the numbers’ list is empty. In practice, you would write the
conditional so it carries out the action when the numbers’ list is not
empty.)
The print-elements-recursively
function (see Recursion with a List) is another example of an every
pattern, except in this case, rather than bring the results together
using cons
, we print each element of output.
The print-elements-recursively
function looks like this:
(setq animals '(gazelle giraffe lion tiger))
(defun print-elements-recursively (list) "Print each element of LIST on a line of its own. Uses recursion." (when list ; do-again-test (print (car list)) ; body (print-elements-recursively ; recursive call (cdr list)))) ; next-step-expression (print-elements-recursively animals)
The pattern for print-elements-recursively
is:
- When the list is empty, do nothing.
- But when the list has at least one element,
- - act on the beginning of the list (the CAR of the list),
- - and make a recursive call on the rest (the CDR) of the list.
Next: Recursive Pattern: keep, Previous: Recursive Pattern: every, Up: Recursive Patterns [Contents][Index]
Recursive Pattern: accumulate
Another recursive pattern is called the accumulate
pattern. In
the accumulate
recursive pattern, an action is performed on
every element of a list and the result of that action is accumulated
with the results of performing the action on the other elements.
This is very like the every
pattern using cons
, except that
cons
is not used, but some other combiner.
The pattern is:
- If a list be empty, return zero or some other constant.
- Else, act on the beginning of the list (the CAR of the list),
- - and combine that acted-on element, using
+
or some other combining function, with - - a recursive call by the function on the rest (the CDR) of the list.
- - and combine that acted-on element, using
Here is an example:
(defun add-elements (numbers-list) "Add the elements of NUMBERS-LIST together." (if (not numbers-list) 0 (+ (car numbers-list) (add-elements (cdr numbers-list)))))
(add-elements '(1 2 3 4)) ⇒ 10
See Making a List of Files, for an example of the accumulate pattern.
Previous: Recursive Pattern: accumulate, Up: Recursive Patterns [Contents][Index]
Recursive Pattern: keep
A third recursive pattern is called the keep
pattern.
In the keep
recursive pattern, each element of a list is tested;
the element is acted on and the results are kept only if the element
meets a criterion.
Again, this is very like the every
pattern, except the element is
skipped unless it meets a criterion.
The pattern has three parts:
- If a list be empty, return
nil
. - Else, if the beginning of the list (the CAR of the list) passes
a test
- - act on that element and combine it, using
cons
with - - a recursive call by the function on the rest (the CDR) of the list.
- - act on that element and combine it, using
- Otherwise, if the beginning of the list (the CAR of the list) fails
the test
- - skip on that element,
- - and, recursively call the function on the rest (the CDR) of the list.
Here is an example that uses cond
:
(defun keep-three-letter-words (word-list) "Keep three letter words in WORD-LIST." (cond ;; First do-again-test: stop-condition ((not word-list) nil) ;; Second do-again-test: when to act ((eq 3 (length (symbol-name (car word-list)))) ;; combine acted-on element with recursive call on shorter list (cons (car word-list) (keep-three-letter-words (cdr word-list)))) ;; Third do-again-test: when to skip element; ;; recursively call shorter list with next-step expression (t (keep-three-letter-words (cdr word-list)))))
(keep-three-letter-words '(one two three four five six)) ⇒ (one two six)
It goes without saying that you need not use nil
as the test for
when to stop; and you can, of course, combine these patterns.
Next: No Deferment Solution, Previous: Recursive Patterns, Up: Recursion [Contents][Index]
11.3.7 Recursion without Deferments
Let’s consider again what happens with the triangle-recursively
function. We will find that the intermediate calculations are
deferred until all can be done.
Here is the function definition:
(defun triangle-recursively (number) "Return the sum of the numbers 1 through NUMBER inclusive. Uses recursion." (if (= number 1) ; do-again-test 1 ; then-part (+ number ; else-part (triangle-recursively ; recursive call (1- number))))) ; next-step-expression
What happens when we call this function with an argument of 7?
The first instance of the triangle-recursively
function adds
the number 7 to the value returned by a second instance of
triangle-recursively
, an instance that has been passed an
argument of 6. That is to say, the first calculation is:
(+ 7 (triangle-recursively 6))
The first instance of triangle-recursively
—you may want to
think of it as a little robot—cannot complete its job. It must hand
off the calculation for (triangle-recursively 6)
to a second
instance of the program, to a second robot. This second individual is
completely different from the first one; it is, in the jargon, a
“different instantiation”. Or, put another way, it is a different
robot. It is the same model as the first; it calculates triangle
numbers recursively; but it has a different serial number.
And what does (triangle-recursively 6)
return? It returns the
number 6 added to the value returned by evaluating
triangle-recursively
with an argument of 5. Using the robot
metaphor, it asks yet another robot to help it.
Now the total is:
(+ 7 6 (triangle-recursively 5))
And what happens next?
(+ 7 6 5 (triangle-recursively 4))
Each time triangle-recursively
is called, except for the last
time, it creates another instance of the program—another robot—and
asks it to make a calculation.
Eventually, the full addition is set up and performed:
(+ 7 6 5 4 3 2 1)
This design for the function defers the calculation of the first step until the second can be done, and defers that until the third can be done, and so on. Each deferment means the computer must remember what is being waited on. This is not a problem when there are only a few steps, as in this example. But it can be a problem when there are more steps.
Previous: Recursion without Deferments, Up: Recursion [Contents][Index]
11.3.8 No Deferment Solution
The solution to the problem of deferred operations is to write in a manner that does not defer operations15. This requires writing to a different pattern, often one that involves writing two function definitions, an initialization function and a helper function.
The initialization function sets up the job; the helper function does the work.
Here are the two function definitions for adding up numbers. They are so simple, I find them hard to understand.
(defun triangle-initialization (number) "Return the sum of the numbers 1 through NUMBER inclusive. This is the initialization component of a two function duo that uses recursion." (triangle-recursive-helper 0 0 number))
(defun triangle-recursive-helper (sum counter number) "Return SUM, using COUNTER, through NUMBER inclusive. This is the helper component of a two function duo that uses recursion." (if (> counter number) sum (triangle-recursive-helper (+ sum counter) ; sum (1+ counter) ; counter number))) ; number
Install both function definitions by evaluating them, then call
triangle-initialization
with 2 rows:
(triangle-initialization 2) ⇒ 3
The initialization function calls the first instance of the helper function with three arguments: zero, zero, and a number which is the number of rows in the triangle.
The first two arguments passed to the helper function are
initialization values. These values are changed when
triangle-recursive-helper
invokes new instances.16
Let’s see what happens when we have a triangle that has one row. (This triangle will have one pebble in it!)
triangle-initialization
will call its helper with
the arguments 0 0 1
. That function will run the conditional
test whether (> counter number)
:
(> 0 1)
and find that the result is false, so it will invoke
the else-part of the if
clause:
(triangle-recursive-helper (+ sum counter) ; sum plus counter ⇒ sum (1+ counter) ; increment counter ⇒ counter number) ; number stays the same
which will first compute:
(triangle-recursive-helper (+ 0 0) ; sum (1+ 0) ; counter 1) ; number
which is:
(triangle-recursive-helper 0 1 1)
Again, (> counter number)
will be false, so again, the Lisp
interpreter will evaluate triangle-recursive-helper
, creating a
new instance with new arguments.
This new instance will be;
(triangle-recursive-helper (+ sum counter) ; sum plus counter ⇒ sum (1+ counter) ; increment counter ⇒ counter number) ; number stays the same
which is:
(triangle-recursive-helper 1 2 1)
In this case, the (> counter number)
test will be true! So the
instance will return the value of the sum, which will be 1, as
expected.
Now, let’s pass triangle-initialization
an argument
of 2, to find out how many pebbles there are in a triangle with two rows.
That function calls (triangle-recursive-helper 0 0 2)
.
In stages, the instances called will be:
sum counter number
(triangle-recursive-helper 0 1 2)
(triangle-recursive-helper 1 2 2)
(triangle-recursive-helper 3 3 2)
When the last instance is called, the (> counter number)
test
will be true, so the instance will return the value of sum
,
which will be 3.
This kind of pattern helps when you are writing functions that can use many resources in a computer.
Previous: Recursion, Up: Loops and Recursion [Contents][Index]
11.4 Looping Exercise
- Write a function similar to
triangle
in which each row has a value which is the square of the row number. Use awhile
loop. - Write a function similar to
triangle
that multiplies instead of adds the values. - Rewrite these two functions recursively. Rewrite these functions
using
cond
. - Write a function for Texinfo mode that creates an index entry at the
beginning of a paragraph for every ‘@dfn’ within the paragraph.
(In a Texinfo file, ‘@dfn’ marks a definition. This book is
written in Texinfo.)
Many of the functions you will need are described in two of the previous chapters, Cutting and Storing Text, and Yanking Text Back. If you use
forward-paragraph
to put the index entry at the beginning of the paragraph, you will have to use C-h f (describe-function
) to find out how to make the command go backwards.For more information, see Indicating in Texinfo Manual, which goes to a Texinfo manual in the current directory. Or, if you are on the Internet, see https://www.gnu.org/software/texinfo/manual/texinfo/
Next: Counting via Repetition and Regexps, Previous: Loops and Recursion, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
12 Regular Expression Searches
Regular expression searches are used extensively in GNU Emacs. The
two functions, forward-sentence
and forward-paragraph
,
illustrate these searches well. They use regular expressions to find
where to move point. The phrase “regular expression” is often written
as “regexp”.
Regular expression searches are described in Regular Expression Search in The GNU Emacs Manual, as well as in
Regular Expressions in The GNU Emacs Lisp Reference
Manual. In writing this chapter, I am presuming that you have at
least a mild acquaintance with them. The major point to remember is
that regular expressions permit you to search for patterns as well as
for literal strings of characters. For example, the code in
forward-sentence
searches for the pattern of possible
characters that could mark the end of a sentence, and moves point to
that spot.
Before looking at the code for the forward-sentence
function, it
is worth considering what the pattern that marks the end of a sentence
must be. The pattern is discussed in the next section; following that
is a description of the regular expression search function,
re-search-forward
. The forward-sentence
function
is described in the section following. Finally, the
forward-paragraph
function is described in the last section of
this chapter. forward-paragraph
is a complex function that
introduces several new features.
- The Regular Expression for
sentence-end
- The
re-search-forward
Function forward-sentence
forward-paragraph
: a Goldmine of Functions- Review
- Exercises with
re-search-forward
12.1 The Regular Expression for sentence-end
The symbol sentence-end
is bound to the pattern that marks the
end of a sentence. What should this regular expression be?
Clearly, a sentence may be ended by a period, a question mark, or an exclamation mark. Indeed, in English, only clauses that end with one of those three characters should be considered the end of a sentence. This means that the pattern should include the character set:
[.?!]
However, we do not want forward-sentence
merely to jump to a
period, a question mark, or an exclamation mark, because such a character
might be used in the middle of a sentence. A period, for example, is
used after abbreviations. So other information is needed.
According to convention, you type two spaces after every sentence, but only one space after a period, a question mark, or an exclamation mark in the body of a sentence. So a period, a question mark, or an exclamation mark followed by two spaces is a good indicator of an end of sentence. However, in a file, the two spaces may instead be a tab or the end of a line. This means that the regular expression should include these three items as alternatives.
This group of alternatives will look like this:
\\($\\| \\| \\) ^ ^^ TAB SPC
Here, ‘$’ indicates the end of the line, and I have pointed out where the tab and two spaces are inserted in the expression. Both are inserted by putting the actual characters into the expression.
Two backslashes, ‘\\’, are required before the parentheses and vertical bars: the first backslash quotes the following backslash in Emacs; and the second indicates that the following character, the parenthesis or the vertical bar, is special.
Also, a sentence may be followed by one or more carriage returns, like this:
[ ]*
Like tabs and spaces, a carriage return is inserted into a regular expression by inserting it literally. The asterisk indicates that the RET is repeated zero or more times.
But a sentence end does not consist only of a period, a question mark or an exclamation mark followed by appropriate space: a closing quotation mark or a closing brace of some kind may precede the space. Indeed more than one such mark or brace may precede the space. These require a expression that looks like this:
[]\"')}]*
In this expression, the first ‘]’ is the first character in the expression; the second character is ‘"’, which is preceded by a ‘\’ to tell Emacs the ‘"’ is not special. The last three characters are ‘'’, ‘)’, and ‘}’.
All this suggests what the regular expression pattern for matching the
end of a sentence should be; and, indeed, if we evaluate
sentence-end
we find that it returns the following value:
sentence-end ⇒ "[.?!][]\"')}]*\\($\\| \\| \\)[ ]*"
(Well, not in GNU Emacs 22; that is because of an effort to make the
process simpler and to handle more glyphs and languages. When the
value of sentence-end
is nil
, then use the value defined
by the function sentence-end
. (Here is a use of the difference
between a value and a function in Emacs Lisp.) The function returns a
value constructed from the variables sentence-end-base
,
sentence-end-double-space
, sentence-end-without-period
,
and sentence-end-without-space
. The critical variable is
sentence-end-base
; its global value is similar to the one
described above but it also contains two additional quotation marks.
These have differing degrees of curliness. The
sentence-end-without-period
variable, when true, tells Emacs
that a sentence may end without a period, such as text in Thai.)
Next: forward-sentence
, Previous: The Regular Expression for sentence-end
, Up: Regular Expression Searches [Contents][Index]
12.2 The re-search-forward
Function
The re-search-forward
function is very like the
search-forward
function. (See The
search-forward
Function.)
re-search-forward
searches for a regular expression. If the
search is successful, it leaves point immediately after the last
character in the target. If the search is backwards, it leaves point
just before the first character in the target. You may tell
re-search-forward
to return t
for true. (Moving point
is therefore a side effect.)
Like search-forward
, the re-search-forward
function takes
four arguments:
- The first argument is the regular expression that the function searches for. The regular expression will be a string between quotation marks.
- The optional second argument limits how far the function will search; it is a bound, which is specified as a position in the buffer.
- The optional third argument specifies how the function responds to
failure:
nil
as the third argument causes the function to signal an error (and print a message) when the search fails; any other value causes it to returnnil
if the search fails andt
if the search succeeds. - The optional fourth argument is the repeat count. A negative repeat
count causes
re-search-forward
to search backwards.
The template for re-search-forward
looks like this:
(re-search-forward "regular-expression" limit-of-search what-to-do-if-search-fails repeat-count)
The second, third, and fourth arguments are optional. However, if you want to pass a value to either or both of the last two arguments, you must also pass a value to all the preceding arguments. Otherwise, the Lisp interpreter will mistake which argument you are passing the value to.
In the forward-sentence
function, the regular expression will be
the value of the variable sentence-end
. In simple form, that is:
"[.?!][]\"')}]*\\($\\| \\| \\)[ ]*"
The limit of the search will be the end of the paragraph (since a
sentence cannot go beyond a paragraph). If the search fails, the
function will return nil
; and the repeat count will be provided
by the argument to the forward-sentence
function.
Next: forward-paragraph
: a Goldmine of Functions, Previous: The re-search-forward
Function, Up: Regular Expression Searches [Contents][Index]
12.3 forward-sentence
The command to move the cursor forward a sentence is a straightforward illustration of how to use regular expression searches in Emacs Lisp. Indeed, the function looks longer and more complicated than it is; this is because the function is designed to go backwards as well as forwards; and, optionally, over more than one sentence. The function is usually bound to the key command M-e.
Next: The while
loops, Up: forward-sentence
[Contents][Index]
Complete forward-sentence
function definition
Here is the code for forward-sentence
:
(defun forward-sentence (&optional arg) "Move forward to next end of sentence. With argument, repeat. With negative argument, move backward repeatedly to start of sentence. The variable `sentence-end' is a regular expression that matches ends of sentences. Also, every paragraph boundary terminates sentences as well."
(interactive "p") (or arg (setq arg 1)) (let ((opoint (point)) (sentence-end (sentence-end))) (while (< arg 0) (let ((pos (point)) (par-beg (save-excursion (start-of-paragraph-text) (point)))) (if (and (re-search-backward sentence-end par-beg t) (or (< (match-end 0) pos) (re-search-backward sentence-end par-beg t))) (goto-char (match-end 0)) (goto-char par-beg))) (setq arg (1+ arg)))
(while (> arg 0) (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) (if (re-search-forward sentence-end par-end t) (skip-chars-backward " \t\n") (goto-char par-end))) (setq arg (1- arg))) (constrain-to-field nil opoint t)))
The function looks long at first sight and it is best to look at its skeleton first, and then its muscle. The way to see the skeleton is to look at the expressions that start in the left-most columns:
(defun forward-sentence (&optional arg) "documentation…" (interactive "p") (or arg (setq arg 1)) (let ((opoint (point)) (sentence-end (sentence-end))) (while (< arg 0) (let ((pos (point)) (par-beg (save-excursion (start-of-paragraph-text) (point)))) rest-of-body-of-while-loop-when-going-backwards (while (> arg 0) (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) rest-of-body-of-while-loop-when-going-forwards handle-forms-and-equivalent
This looks much simpler! The function definition consists of
documentation, an interactive
expression, an or
expression, a let
expression, and while
loops.
Let’s look at each of these parts in turn.
We note that the documentation is thorough and understandable.
The function has an interactive "p"
declaration. This means
that the processed prefix argument, if any, is passed to the
function as its argument. (This will be a number.) If the function
is not passed an argument (it is optional) then the argument
arg
will be bound to 1.
When forward-sentence
is called non-interactively without an
argument, arg
is bound to nil
. The or
expression
handles this. What it does is either leave the value of arg
as
it is, but only if arg
is bound to a value; or it sets the
value of arg
to 1, in the case when arg
is bound to
nil
.
Next is a let
. That specifies the values of two local
variables, opoint
and sentence-end
. The local value of
point, from before the search, is used in the
constrain-to-field
function which handles forms and
equivalents. The sentence-end
variable is set by the
sentence-end
function.
Next: The regular expression search, Previous: Complete forward-sentence
function definition, Up: forward-sentence
[Contents][Index]
The while
loops
Two while
loops follow. The first while
has a
true-or-false-test that tests true if the prefix argument for
forward-sentence
is a negative number. This is for going
backwards. The body of this loop is similar to the body of the second
while
clause, but it is not exactly the same. We will skip
this while
loop and concentrate on the second while
loop.
The second while
loop is for moving point forward. Its skeleton
looks like this:
(while (> arg 0) ; true-or-false-test
(let varlist
(if (true-or-false-test)
then-part
else-part
(setq arg (1- arg)))) ; while
loop decrementer
The while
loop is of the decrementing kind.
(See A Loop with a Decrementing Counter.) It
has a true-or-false-test that tests true so long as the counter (in
this case, the variable arg
) is greater than zero; and it has a
decrementer that subtracts 1 from the value of the counter every time
the loop repeats.
If no prefix argument is given to forward-sentence
, which is
the most common way the command is used, this while
loop will
run once, since the value of arg
will be 1.
The body of the while
loop consists of a let
expression,
which creates and binds a local variable, and has, as its body, an
if
expression.
The body of the while
loop looks like this:
(let ((par-end (save-excursion (end-of-paragraph-text) (point)))) (if (re-search-forward sentence-end par-end t) (skip-chars-backward " \t\n") (goto-char par-end)))
The let
expression creates and binds the local variable
par-end
. As we shall see, this local variable is designed to
provide a bound or limit to the regular expression search. If the
search fails to find a proper sentence ending in the paragraph, it will
stop on reaching the end of the paragraph.
But first, let us examine how par-end
is bound to the value of
the end of the paragraph. What happens is that the let
sets the
value of par-end
to the value returned when the Lisp interpreter
evaluates the expression
(save-excursion (end-of-paragraph-text) (point))
In this expression, (end-of-paragraph-text)
moves point to the
end of the paragraph, (point)
returns the value of point, and then
save-excursion
restores point to its original position. Thus,
the let
binds par-end
to the value returned by the
save-excursion
expression, which is the position of the end of
the paragraph. (The end-of-paragraph-text
function uses
forward-paragraph
, which we will discuss shortly.)
Emacs next evaluates the body of the let
, which is an if
expression that looks like this:
(if (re-search-forward sentence-end par-end t) ; if-part (skip-chars-backward " \t\n") ; then-part (goto-char par-end))) ; else-part
The if
tests whether its first argument is true and if so,
evaluates its then-part; otherwise, the Emacs Lisp interpreter
evaluates the else-part. The true-or-false-test of the if
expression is the regular expression search.
It may seem odd to have what looks like the real work of
the forward-sentence
function buried here, but this is a common
way this kind of operation is carried out in Lisp.
Previous: The while
loops, Up: forward-sentence
[Contents][Index]
The regular expression search
The re-search-forward
function searches for the end of the
sentence, that is, for the pattern defined by the sentence-end
regular expression. If the pattern is found—if the end of the sentence is
found—then the re-search-forward
function does two things:
- The
re-search-forward
function carries out a side effect, which is to move point to the end of the occurrence found. - The
re-search-forward
function returns a value of true. This is the value received by theif
, and means that the search was successful.
The side effect, the movement of point, is completed before the
if
function is handed the value returned by the successful
conclusion of the search.
When the if
function receives the value of true from a successful
call to re-search-forward
, the if
evaluates the then-part,
which is the expression (skip-chars-backward " \t\n")
. This
expression moves backwards over any blank spaces, tabs or carriage
returns until a printed character is found and then leaves point after
the character. Since point has already been moved to the end of the
pattern that marks the end of the sentence, this action leaves point
right after the closing printed character of the sentence, which is
usually a period.
On the other hand, if the re-search-forward
function fails to
find a pattern marking the end of the sentence, the function returns
false. The false then causes the if
to evaluate its third
argument, which is (goto-char par-end)
: it moves point to the
end of the paragraph.
(And if the text is in a form or equivalent, and point may not move
fully, then the constrain-to-field
function comes into play.)
Regular expression searches are exceptionally useful and the pattern
illustrated by re-search-forward
, in which the search is the
test of an if
expression, is handy. You will see or write code
incorporating this pattern often.
Next: Review, Previous: forward-sentence
, Up: Regular Expression Searches [Contents][Index]
12.4 forward-paragraph
: a Goldmine of Functions
The forward-paragraph
function moves point forward to the end
of the paragraph. It is usually bound to M-} and makes use of a
number of functions that are important in themselves, including
let*
, match-beginning
, and looking-at
.
The function definition for forward-paragraph
is considerably
longer than the function definition for forward-sentence
because it works with a paragraph, each line of which may begin with a
fill prefix.
A fill prefix consists of a string of characters that are repeated at the beginning of each line. For example, in Lisp code, it is a convention to start each line of a paragraph-long comment with ‘;;; ’. In Text mode, four blank spaces make up another common fill prefix, creating an indented paragraph. (See Fill Prefix in The GNU Emacs Manual, for more information about fill prefixes.)
The existence of a fill prefix means that in addition to being able to
find the end of a paragraph whose lines begin on the left-most
column, the forward-paragraph
function must be able to find the
end of a paragraph when all or many of the lines in the buffer begin
with the fill prefix.
Moreover, it is sometimes practical to ignore a fill prefix that exists, especially when blank lines separate paragraphs. This is an added complication.
Shortened forward-paragraph
function definition
Rather than print all of the forward-paragraph
function, we
will only print parts of it. Read without preparation, the function
can be daunting!
In outline, the function looks like this:
(defun forward-paragraph (&optional arg) "documentation…" (interactive "p") (or arg (setq arg 1)) (let* varlist (while (and (< arg 0) (not (bobp))) ; backward-moving-code … (while (and (> arg 0) (not (eobp))) ; forward-moving-code …
The first parts of the function are routine: the function’s argument list consists of one optional argument. Documentation follows.
The lower case ‘p’ in the interactive
declaration means
that the processed prefix argument, if any, is passed to the function.
This will be a number, and is the repeat count of how many paragraphs
point will move. The or
expression in the next line handles
the common case when no argument is passed to the function, which occurs
if the function is called from other code rather than interactively.
This case was described earlier. (See The
forward-sentence
function.) Now we reach the end of the
familiar part of this function.
Next: The forward motion while
loop, Previous: Shortened forward-paragraph
function definition, Up: forward-paragraph
: a Goldmine of Functions [Contents][Index]
The let*
expression
The next line of the forward-paragraph
function begins a
let*
expression (see let*
introduced), in which Emacs binds a total of seven variables:
opoint
, fill-prefix-regexp
, parstart
,
parsep
, sp-parstart
, start
, and
found-start
. The first part of the let*
expression
looks like below:
(let* ((opoint (point)) (fill-prefix-regexp (and fill-prefix (not (equal fill-prefix "")) (not paragraph-ignore-fill-prefix) (regexp-quote fill-prefix))) ;; Remove ^ from paragraph-start and paragraph-sep if they are there. ;; These regexps shouldn't be anchored, because we look for them ;; starting at the left-margin. This allows paragraph commands to ;; work normally with indented text. ;; This hack will not find problem cases like "whatever\\|^something". (parstart (if (and (not (equal "" paragraph-start)) (equal ?^ (aref paragraph-start 0))) (substring paragraph-start 1) paragraph-start)) (parsep (if (and (not (equal "" paragraph-separate)) (equal ?^ (aref paragraph-separate 0))) (substring paragraph-separate 1) paragraph-separate)) (parsep (if fill-prefix-regexp (concat parsep "\\|" fill-prefix-regexp "[ \t]*$") parsep)) ;; This is used for searching. (sp-parstart (concat "^[ \t]*\\(?:" parstart "\\|" parsep "\\)")) start found-start) ...)
The variable parsep
appears twice, first, to remove instances
of ‘^’, and second, to handle fill prefixes.
The variable opoint
is just the value of point
. As you
can guess, it is used in a constrain-to-field
expression, just
as in forward-sentence
.
The variable fill-prefix-regexp
is set to the value returned by
evaluating the following list:
(and fill-prefix (not (equal fill-prefix "")) (not paragraph-ignore-fill-prefix) (regexp-quote fill-prefix))
This is an expression whose first element is the and
special form.
As we learned earlier (see The kill-new
function), the and
special form evaluates each of its
arguments until one of the arguments returns a value of nil
, in
which case the and
expression returns nil
; however, if
none of the arguments returns a value of nil
, the value
resulting from evaluating the last argument is returned. (Since such
a value is not nil
, it is considered true in Lisp.) In other
words, an and
expression returns a true value only if all its
arguments are true.
In this case, the variable fill-prefix-regexp
is bound to a
non-nil
value only if the following four expressions produce a
true (i.e., a non-nil
) value when they are evaluated; otherwise,
fill-prefix-regexp
is bound to nil
.
fill-prefix
When this variable is evaluated, the value of the fill prefix, if any, is returned. If there is no fill prefix, this variable returns
nil
.(not (equal fill-prefix "")
This expression checks whether an existing fill prefix is an empty string, that is, a string with no characters in it. An empty string is not a useful fill prefix.
(not paragraph-ignore-fill-prefix)
This expression returns
nil
if the variableparagraph-ignore-fill-prefix
has been turned on by being set to a true value such ast
.(regexp-quote fill-prefix)
This is the last argument to the
and
special form. If all the arguments to theand
are true, the value resulting from evaluating this expression will be returned by theand
expression and bound to the variablefill-prefix-regexp
,
The result of evaluating this and
expression successfully is that
fill-prefix-regexp
will be bound to the value of
fill-prefix
as modified by the regexp-quote
function.
What regexp-quote
does is read a string and return a regular
expression that will exactly match the string and match nothing else.
This means that fill-prefix-regexp
will be set to a value that
will exactly match the fill prefix if the fill prefix exists.
Otherwise, the variable will be set to nil
.
The next two local variables in the let*
expression are
designed to remove instances of ‘^’ from parstart
and
parsep
, the local variables which indicate the paragraph start
and the paragraph separator. The next expression sets parsep
again. That is to handle fill prefixes.
This is the setting that requires the definition call let*
rather than let
. The true-or-false-test for the if
depends on whether the variable fill-prefix-regexp
evaluates to
nil
or some other value.
If fill-prefix-regexp
does not have a value, Emacs evaluates
the else-part of the if
expression and binds parsep
to
its local value. (parsep
is a regular expression that matches
what separates paragraphs.)
But if fill-prefix-regexp
does have a value, Emacs evaluates
the then-part of the if
expression and binds parsep
to a
regular expression that includes the fill-prefix-regexp
as part
of the pattern.
Specifically, parsep
is set to the original value of the
paragraph separate regular expression concatenated with an alternative
expression that consists of the fill-prefix-regexp
followed by
optional whitespace to the end of the line. The whitespace is defined
by "[ \t]*$"
.) The ‘\\|’ defines this portion of the
regexp as an alternative to parsep
.
According to a comment in the code, the next local variable,
sp-parstart
, is used for searching, and then the final two,
start
and found-start
, are set to nil
.
Now we get into the body of the let*
. The first part of the body
of the let*
deals with the case when the function is given a
negative argument and is therefore moving backwards. We will skip this
section.
Previous: The let*
expression, Up: forward-paragraph
: a Goldmine of Functions [Contents][Index]
The forward motion while
loop
The second part of the body of the let*
deals with forward
motion. It is a while
loop that repeats itself so long as the
value of arg
is greater than zero. In the most common use of
the function, the value of the argument is 1, so the body of the
while
loop is evaluated exactly once, and the cursor moves
forward one paragraph.
This part handles three situations: when point is between paragraphs, when there is a fill prefix and when there is no fill prefix.
The while
loop looks like this:
;; going forwards and not at the end of the buffer (while (and (> arg 0) (not (eobp))) ;; between paragraphs ;; Move forward over separator lines... (while (and (not (eobp)) (progn (move-to-left-margin) (not (eobp))) (looking-at parsep)) (forward-line 1)) ;; This decrements the loop (unless (eobp) (setq arg (1- arg))) ;; ... and one more line. (forward-line 1)
(if fill-prefix-regexp ;; There is a fill prefix; it overrides parstart; ;; we go forward line by line (while (and (not (eobp)) (progn (move-to-left-margin) (not (eobp))) (not (looking-at parsep)) (looking-at fill-prefix-regexp)) (forward-line 1))
;; There is no fill prefix; ;; we go forward character by character (while (and (re-search-forward sp-parstart nil 1) (progn (setq start (match-beginning 0)) (goto-char start) (not (eobp))) (progn (move-to-left-margin) (not (looking-at parsep))) (or (not (looking-at parstart)) (and use-hard-newlines (not (get-text-property (1- start) 'hard))))) (forward-char 1))
;; and if there is no fill prefix and if we are not at the end, ;; go to whatever was found in the regular expression search ;; for sp-parstart (if (< (point) (point-max)) (goto-char start))))
We can see that this is a decrementing counter while
loop,
using the expression (setq arg (1- arg))
as the decrementer.
That expression is not far from the while
, but is hidden in
another Lisp macro, an unless
macro. Unless we are at the end
of the buffer—that is what the eobp
function determines; it
is an abbreviation of ‘End Of Buffer P’—we decrease the value
of arg
by one.
(If we are at the end of the buffer, we cannot go forward any more and
the next loop of the while
expression will test false since the
test is an and
with (not (eobp))
. The not
function means exactly as you expect; it is another name for
null
, a function that returns true when its argument is false.)
Interestingly, the loop count is not decremented until we leave the space between paragraphs, unless we come to the end of buffer or stop seeing the local value of the paragraph separator.
That second while
also has a (move-to-left-margin)
expression. The function is self-explanatory. It is inside a
progn
expression and not the last element of its body, so it is
only invoked for its side effect, which is to move point to the left
margin of the current line.
The looking-at
function is also self-explanatory; it returns
true if the text after point matches the regular expression given as
its argument.
The rest of the body of the loop looks difficult at first, but makes sense as you come to understand it.
First consider what happens if there is a fill prefix:
(if fill-prefix-regexp ;; There is a fill prefix; it overrides parstart; ;; we go forward line by line (while (and (not (eobp)) (progn (move-to-left-margin) (not (eobp))) (not (looking-at parsep)) (looking-at fill-prefix-regexp)) (forward-line 1))
This expression moves point forward line by line so long as four conditions are true:
- Point is not at the end of the buffer.
- We can move to the left margin of the text and are not at the end of the buffer.
- The text following point does not separate paragraphs.
- The pattern following point is the fill prefix regular expression.
The last condition may be puzzling, until you remember that point was
moved to the beginning of the line early in the forward-paragraph
function. This means that if the text has a fill prefix, the
looking-at
function will see it.
Consider what happens when there is no fill prefix.
(while (and (re-search-forward sp-parstart nil 1) (progn (setq start (match-beginning 0)) (goto-char start) (not (eobp))) (progn (move-to-left-margin) (not (looking-at parsep))) (or (not (looking-at parstart)) (and use-hard-newlines (not (get-text-property (1- start) 'hard))))) (forward-char 1))
This while
loop has us searching forward for
sp-parstart
, which is the combination of possible whitespace
with the local value of the start of a paragraph or of a paragraph
separator. (The latter two are within an expression starting
\(?:
so that they are not referenced by the
match-beginning
function.)
The two expressions,
(setq start (match-beginning 0)) (goto-char start)
mean go to the start of the text matched by the regular expression search.
The (match-beginning 0)
expression is new. It returns a number
specifying the location of the start of the text that was matched by
the last search.
The match-beginning
function is used here because of a
characteristic of a forward search: a successful forward search,
regardless of whether it is a plain search or a regular expression
search, moves point to the end of the text that is found. In this
case, a successful search moves point to the end of the pattern for
sp-parstart
.
However, we want to put point at the end of the current paragraph, not
somewhere else. Indeed, since the search possibly includes the
paragraph separator, point may end up at the beginning of the next one
unless we use an expression that includes match-beginning
.
When given an argument of 0, match-beginning
returns the
position that is the start of the text matched by the most recent
search. In this case, the most recent search looks for
sp-parstart
. The (match-beginning 0)
expression returns
the beginning position of that pattern, rather than the end position
of that pattern.
(Incidentally, when passed a positive number as an argument, the
match-beginning
function returns the location of point at that
parenthesized expression in the last search unless that parenthesized
expression begins with \(?:
. I don’t know why \(?:
appears here since the argument is 0.)
The last expression when there is no fill prefix is
(if (< (point) (point-max)) (goto-char start))))
(Note that this code snippet is copied verbatim from the original code,
so the two extra ending parentheses are matching the previous if
and while
.)
This says that if there is no fill prefix and if we are not at the
end, point should move to the beginning of whatever was found by the
regular expression search for sp-parstart
.
The full definition for the forward-paragraph
function not only
includes code for going forwards, but also code for going backwards.
If you are reading this inside of GNU Emacs and you want to see the
whole function, you can type C-h f (describe-function
)
and the name of the function. This gives you the function
documentation and the name of the library containing the function’s
source. Place point over the name of the library and press the RET
key; you will be taken directly to the source. (Be sure to install
your sources! Without them, you are like a person who tries to drive
a car with his eyes shut!)
Next: Exercises with re-search-forward
, Previous: forward-paragraph
: a Goldmine of Functions, Up: Regular Expression Searches [Contents][Index]
12.5 Review
Here is a brief summary of some recently introduced functions.
while
Repeatedly evaluate the body of the expression so long as the first element of the body tests true. Then return
nil
. (The expression is evaluated only for its side effects.)For example:
(let ((foo 2)) (while (> foo 0) (insert (format "foo is %d.\n" foo)) (setq foo (1- foo)))) ⇒ foo is 2. foo is 1. nil
(The
insert
function inserts its arguments at point; theformat
function returns a string formatted from its arguments the waymessage
formats its arguments;\n
produces a new line.)re-search-forward
Search for a pattern, and if the pattern is found, move point to rest just after it.
Takes four arguments, like
search-forward
:- A regular expression that specifies the pattern to search for. (Remember to put quotation marks around this argument!)
- Optionally, the limit of the search.
- Optionally, what to do if the search fails, return
nil
or an error message. - Optionally, how many times to repeat the search; if negative, the search goes backwards.
let*
Bind some variables locally to particular values, and then evaluate the remaining arguments, returning the value of the last one. While binding the local variables, use the local values of variables bound earlier, if any.
For example:
(let* ((foo 7) (bar (* 3 foo))) (message "`bar' is %d." bar)) ⇒ ‘bar’ is 21.
match-beginning
Return the position of the start of the text found by the last regular expression search.
looking-at
Return
t
for true if the text after point matches the argument, which should be a regular expression.eobp
Return
t
for true if point is at the end of the accessible part of a buffer. The end of the accessible part is the end of the buffer if the buffer is not narrowed; it is the end of the narrowed part if the buffer is narrowed.
Previous: Review, Up: Regular Expression Searches [Contents][Index]
12.6 Exercises with re-search-forward
- Write a function to search for a regular expression that matches two or more blank lines in sequence.
- Write a function to search for duplicated words, such as “the the”.
See Syntax of Regular Expressions in The GNU Emacs
Manual, for information on how to write a regexp (a regular
expression) to match a string that is composed of two identical
halves. You can devise several regexps; some are better than others.
The function I use is described in an appendix, along with several
regexps. See
the-the
Duplicated Words Function.
Next: Counting Words in a defun
, Previous: Regular Expression Searches, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
13 Counting via Repetition and Regexps
Repetition and regular expression searches are powerful tools that you
often use when you write code in Emacs Lisp. This chapter illustrates
the use of regular expression searches through the construction of
word count commands using while
loops and recursion.
- Counting words
- The
count-words-example
Function - Count Words Recursively
- Exercise: Counting Punctuation
Counting words
The standard Emacs distribution contains functions for counting the number of lines and words within a region.
Certain types of writing ask you to count words. Thus, if you write
an essay, you may be limited to 800 words; if you write a novel, you
may discipline yourself to write 1000 words a day. It seems odd, but
for a long time, Emacs lacked a word count command. Perhaps people used
Emacs mostly for code or types of documentation that did not require
word counts; or perhaps they restricted themselves to the operating
system word count command, wc
. Alternatively, people may have
followed the publishers’ convention and computed a word count by
dividing the number of characters in a document by five.
There are many ways to implement a command to count words. Here are
some examples, which you may wish to compare with the standard Emacs
command, count-words-region
.
Next: Count Words Recursively, Previous: Counting words, Up: Counting via Repetition and Regexps [Contents][Index]
13.1 The count-words-example
Function
A word count command could count words in a line, paragraph, region,
or buffer. What should the command cover? You could design the
command to count the number of words in a complete buffer. However,
the Emacs tradition encourages flexibility—you may want to count
words in just a section, rather than all of a buffer. So it makes
more sense to design the command to count the number of words in a
region. Once you have a command to count words in a region, you can,
if you wish, count words in a whole buffer by marking it with
C-x h (mark-whole-buffer
).
Clearly, counting words is a repetitive act: starting from the
beginning of the region, you count the first word, then the second
word, then the third word, and so on, until you reach the end of the
region. This means that word counting is ideally suited to recursion
or to a while
loop.
Next: The Whitespace Bug in count-words-example
, Up: The count-words-example
Function [Contents][Index]
Designing count-words-example
First, we will implement the word count command with a while
loop, then with recursion. The command will, of course, be
interactive.
The template for an interactive function definition is, as always:
(defun name-of-function (argument-list) "documentation…" (interactive-expression…) body…)
What we need to do is fill in the slots.
The name of the function should be self-explanatory and easy
to remember. count-words-region
is the obvious choice. Since
that name is used for the standard Emacs command to count words, we
will name our implementation count-words-example
.
The function counts words within a region. This means that the
argument list must contain symbols that are bound to the two
positions, the beginning and end of the region. These two positions
can be called ‘beginning’ and ‘end’ respectively. The first
line of the documentation should be a single sentence, since that is
all that is printed as documentation by a command such as
apropos
. The interactive expression will be of the form
‘(interactive "r")’, since that will cause Emacs to pass the
beginning and end of the region to the function’s argument list. All
this is routine.
The body of the function needs to be written to do three tasks:
first, to set up conditions under which the while
loop can
count words, second, to run the while
loop, and third, to send
a message to the user.
When a user calls count-words-example
, point may be at the
beginning or the end of the region. However, the counting process
must start at the beginning of the region. This means we will want
to put point there if it is not already there. Executing
(goto-char beginning)
ensures this. Of course, we will want to
return point to its expected position when the function finishes its
work. For this reason, the body must be enclosed in a
save-excursion
expression.
The central part of the body of the function consists of a
while
loop in which one expression jumps point forward word by
word, and another expression counts those jumps. The true-or-false-test
of the while
loop should test true so long as point should jump
forward, and false when point is at the end of the region.
We could use (forward-word 1)
as the expression for moving point
forward word by word, but it is easier to see what Emacs identifies as a
“word” if we use a regular expression search.
A regular expression search that finds the pattern for which it is searching leaves point after the last character matched. This means that a succession of successful word searches will move point forward word by word.
As a practical matter, we want the regular expression search to jump over whitespace and punctuation between words as well as over the words themselves. A regexp that refuses to jump over interword whitespace would never jump more than one word! This means that the regexp should include the whitespace and punctuation that follows a word, if any, as well as the word itself. (A word may end a buffer and not have any following whitespace or punctuation, so that part of the regexp must be optional.)
Thus, what we want for the regexp is a pattern defining one or more word constituent characters followed, optionally, by one or more characters that are not word constituents. The regular expression for this is:
\w+\W*
The buffer’s syntax table determines which characters are and are not word constituents. For more information about syntax, see Syntax Tables in The GNU Emacs Lisp Reference Manual.
The search expression looks like this:
(re-search-forward "\\w+\\W*")
(Note that paired backslashes precede the ‘w’ and ‘W’. A single backslash has special meaning to the Emacs Lisp interpreter. It indicates that the following character is interpreted differently than usual. For example, the two characters, ‘\n’, stand for ‘newline’, rather than for a backslash followed by ‘n’. Two backslashes in a row stand for an ordinary, unspecial backslash, so Emacs Lisp interpreter ends of seeing a single backslash followed by a letter. So it discovers the letter is special.)
We need a counter to count how many words there are; this variable
must first be set to 0 and then incremented each time Emacs goes
around the while
loop. The incrementing expression is simply:
(setq count (1+ count))
Finally, we want to tell the user how many words there are in the
region. The message
function is intended for presenting this
kind of information to the user. The message has to be phrased so
that it reads properly regardless of how many words there are in the
region: we don’t want to say that “there are 1 words in the region”.
The conflict between singular and plural is ungrammatical. We can
solve this problem by using a conditional expression that evaluates
different messages depending on the number of words in the region.
There are three possibilities: no words in the region, one word in the
region, and more than one word. This means that the cond
special form is appropriate.
All this leads to the following function definition:
;;; First version; has bugs!
(defun count-words-example (beginning end)
"Print number of words in the region.
Words are defined as at least one word-constituent
character followed by at least one character that
is not a word-constituent. The buffer's syntax
table determines which characters these are."
(interactive "r")
(message "Counting words in region ... ")
;;; 1. Set up appropriate conditions.
(save-excursion
(goto-char beginning)
(let ((count 0))
;;; 2. Run the while loop. (while (< (point) end) (re-search-forward "\\w+\\W*") (setq count (1+ count)))
;;; 3. Send a message to the user.
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message
"The region has 1 word."))
(t
(message
"The region has %d words." count))))))
As written, the function works, but not in all circumstances.
Previous: Designing count-words-example
, Up: The count-words-example
Function [Contents][Index]
13.1.1 The Whitespace Bug in count-words-example
The count-words-example
command described in the preceding
section has two bugs, or rather, one bug with two manifestations.
First, if you mark a region containing only whitespace in the middle
of some text, the count-words-example
command tells you that the
region contains one word! Second, if you mark a region containing
only whitespace at the end of the buffer or the accessible portion of
a narrowed buffer, the command displays an error message that looks
like this:
Search failed: "\\w+\\W*"
If you are reading this in Info in GNU Emacs, you can test for these bugs yourself.
First, evaluate the function in the usual manner to install it.
If you wish, you can also install this key binding by evaluating it:
(global-set-key "\C-c=" 'count-words-example)
To conduct the first test, set mark and point to the beginning and end of the following line and then type C-c = (or M-x count-words-example if you have not bound C-c =):
one two three
Emacs will tell you, correctly, that the region has three words.
Repeat the test, but place mark at the beginning of the line and place point just before the word ‘one’. Again type the command C-c = (or M-x count-words-example). Emacs should tell you that the region has no words, since it is composed only of the whitespace at the beginning of the line. But instead Emacs tells you that the region has one word!
For the third test, copy the sample line to the end of the *scratch* buffer and then type several spaces at the end of the line. Place mark right after the word ‘three’ and point at the end of line. (The end of the line will be the end of the buffer.) Type C-c = (or M-x count-words-example) as you did before. Again, Emacs should tell you that the region has no words, since it is composed only of the whitespace at the end of the line. Instead, Emacs displays an error message saying ‘Search failed’.
The two bugs stem from the same problem.
Consider the first manifestation of the bug, in which the command
tells you that the whitespace at the beginning of the line contains
one word. What happens is this: The M-x count-words-example
command moves point to the beginning of the region. The while
tests whether the value of point is smaller than the value of
end
, which it is. Consequently, the regular expression search
looks for and finds the first word. It leaves point after the word.
count
is set to one. The while
loop repeats; but this
time the value of point is larger than the value of end
, the
loop is exited; and the function displays a message saying the number
of words in the region is one. In brief, the regular expression
search looks for and finds the word even though it is outside
the marked region.
In the second manifestation of the bug, the region is whitespace at
the end of the buffer. Emacs says ‘Search failed’. What happens
is that the true-or-false-test in the while
loop tests true, so
the search expression is executed. But since there are no more words
in the buffer, the search fails.
In both manifestations of the bug, the search extends or attempts to extend outside of the region.
The solution is to limit the search to the region—this is a fairly simple action, but as you may have come to expect, it is not quite as simple as you might think.
As we have seen, the re-search-forward
function takes a search
pattern as its first argument. But in addition to this first,
mandatory argument, it accepts three optional arguments. The optional
second argument bounds the search. The optional third argument, if
t
, causes the function to return nil
rather than signal
an error if the search fails. The optional fourth argument is a
repeat count. (In Emacs, you can see a function’s documentation by
typing C-h f, the name of the function, and then RET.)
In the count-words-example
definition, the value of the end of
the region is held by the variable end
which is passed as an
argument to the function. Thus, we can add end
as an argument
to the regular expression search expression:
(re-search-forward "\\w+\\W*" end)
However, if you make only this change to the count-words-example
definition and then test the new version of the definition on a
stretch of whitespace, you will receive an error message saying
‘Search failed’.
What happens is this: the search is limited to the region, and fails as you expect because there are no word-constituent characters in the region. Since it fails, we receive an error message. But we do not want to receive an error message in this case; we want to receive the message “The region does NOT have any words.”
The solution to this problem is to provide re-search-forward
with a third argument of t
, which causes the function to return
nil
rather than signal an error if the search fails.
However, if you make this change and try it, you will see the message
“Counting words in region ... ” and … you will keep on seeing
that message …, until you type C-g (keyboard-quit
).
Here is what happens: the search is limited to the region, as before,
and it fails because there are no word-constituent characters in the
region, as expected. Consequently, the re-search-forward
expression returns nil
. It does nothing else. In particular,
it does not move point, which it does as a side effect if it finds the
search target. After the re-search-forward
expression returns
nil
, the next expression in the while
loop is evaluated.
This expression increments the count. Then the loop repeats. The
true-or-false-test tests true because the value of point is still less
than the value of end, since the re-search-forward
expression
did not move point. … and the cycle repeats …
The count-words-example
definition requires yet another
modification, to cause the true-or-false-test of the while
loop
to test false if the search fails. Put another way, there are two
conditions that must be satisfied in the true-or-false-test before the
word count variable is incremented: point must still be within the
region and the search expression must have found a word to count.
Since both the first condition and the second condition must be true
together, the two expressions, the region test and the search
expression, can be joined with an and
special form and embedded in
the while
loop as the true-or-false-test, like this:
(and (< (point) end) (re-search-forward "\\w+\\W*" end t))
The re-search-forward
expression returns t
if the search
succeeds and as a side effect moves point. Consequently, as words are
found, point is moved through the region. When the search expression
fails to find another word, or when point reaches the end of the
region, the true-or-false-test tests false, the while
loop
exits, and the count-words-example
function displays one or
other of its messages.
After incorporating these final changes, the count-words-example
works without bugs (or at least, without bugs that I have found!).
Here is what it looks like:
;;; Final version: while
(defun count-words-example (beginning end)
"Print number of words in the region."
(interactive "r")
(message "Counting words in region ... ")
;;; 1. Set up appropriate conditions.
(save-excursion
(let ((count 0))
(goto-char beginning)
;;; 2. Run the while loop. (while (and (< (point) end) (re-search-forward "\\w+\\W*" end t)) (setq count (1+ count)))
;;; 3. Send a message to the user.
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message
"The region has 1 word."))
(t
(message
"The region has %d words." count))))))
Next: Exercise: Counting Punctuation, Previous: The count-words-example
Function, Up: Counting via Repetition and Regexps [Contents][Index]
13.2 Count Words Recursively
You can write the function for counting words recursively as well as
with a while
loop. Let’s see how this is done.
First, we need to recognize that the count-words-example
function has three jobs: it sets up the appropriate conditions for
counting to occur; it counts the words in the region; and it sends a
message to the user telling how many words there are.
If we write a single recursive function to do everything, we will receive a message for every recursive call. If the region contains 13 words, we will receive thirteen messages, one right after the other. We don’t want this! Instead, we must write two functions to do the job, one of which (the recursive function) will be used inside of the other. One function will set up the conditions and display the message; the other will return the word count.
Let us start with the function that causes the message to be displayed.
We can continue to call this count-words-example
.
This is the function that the user will call. It will be interactive.
Indeed, it will be similar to our previous versions of this
function, except that it will call recursive-count-words
to
determine how many words are in the region.
We can readily construct a template for this function, based on our previous versions:
;; Recursive version; uses regular expression search
(defun count-words-example (beginning end)
"documentation…"
(interactive-expression…)
;;; 1. Set up appropriate conditions.
(explanatory message)
(set-up functions…
;;; 2. Count the words.
recursive call
;;; 3. Send a message to the user.
message providing word count))
The definition looks straightforward, except that somehow the count
returned by the recursive call must be passed to the message
displaying the word count. A little thought suggests that this can be
done by making use of a let
expression: we can bind a variable
in the varlist of a let
expression to the number of words in
the region, as returned by the recursive call; and then the
cond
expression, using binding, can display the value to the
user.
Often, one thinks of the binding within a let
expression as
somehow secondary to the primary work of a function. But in this
case, what you might consider the primary job of the function,
counting words, is done within the let
expression.
Using let
, the function definition looks like this:
(defun count-words-example (beginning end) "Print number of words in the region." (interactive "r")
;;; 1. Set up appropriate conditions.
(message "Counting words in region ... ")
(save-excursion
(goto-char beginning)
;;; 2. Count the words.
(let ((count (recursive-count-words end)))
;;; 3. Send a message to the user.
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message
"The region has 1 word."))
(t
(message
"The region has %d words." count))))))
Next, we need to write the recursive counting function.
A recursive function has at least three parts: the do-again-test, the next-step-expression, and the recursive call.
The do-again-test determines whether the function will or will not be
called again. Since we are counting words in a region and can use a
function that moves point forward for every word, the do-again-test
can check whether point is still within the region. The do-again-test
should find the value of point and determine whether point is before,
at, or after the value of the end of the region. We can use the
point
function to locate point. Clearly, we must pass the
value of the end of the region to the recursive counting function as an
argument.
In addition, the do-again-test should also test whether the search finds a word. If it does not, the function should not call itself again.
The next-step-expression changes a value so that when the recursive function is supposed to stop calling itself, it stops. More precisely, the next-step-expression changes a value so that at the right time, the do-again-test stops the recursive function from calling itself again. In this case, the next-step-expression can be the expression that moves point forward, word by word.
The third part of a recursive function is the recursive call.
Somewhere, we also need a part that does the work of the function, a part that does the counting. A vital part!
But already, we have an outline of the recursive counting function:
(defun recursive-count-words (region-end) "documentation…" do-again-test next-step-expression recursive call)
Now we need to fill in the slots. Let’s start with the simplest cases first: if point is at or beyond the end of the region, there cannot be any words in the region, so the function should return zero. Likewise, if the search fails, there are no words to count, so the function should return zero.
On the other hand, if point is within the region and the search succeeds, the function should call itself again.
Thus, the do-again-test should look like this:
(and (< (point) region-end) (re-search-forward "\\w+\\W*" region-end t))
Note that the search expression is part of the do-again-test—the
function returns t
if its search succeeds and nil
if it
fails. (See The Whitespace Bug in
count-words-example
, for an explanation of how
re-search-forward
works.)
The do-again-test is the true-or-false test of an if
clause.
Clearly, if the do-again-test succeeds, the then-part of the if
clause should call the function again; but if it fails, the else-part
should return zero since either point is outside the region or the
search failed because there were no words to find.
But before considering the recursive call, we need to consider the next-step-expression. What is it? Interestingly, it is the search part of the do-again-test.
In addition to returning t
or nil
for the
do-again-test, re-search-forward
moves point forward as a side
effect of a successful search. This is the action that changes the
value of point so that the recursive function stops calling itself
when point completes its movement through the region. Consequently,
the re-search-forward
expression is the next-step-expression.
In outline, then, the body of the recursive-count-words
function looks like this:
(if do-again-test-and-next-step-combined ;; then recursive-call-returning-count ;; else return-zero)
How to incorporate the mechanism that counts?
If you are not used to writing recursive functions, a question like this can be troublesome. But it can and should be approached systematically.
We know that the counting mechanism should be associated in some way
with the recursive call. Indeed, since the next-step-expression moves
point forward by one word, and since a recursive call is made for
each word, the counting mechanism must be an expression that adds one
to the value returned by a call to recursive-count-words
.
Consider several cases:
- If there are two words in the region, the function should return a value resulting from adding one to the value returned when it counts the first word, plus the number returned when it counts the remaining words in the region, which in this case is one.
- If there is one word in the region, the function should return a value resulting from adding one to the value returned when it counts that word, plus the number returned when it counts the remaining words in the region, which in this case is zero.
- If there are no words in the region, the function should return zero.
From the sketch we can see that the else-part of the if
returns
zero for the case of no words. This means that the then-part of the
if
must return a value resulting from adding one to the value
returned from a count of the remaining words.
The expression will look like this, where 1+
is a function that
adds one to its argument.
(1+ (recursive-count-words region-end))
The whole recursive-count-words
function will then look like
this:
(defun recursive-count-words (region-end)
"documentation…"
;;; 1. do-again-test
(if (and (< (point) region-end)
(re-search-forward "\\w+\\W*" region-end t))
;;; 2. then-part: the recursive call (1+ (recursive-count-words region-end)) ;;; 3. else-part 0))
Let’s examine how this works:
If there are no words in the region, the else part of the if
expression is evaluated and consequently the function returns zero.
If there is one word in the region, the value of point is less than
the value of region-end
and the search succeeds. In this case,
the true-or-false-test of the if
expression tests true, and the
then-part of the if
expression is evaluated. The counting
expression is evaluated. This expression returns a value (which will
be the value returned by the whole function) that is the sum of one
added to the value returned by a recursive call.
Meanwhile, the next-step-expression has caused point to jump over the
first (and in this case only) word in the region. This means that
when (recursive-count-words region-end)
is evaluated a second
time, as a result of the recursive call, the value of point will be
equal to or greater than the value of region end. So this time,
recursive-count-words
will return zero. The zero will be added
to one, and the original evaluation of recursive-count-words
will return one plus zero, which is one, which is the correct amount.
Clearly, if there are two words in the region, the first call to
recursive-count-words
returns one added to the value returned
by calling recursive-count-words
on a region containing the
remaining word—that is, it adds one to one, producing two, which is
the correct amount.
Similarly, if there are three words in the region, the first call to
recursive-count-words
returns one added to the value returned
by calling recursive-count-words
on a region containing the
remaining two words—and so on and so on.
With full documentation the two functions look like this:
The recursive function:
(defun recursive-count-words (region-end) "Number of words between point and REGION-END."
;;; 1. do-again-test
(if (and (< (point) region-end)
(re-search-forward "\\w+\\W*" region-end t))
;;; 2. then-part: the recursive call (1+ (recursive-count-words region-end)) ;;; 3. else-part 0))
The wrapper:
;;; Recursive version
(defun count-words-example (beginning end)
"Print number of words in the region.
Words are defined as at least one word-constituent character followed by at least one character that is not a word-constituent. The buffer's syntax table determines which characters these are."
(interactive "r") (message "Counting words in region ... ") (save-excursion (goto-char beginning) (let ((count (recursive-count-words end)))
(cond ((zerop count) (message "The region does NOT have any words."))
((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count))))))
Previous: Count Words Recursively, Up: Counting via Repetition and Regexps [Contents][Index]
13.3 Exercise: Counting Punctuation
Using a while
loop, write a function to count the number of
punctuation marks in a region—period, comma, semicolon, colon,
exclamation mark, and question mark. Do the same using recursion.
Next: Readying a Graph, Previous: Counting via Repetition and Regexps, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
14 Counting Words in a defun
Our next project is to count the number of words in a function
definition. Clearly, this can be done using some variant of
count-words-example
. See Counting via
Repetition and Regexps. If we are just going to count the words in
one definition, it is easy enough to mark the definition with the
C-M-h (mark-defun
) command, and then call
count-words-example
.
However, I am more ambitious: I want to count the words and symbols in every definition in the Emacs sources and then print a graph that shows how many functions there are of each length: how many contain 40 to 49 words or symbols, how many contain 50 to 59 words or symbols, and so on. I have often been curious how long a typical function is, and this will tell.
- Divide and Conquer
- What to Count?
- What Constitutes a Word or Symbol?
- The
count-words-in-defun
Function - Count Several
defuns
Within a File - Find a File
lengths-list-file
in Detail- Count Words in
defuns
in Different Files - Recursively Count Words in Different Files
- Prepare the Data for Display in a Graph
Next: What to Count?, Up: Counting Words in a defun
[Contents][Index]
Divide and Conquer
Described in one phrase, the histogram project is daunting; but divided into numerous small steps, each of which we can take one at a time, the project becomes less fearsome. Let us consider what the steps must be:
- First, write a function to count the words in one definition. This includes the problem of handling symbols as well as words.
- Second, write a function to list the number of words in each function
in a file. This function can use the
count-words-in-defun
function. - Third, write a function to list the number of words in each function in each of several files. This entails automatically finding the various files, switching to them, and counting the words in the definitions within them.
- Fourth, write a function to convert the list of numbers that we created in step three to a form that will be suitable for printing as a graph.
- Fifth, write a function to print the results as a graph.
This is quite a project! But if we take each step slowly, it will not be difficult.
Next: What Constitutes a Word or Symbol?, Previous: Divide and Conquer, Up: Counting Words in a defun
[Contents][Index]
14.1 What to Count?
When we first start thinking about how to count the words in a
function definition, the first question is (or ought to be) what are
we going to count? When we speak of “words” with respect to a Lisp
function definition, we are actually speaking, in large part, of
symbols. For example, the following multiply-by-seven
function contains the five symbols defun
,
multiply-by-seven
, number
, *
, and 7
. In
addition, in the documentation string, it contains the four words
‘Multiply’, ‘NUMBER’, ‘by’, and ‘seven’. The
symbol ‘number’ is repeated, so the definition contains a total
of ten words and symbols.
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
However, if we mark the multiply-by-seven
definition with
C-M-h (mark-defun
), and then call
count-words-example
on it, we will find that
count-words-example
claims the definition has eleven words, not
ten! Something is wrong!
The problem is twofold: count-words-example
does not count the
‘*’ as a word, and it counts the single symbol,
multiply-by-seven
, as containing three words. The hyphens are
treated as if they were interword spaces rather than intraword
connectors: ‘multiply-by-seven’ is counted as if it were written
‘multiply by seven’.
The cause of this confusion is the regular expression search within
the count-words-example
definition that moves point forward word
by word. In the canonical version of count-words-example
, the
regexp is:
"\\w+\\W*"
This regular expression is a pattern defining one or more word constituent characters possibly followed by one or more characters that are not word constituents. What is meant by “word constituent characters” brings us to the issue of syntax, which is worth a section of its own.
Next: The count-words-in-defun
Function, Previous: What to Count?, Up: Counting Words in a defun
[Contents][Index]
14.2 What Constitutes a Word or Symbol?
Emacs treats different characters as belonging to different syntax categories. For example, the regular expression, ‘\\w+’, is a pattern specifying one or more word constituent characters. Word constituent characters are members of one syntax category. Other syntax categories include the class of punctuation characters, such as the period and the comma, and the class of whitespace characters, such as the blank space and the tab character. (For more information, see Syntax Tables in The GNU Emacs Lisp Reference Manual.)
Syntax tables specify which characters belong to which categories.
Usually, a hyphen is not specified as a word constituent character.
Instead, it is specified as being in the class of characters that are
part of symbol names but not words. This means that the
count-words-example
function treats it in the same way it treats
an interword white space, which is why count-words-example
counts ‘multiply-by-seven’ as three words.
There are two ways to cause Emacs to count ‘multiply-by-seven’ as one symbol: modify the syntax table or modify the regular expression.
We could redefine a hyphen as a word constituent character by modifying the syntax table that Emacs keeps for each mode. This action would serve our purpose, except that a hyphen is merely the most common character within symbols that is not typically a word constituent character; there are others, too.
Alternatively, we can redefine the regexp used in the
count-words-example
definition so as to include symbols. This
procedure has the merit of clarity, but the task is a little tricky.
The first part is simple enough: the pattern must match at least one character that is a word or symbol constituent. Thus:
"\\(\\w\\|\\s_\\)+"
The ‘\\(’ is the first part of the grouping construct that includes the ‘\\w’ and the ‘\\s_’ as alternatives, separated by the ‘\\|’. The ‘\\w’ matches any word-constituent character and the ‘\\s_’ matches any character that is part of a symbol name but not a word-constituent character. The ‘+’ following the group indicates that the word or symbol constituent characters must be matched at least once.
However, the second part of the regexp is more difficult to design. What we want is to follow the first part with optionally one or more characters that are not constituents of a word or symbol. At first, I thought I could define this with the following:
"\\(\\W\\|\\S_\\)*"
The upper case ‘W’ and ‘S’ match characters that are not word or symbol constituents. Unfortunately, this expression matches any character that is either not a word constituent or not a symbol constituent. This matches any character!
I then noticed that every word or symbol in my test region was followed by white space (blank space, tab, or newline). So I tried placing a pattern to match one or more blank spaces after the pattern for one or more word or symbol constituents. This failed, too. Words and symbols are often separated by whitespace, but in actual code parentheses may follow symbols and punctuation may follow words. So finally, I designed a pattern in which the word or symbol constituents are followed optionally by characters that are not white space and then followed optionally by white space.
Here is the full regular expression:
"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
Next: Count Several defuns
Within a File, Previous: What Constitutes a Word or Symbol?, Up: Counting Words in a defun
[Contents][Index]
14.3 The count-words-in-defun
Function
We have seen that there are several ways to write a
count-words-region
function. To write a
count-words-in-defun
, we need merely adapt one of these
versions.
The version that uses a while
loop is easy to understand, so I
am going to adapt that. Because count-words-in-defun
will be
part of a more complex program, it need not be interactive and it need
not display a message but just return the count. These considerations
simplify the definition a little.
On the other hand, count-words-in-defun
will be used within a
buffer that contains function definitions. Consequently, it is
reasonable to ask that the function determine whether it is called
when point is within a function definition, and if it is, to return
the count for that definition. This adds complexity to the
definition, but saves us from needing to pass arguments to the
function.
These considerations lead us to prepare the following template:
(defun count-words-in-defun () "documentation…" (set up… (while loop…) return count)
As usual, our job is to fill in the slots.
First, the set up.
We are presuming that this function will be called within a buffer
containing function definitions. Point will either be within a
function definition or not. For count-words-in-defun
to work,
point must move to the beginning of the definition, a counter must
start at zero, and the counting loop must stop when point reaches the
end of the definition.
The beginning-of-defun
function searches backwards for an
opening delimiter such as a ‘(’ at the beginning of a line, and
moves point to that position, or else to the limit of the search. In
practice, this means that beginning-of-defun
moves point to the
beginning of an enclosing or preceding function definition, or else to
the beginning of the buffer. We can use beginning-of-defun
to
place point where we wish to start.
The while
loop requires a counter to keep track of the words or
symbols being counted. A let
expression can be used to create
a local variable for this purpose, and bind it to an initial value of zero.
The end-of-defun
function works like beginning-of-defun
except that it moves point to the end of the definition.
end-of-defun
can be used as part of an expression that
determines the position of the end of the definition.
The set up for count-words-in-defun
takes shape rapidly: first
we move point to the beginning of the definition, then we create a
local variable to hold the count, and finally, we record the position
of the end of the definition so the while
loop will know when to stop
looping.
The code looks like this:
(beginning-of-defun) (let ((count 0) (end (save-excursion (end-of-defun) (point))))
The code is simple. The only slight complication is likely to concern
end
: it is bound to the position of the end of the definition
by a save-excursion
expression that returns the value of point
after end-of-defun
temporarily moves it to the end of the
definition.
The second part of the count-words-in-defun
, after the set up,
is the while
loop.
The loop must contain an expression that jumps point forward word by
word and symbol by symbol, and another expression that counts the
jumps. The true-or-false-test for the while
loop should test
true so long as point should jump forward, and false when point is at
the end of the definition. We have already redefined the regular
expression for this, so the loop is straightforward:
(while (and (< (point) end) (re-search-forward "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t)) (setq count (1+ count)))
The third part of the function definition returns the count of words
and symbols. This part is the last expression within the body of the
let
expression, and can be, very simply, the local variable
count
, which when evaluated returns the count.
Put together, the count-words-in-defun
definition looks like this:
(defun count-words-in-defun () "Return the number of words and symbols in a defun." (beginning-of-defun) (let ((count 0) (end (save-excursion (end-of-defun) (point))))
(while (and (< (point) end) (re-search-forward "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t)) (setq count (1+ count))) count))
How to test this? The function is not interactive, but it is easy to
put a wrapper around the function to make it interactive; we can use
almost the same code as for the recursive version of
count-words-example
:
;;; Interactive version.
(defun count-words-defun ()
"Number of words and symbols in a function definition."
(interactive)
(message
"Counting words and symbols in function definition ... ")
(let ((count (count-words-in-defun))) (cond ((zerop count) (message "The definition does NOT have any words or symbols."))
((= 1 count) (message "The definition has 1 word or symbol.")) (t (message "The definition has %d words or symbols." count)))))
Let’s reuse C-c = as a convenient key binding:
(global-set-key "\C-c=" 'count-words-defun)
Now we can try out count-words-defun
: install both
count-words-in-defun
and count-words-defun
, and set the
key binding. Then copy the following to an Emacs Lisp buffer (like,
for instance, *scratch*), place the cursor within the
definition, and use the C-c = command.
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number)) ⇒ 10
Success! The definition has 10 words and symbols.
The next problem is to count the numbers of words and symbols in several definitions within a single file.
Next: Find a File, Previous: The count-words-in-defun
Function, Up: Counting Words in a defun
[Contents][Index]
14.4 Count Several defuns
Within a File
A file such as simple.el may have a hundred or more function definitions within it. Our long term goal is to collect statistics on many files, but as a first step, our immediate goal is to collect statistics on one file.
The information will be a series of numbers, each number being the length of a function definition. We can store the numbers in a list.
We know that we will want to incorporate the information regarding one file with information about many other files; this means that the function for counting definition lengths within one file need only return the list of lengths. It need not and should not display any messages.
The word count commands contain one expression to jump point forward word by word and another expression to count the jumps. The function to return the lengths of definitions can be designed to work the same way, with one expression to jump point forward definition by definition and another expression to construct the lengths’ list.
This statement of the problem makes it elementary to write the
function definition. Clearly, we will start the count at the
beginning of the file, so the first command will be (goto-char
(point-min))
. Next, we start the while
loop; and the
true-or-false test of the loop can be a regular expression search for
the next function definition—so long as the search succeeds, point
is moved forward and then the body of the loop is evaluated. The body
needs an expression that constructs the lengths’ list. cons
,
the list construction command, can be used to create the list. That
is almost all there is to it.
Here is what this fragment of code looks like:
(goto-char (point-min)) (while (re-search-forward "^(defun" nil t) (setq lengths-list (cons (count-words-in-defun) lengths-list)))
What we have left out is the mechanism for finding the file that contains the function definitions.
In previous examples, we either used this, the Info file, or we switched back and forth to some other buffer, such as the *scratch* buffer.
Finding a file is a new process that we have not yet discussed.
Next: lengths-list-file
in Detail, Previous: Count Several defuns
Within a File, Up: Counting Words in a defun
[Contents][Index]
14.5 Find a File
To find a file in Emacs, you use the C-x C-f (find-file
)
command. This command is almost, but not quite right for the lengths
problem.
Let’s look at the source for find-file
:
(defun find-file (filename) "Edit file FILENAME. Switch to a buffer visiting file FILENAME, creating one if none already exists." (interactive "FFind file: ") (switch-to-buffer (find-file-noselect filename)))
(The most recent version of the find-file
function definition
permits you to specify optional wildcards to visit multiple files;
that makes the definition more complex and we will not discuss it
here, since it is not relevant. You can see its source using either
M-. (xref-find-definitions
) or C-h f
(describe-function
).)
The definition I am showing possesses short but complete documentation
and an interactive specification that prompts you for a file name when
you use the command interactively. The body of the definition
contains two functions, find-file-noselect
and
switch-to-buffer
.
According to its documentation as shown by C-h f (the
describe-function
command), the find-file-noselect
function reads the named file into a buffer and returns the buffer.
(Its most recent version includes an optional wildcards argument,
too, as well as another to read a file literally and another to
suppress warning messages. These optional arguments are irrelevant.)
However, the find-file-noselect
function does not select the
buffer in which it puts the file. Emacs does not switch its attention
(or yours if you are using find-file-noselect
) to the selected
buffer. That is what switch-to-buffer
does: it switches the
buffer to which Emacs attention is directed; and it switches the
buffer displayed in the window to the new buffer. We have discussed
buffer switching elsewhere. (See Switching Buffers.)
In this histogram project, we do not need to display each file on the
screen as the program determines the length of each definition within
it. Instead of employing switch-to-buffer
, we can work with
set-buffer
, which redirects the attention of the computer
program to a different buffer but does not redisplay it on the screen.
So instead of calling on find-file
to do the job, we must write
our own expression.
The task is easy: use find-file-noselect
and set-buffer
.
Next: Count Words in defuns
in Different Files, Previous: Find a File, Up: Counting Words in a defun
[Contents][Index]
14.6 lengths-list-file
in Detail
The core of the lengths-list-file
function is a while
loop containing a function to move point forward defun by defun, and
a function to count the number of words and symbols in each defun.
This core must be surrounded by functions that do various other tasks,
including finding the file, and ensuring that point starts out at the
beginning of the file. The function definition looks like this:
(defun lengths-list-file (filename) "Return list of definitions' lengths within FILE. The returned list is a list of numbers. Each number is the number of words or symbols in one function definition."
(message "Working on `%s' ... " filename) (save-excursion (let ((buffer (find-file-noselect filename)) (lengths-list)) (set-buffer buffer) (setq buffer-read-only t) (widen) (goto-char (point-min)) (while (re-search-forward "^(defun" nil t) (setq lengths-list (cons (count-words-in-defun) lengths-list))) (kill-buffer buffer) lengths-list)))
The function is passed one argument, the name of the file on which it will work. It has four lines of documentation, but no interactive specification. Since people worry that a computer is broken if they don’t see anything going on, the first line of the body is a message.
The next line contains a save-excursion
that returns Emacs’s
attention to the current buffer when the function completes. This is
useful in case you embed this function in another function that
presumes point is restored to the original buffer.
In the varlist of the let
expression, Emacs finds the file and
binds the local variable buffer
to the buffer containing the
file. At the same time, Emacs creates lengths-list
as a local
variable.
Next, Emacs switches its attention to the buffer.
In the following line, Emacs makes the buffer read-only. Ideally, this line is not necessary. None of the functions for counting words and symbols in a function definition should change the buffer. Besides, the buffer is not going to be saved, even if it were changed. This line is entirely the consequence of great, perhaps excessive, caution. The reason for the caution is that this function and those it calls work on the sources for Emacs and it is inconvenient if they are inadvertently modified. It goes without saying that I did not realize a need for this line until an experiment went awry and started to modify my Emacs source files …
Next comes a call to widen the buffer if it is narrowed. This function is usually not needed—Emacs creates a fresh buffer if none already exists; but if a buffer visiting the file already exists Emacs returns that one. In this case, the buffer may be narrowed and must be widened. If we wanted to be fully user-friendly, we would arrange to save the restriction and the location of point, but we won’t.
The (goto-char (point-min))
expression moves point to the
beginning of the buffer.
Then comes a while
loop in which the work of the function is
carried out. In the loop, Emacs determines the length of each
definition and constructs a lengths’ list containing the information.
Emacs kills the buffer after working through it. This is to save
space inside of Emacs. My version of GNU Emacs 19 contained over 300
source files of interest; GNU Emacs 22 contains over a thousand source
files. Another function will apply lengths-list-file
to each
of the files.
Finally, the last expression within the let
expression is the
lengths-list
variable; its value is returned as the value of
the whole function.
You can try this function by installing it in the usual fashion. Then
place your cursor after the following expression and type C-x
C-e (eval-last-sexp
).
(lengths-list-file "/usr/local/share/emacs/22.1/lisp/emacs-lisp/debug.el")
You may need to change the pathname of the file; the one here is for GNU Emacs version 22.1. To change the expression, copy it to the *scratch* buffer and edit it.
Also, to see the full length of the list, rather than a truncated version, you may have to evaluate the following:
(custom-set-variables '(eval-expression-print-length nil))
(See Specifying Variables using defcustom
.
Then evaluate the lengths-list-file
expression.)
The lengths’ list for debug.el takes less than a second to produce and looks like this in GNU Emacs 22:
(83 113 105 144 289 22 30 97 48 89 25 52 52 88 28 29 77 49 43 290 232 587)
(Using my old machine, the version 19 lengths’ list for debug.el took seven seconds to produce and looked like this:
(75 41 80 62 20 45 44 68 45 12 34 235)
The newer version of debug.el contains more defuns than the earlier one; and my new machine is much faster than the old one.)
Note that the length of the last definition in the file is first in the list.
Next: Recursively Count Words in Different Files, Previous: lengths-list-file
in Detail, Up: Counting Words in a defun
[Contents][Index]
14.7 Count Words in defuns
in Different Files
In the previous section, we created a function that returns a list of the lengths of each definition in a file. Now, we want to define a function to return a master list of the lengths of the definitions in a list of files.
Working on each of a list of files is a repetitious act, so we can use
either a while
loop or recursion.
Determine the lengths of defuns
The design using a while
loop is routine. The argument passed
to the function is a list of files. As we saw earlier (see A while
Loop and a List), you can write a while
loop so that the body of the
loop is evaluated if such a list contains elements, but to exit the
loop if the list is empty. For this design to work, the body of the
loop must contain an expression that shortens the list each time the
body is evaluated, so that eventually the list is empty. The usual
technique is to set the value of the list to the value of the CDR
of the list each time the body is evaluated.
The template looks like this:
(while test-whether-list-is-empty body… set-list-to-cdr-of-list)
Also, we remember that a while
loop returns nil
(the
result of evaluating the true-or-false-test), not the result of any
evaluation within its body. (The evaluations within the body of the
loop are done for their side effects.) However, the expression that
sets the lengths’ list is part of the body—and that is the value
that we want returned by the function as a whole. To do this, we
enclose the while
loop within a let
expression, and
arrange that the last element of the let
expression contains
the value of the lengths’ list. (See Loop
Example with an Incrementing Counter.)
These considerations lead us directly to the function itself:
;;; Use while
loop.
(defun lengths-list-many-files (list-of-files)
"Return list of lengths of defuns in LIST-OF-FILES."
(let (lengths-list) ;;; true-or-false-test (while list-of-files (setq lengths-list (append lengths-list ;;; Generate a lengths’ list. (lengths-list-file (expand-file-name (car list-of-files)))))
;;; Make files’ list shorter. (setq list-of-files (cdr list-of-files))) ;;; Return final value of lengths’ list. lengths-list))
expand-file-name
is a built-in function that converts a file
name to the absolute, long, path name form. The function employs the
name of the directory in which the function is called.
Thus, if expand-file-name
is called on debug.el
when
Emacs is visiting the
/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/ directory,
debug.el
becomes
/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/debug.el
The only other new element of this function definition is the as yet
unstudied function append
, which merits a short section for
itself.
Previous: Determine the lengths of defuns
, Up: Count Words in defuns
in Different Files [Contents][Index]
14.7.1 The append
Function
The append
function attaches one list to another. Thus,
(append '(1 2 3 4) '(5 6 7 8))
produces the list
(1 2 3 4 5 6 7 8)
This is exactly how we want to attach two lengths’ lists produced by
lengths-list-file
to each other. The results contrast with
cons
,
(cons '(1 2 3 4) '(5 6 7 8))
which constructs a new list in which the first argument to cons
becomes the first element of the new list:
((1 2 3 4) 5 6 7 8)
Next: Prepare the Data for Display in a Graph, Previous: Count Words in defuns
in Different Files, Up: Counting Words in a defun
[Contents][Index]
14.8 Recursively Count Words in Different Files
Besides a while
loop, you can work on each of a list of files
with recursion. A recursive version of lengths-list-many-files
is short and simple.
The recursive function has the usual parts: the do-again-test, the
next-step-expression, and the recursive call. The do-again-test
determines whether the function should call itself again, which it
will do if the list-of-files
contains any remaining elements;
the next-step-expression resets the list-of-files
to the
CDR of itself, so eventually the list will be empty; and the
recursive call calls itself on the shorter list. The complete
function is shorter than this description!
(defun recursive-lengths-list-many-files (list-of-files)
"Return list of lengths of each defun in LIST-OF-FILES."
(if list-of-files ; do-again-test
(append
(lengths-list-file
(expand-file-name (car list-of-files)))
(recursive-lengths-list-many-files
(cdr list-of-files)))))
In a sentence, the function returns the lengths’ list for the first of
the list-of-files
appended to the result of calling itself on
the rest of the list-of-files
.
Here is a test of recursive-lengths-list-many-files
, along with
the results of running lengths-list-file
on each of the files
individually.
Install recursive-lengths-list-many-files
and
lengths-list-file
, if necessary, and then evaluate the
following expressions. You may need to change the files’ pathnames;
those here work when this Info file and the Emacs sources are located
in their customary places. To change the expressions, copy them to
the *scratch* buffer, edit them, and then evaluate them.
The results are shown after the ‘⇒’. (These results are for files from Emacs version 22.1.1; files from other versions of Emacs may produce different results.)
(cd "/usr/local/share/emacs/22.1.1/") (lengths-list-file "./lisp/macros.el") ⇒ (283 263 480 90)
(lengths-list-file "./lisp/mail/mailalias.el") ⇒ (38 32 29 95 178 180 321 218 324)
(lengths-list-file "./lisp/hex-util.el") ⇒ (82 71)
(recursive-lengths-list-many-files '("./lisp/macros.el" "./lisp/mail/mailalias.el" "./lisp/hex-util.el")) ⇒ (283 263 480 90 38 32 29 95 178 180 321 218 324 82 71)
The recursive-lengths-list-many-files
function produces the
output we want.
The next step is to prepare the data in the list for display in a graph.
Previous: Recursively Count Words in Different Files, Up: Counting Words in a defun
[Contents][Index]
14.9 Prepare the Data for Display in a Graph
The recursive-lengths-list-many-files
function returns a list
of numbers. Each number records the length of a function definition.
What we need to do now is transform this data into a list of numbers
suitable for generating a graph. The new list will tell how many
functions definitions contain less than 10 words and
symbols, how many contain between 10 and 19 words and symbols, how
many contain between 20 and 29 words and symbols, and so on.
In brief, we need to go through the lengths’ list produced by the
recursive-lengths-list-many-files
function and count the number
of defuns within each range of lengths, and produce a list of those
numbers.
Next: Sorting Lists, Up: Prepare the Data for Display in a Graph [Contents][Index]
The Data for Display in Detail
Based on what we have done before, we can readily foresee that it should not be too hard to write a function that CDRs down the lengths’ list, looks at each element, determines which length range it is in, and increments a counter for that range.
However, before beginning to write such a function, we should consider the advantages of sorting the lengths’ list first, so the numbers are ordered from smallest to largest. First, sorting will make it easier to count the numbers in each range, since two adjacent numbers will either be in the same length range or in adjacent ranges. Second, by inspecting a sorted list, we can discover the highest and lowest number, and thereby determine the largest and smallest length range that we will need.
Next: Making a List of Files, Previous: The Data for Display in Detail, Up: Prepare the Data for Display in a Graph [Contents][Index]
14.9.1 Sorting Lists
Emacs contains a function to sort lists, called (as you might guess)
sort
. The sort
function takes two arguments, the list
to be sorted, and a predicate that determines whether the first of
two list elements is less than the second.
As we saw earlier (see Using the Wrong
Type Object as an Argument), a predicate is a function that
determines whether some property is true or false. The sort
function will reorder a list according to whatever property the
predicate uses; this means that sort
can be used to sort
non-numeric lists by non-numeric criteria—it can, for example,
alphabetize a list.
The <
function is used when sorting a numeric list. For example,
(sort '(4 8 21 17 33 7 21 7) '<)
produces this:
(4 7 7 8 17 21 21 33)
(Note that in this example, both the arguments are quoted so that the
symbols are not evaluated before being passed to sort
as
arguments.)
Sorting the list returned by the
recursive-lengths-list-many-files
function is straightforward;
it uses the <
function:
(sort (recursive-lengths-list-many-files '("./lisp/macros.el" "./lisp/mailalias.el" "./lisp/hex-util.el")) '<)
which produces:
(29 32 38 71 82 90 95 178 180 218 263 283 321 324 480)
(Note that in this example, the first argument to sort
is not
quoted, since the expression must be evaluated so as to produce the
list that is passed to sort
.)
Next: Counting function definitions, Previous: Sorting Lists, Up: Prepare the Data for Display in a Graph [Contents][Index]
14.9.2 Making a List of Files
The recursive-lengths-list-many-files
function requires a list
of files as its argument. For our test examples, we constructed such
a list by hand; but the Emacs Lisp source directory is too large for
us to do for that. Instead, we will write a function to do the job
for us. In this function, we will use both a while
loop and a
recursive call.
We did not have to write a function like this for older versions of
GNU Emacs, since they placed all the ‘.el’ files in one
directory. Instead, we were able to use the directory-files
function, which lists the names of files that match a specified
pattern within a single directory.
However, recent versions of Emacs place Emacs Lisp files in sub-directories of the top level lisp directory. This re-arrangement eases navigation. For example, all the mail related files are in a lisp sub-directory called mail. But at the same time, this arrangement forces us to create a file listing function that descends into the sub-directories.
We can create this function, called files-in-below-directory
,
using familiar functions such as car
, nthcdr
, and
substring
in conjunction with an existing function called
directory-files-and-attributes
. This latter function not only
lists all the filenames in a directory, including the names
of sub-directories, but also their attributes.
To restate our goal: to create a function that will enable us
to feed filenames to recursive-lengths-list-many-files
as a list that looks like this (but with more elements):
("./lisp/macros.el" "./lisp/mail/rmail.el" "./lisp/hex-util.el")
The directory-files-and-attributes
function returns a list of
lists. Each of the lists within the main list consists of 13
elements. The first element is a string that contains the name of the
file—which, in GNU/Linux, may be a directory file, that is to
say, a file with the special attributes of a directory. The second
element of the list is t
for a directory, a string
for symbolic link (the string is the name linked to), or nil
.
For example, the first ‘.el’ file in the lisp/ directory is abbrev.el. Its name is /usr/local/share/emacs/22.1.1/lisp/abbrev.el and it is not a directory or a symbolic link.
This is how directory-files-and-attributes
lists that file and
its attributes:
("abbrev.el" nil 1 1000 100
(20615 27034 579989 697000) (17905 55681 0 0) (20615 26327 734791 805000)(17) 13188 "-rw-r--r--"
t 2971624 773)
On the other hand, mail/ is a directory within the lisp/ directory. The beginning of its listing looks like this:
("mail" t … )
(To learn about the different attributes, look at the documentation of
file-attributes
. Bear in mind that the file-attributes
function does not list the filename, so its first element is
directory-files-and-attributes
’s second element.)
We will want our new function, files-in-below-directory
, to
list the ‘.el’ files in the directory it is told to check, and in
any directories below that directory.
This gives us a hint on how to construct
files-in-below-directory
: within a directory, the function
should add ‘.el’ filenames to a list; and if, within a directory,
the function comes upon a sub-directory, it should go into that
sub-directory and repeat its actions.
However, we should note that every directory contains a name that
refers to itself, called . (“dot”), and a name that refers to
its parent directory, called .. (“dot dot”). (In
/, the root directory, .. refers to itself, since
/ has no parent.) Clearly, we do not want our
files-in-below-directory
function to enter those directories,
since they always lead us, directly or indirectly, to the current
directory.
Consequently, our files-in-below-directory
function must do
several tasks:
- Check to see whether it is looking at a filename that ends in ‘.el’; and if so, add its name to a list.
- Check to see whether it is looking at a filename that is the name of a
directory; and if so,
- - Check to see whether it is looking at . or ..; and if so skip it.
- - Or else, go into that directory and repeat the process.
Let’s write a function definition to do these tasks. We will use a
while
loop to move from one filename to another within a
directory, checking what needs to be done; and we will use a recursive
call to repeat the actions on each sub-directory. The recursive
pattern is Accumulate
(see Recursive Pattern: accumulate),
using append
as the combiner.
Here is the function:
(defun files-in-below-directory (directory) "List the .el files in DIRECTORY and in its sub-directories." ;; Although the function will be used non-interactively, ;; it will be easier to test if we make it interactive. ;; The directory will have a name such as ;; "/usr/local/share/emacs/22.1.1/lisp/" (interactive "DDirectory name: ")
(let (el-files-list (current-directory-list (directory-files-and-attributes directory t))) ;; while we are in the current directory (while current-directory-list
(cond ;; check to see whether filename ends in '.el' ;; and if so, add its name to a list. ((equal ".el" (substring (car (car current-directory-list)) -3)) (setq el-files-list (cons (car (car current-directory-list)) el-files-list)))
;; check whether filename is that of a directory ((eq t (car (cdr (car current-directory-list)))) ;; decide whether to skip or recurse (if (equal "." (substring (car (car current-directory-list)) -1)) ;; then do nothing since filename is that of ;; current directory or parent, "." or ".." ()
;; else descend into the directory and repeat the process (setq el-files-list (append (files-in-below-directory (car (car current-directory-list))) el-files-list))))) ;; move to the next filename in the list; this also ;; shortens the list so the while loop eventually comes to an end (setq current-directory-list (cdr current-directory-list))) ;; return the filenames el-files-list))
The files-in-below-directory
directory-files
function
takes one argument, the name of a directory.
Thus, on my system,
(length (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/"))
tells me that in and below my Lisp sources directory are 1031 ‘.el’ files.
files-in-below-directory
returns a list in reverse alphabetical
order. An expression to sort the list in alphabetical order looks
like this:
(sort (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/") 'string-lessp)
Previous: Making a List of Files, Up: Prepare the Data for Display in a Graph [Contents][Index]
14.9.3 Counting function definitions
Our immediate goal is to generate a list that tells us how many function definitions contain fewer than 10 words and symbols, how many contain between 10 and 19 words and symbols, how many contain between 20 and 29 words and symbols, and so on.
With a sorted list of numbers, this is easy: count how many elements
of the list are smaller than 10, then, after moving past the numbers
just counted, count how many are smaller than 20, then, after moving
past the numbers just counted, count how many are smaller than 30, and
so on. Each of the numbers, 10, 20, 30, 40, and the like, is one
larger than the top of that range. We can call the list of such
numbers the top-of-ranges
list.
If we wished, we could generate this list automatically, but it is simpler to write a list manually. Here it is:
(defvar top-of-ranges '(10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300) "List specifying ranges for `defuns-per-range'.")
To change the ranges, we edit this list.
Next, we need to write the function that creates the list of the
number of definitions within each range. Clearly, this function must
take the sorted-lengths
and the top-of-ranges
lists
as arguments.
The defuns-per-range
function must do two things again and
again: it must count the number of definitions within a range
specified by the current top-of-range value; and it must shift to the
next higher value in the top-of-ranges
list after counting the
number of definitions in the current range. Since each of these
actions is repetitive, we can use while
loops for the job.
One loop counts the number of definitions in the range defined by the
current top-of-range value, and the other loop selects each of the
top-of-range values in turn.
Several entries of the sorted-lengths
list are counted for each
range; this means that the loop for the sorted-lengths
list
will be inside the loop for the top-of-ranges
list, like a
small gear inside a big gear.
The inner loop counts the number of definitions within the range. It
is a simple counting loop of the type we have seen before.
(See A loop with an incrementing counter.)
The true-or-false test of the loop tests whether the value from the
sorted-lengths
list is smaller than the current value of the
top of the range. If it is, the function increments the counter and
tests the next value from the sorted-lengths
list.
The inner loop looks like this:
(while length-element-smaller-than-top-of-range (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths)))
The outer loop must start with the lowest value of the
top-of-ranges
list, and then be set to each of the succeeding
higher values in turn. This can be done with a loop like this:
(while top-of-ranges body-of-loop… (setq top-of-ranges (cdr top-of-ranges)))
Put together, the two loops look like this:
(while top-of-ranges ;; Count the number of elements within the current range. (while length-element-smaller-than-top-of-range (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths))) ;; Move to next range. (setq top-of-ranges (cdr top-of-ranges)))
In addition, in each circuit of the outer loop, Emacs should record
the number of definitions within that range (the value of
number-within-range
) in a list. We can use cons
for
this purpose. (See cons
.)
The cons
function works fine, except that the list it
constructs will contain the number of definitions for the highest
range at its beginning and the number of definitions for the lowest
range at its end. This is because cons
attaches new elements
of the list to the beginning of the list, and since the two loops are
working their way through the lengths’ list from the lower end first,
the defuns-per-range-list
will end up largest number first.
But we will want to print our graph with smallest values first and the
larger later. The solution is to reverse the order of the
defuns-per-range-list
. We can do this using the
nreverse
function, which reverses the order of a list.
For example,
(nreverse '(1 2 3 4))
produces:
(4 3 2 1)
Note that the nreverse
function is destructive—that is,
it changes the list to which it is applied; this contrasts with the
car
and cdr
functions, which are non-destructive. In
this case, we do not want the original defuns-per-range-list
,
so it does not matter that it is destroyed. (The reverse
function provides a reversed copy of a list, leaving the original list
as is.)
Put all together, the defuns-per-range
looks like this:
(defun defuns-per-range (sorted-lengths top-of-ranges) "SORTED-LENGTHS defuns in each TOP-OF-RANGES range." (let ((top-of-range (car top-of-ranges)) (number-within-range 0) defuns-per-range-list)
;; Outer loop.
(while top-of-ranges
;; Inner loop. (while (and ;; Need number for numeric test. (car sorted-lengths) (< (car sorted-lengths) top-of-range))
;; Count number of definitions within current range. (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths))) ;; Exit inner loop but remain within outer loop.
(setq defuns-per-range-list
(cons number-within-range defuns-per-range-list))
(setq number-within-range 0) ; Reset count to zero.
;; Move to next range. (setq top-of-ranges (cdr top-of-ranges)) ;; Specify next top of range value. (setq top-of-range (car top-of-ranges)))
;; Exit outer loop and count the number of defuns larger than ;; the largest top-of-range value. (setq defuns-per-range-list (cons (length sorted-lengths) defuns-per-range-list))
;; Return a list of the number of definitions within each range, ;; smallest to largest. (nreverse defuns-per-range-list)))
The function is straightforward except for one subtle feature. The true-or-false test of the inner loop looks like this:
(and (car sorted-lengths) (< (car sorted-lengths) top-of-range))
instead of like this:
(< (car sorted-lengths) top-of-range)
The purpose of the test is to determine whether the first item in the
sorted-lengths
list is less than the value of the top of the
range.
The simple version of the test works fine unless the
sorted-lengths
list has a nil
value. In that case, the
(car sorted-lengths)
expression function returns
nil
. The <
function cannot compare a number to
nil
, which is an empty list, so Emacs signals an error and
stops the function from attempting to continue to execute.
The sorted-lengths
list always becomes nil
when the
counter reaches the end of the list. This means that any attempt to
use the defuns-per-range
function with the simple version of
the test will fail.
We solve the problem by using the (car sorted-lengths)
expression in conjunction with the and
expression. The
(car sorted-lengths)
expression returns a non-nil
value so long as the list has at least one number within it, but
returns nil
if the list is empty. The and
expression
first evaluates the (car sorted-lengths)
expression, and
if it is nil
, returns false without evaluating the
<
expression. But if the (car sorted-lengths)
expression returns a non-nil
value, the and
expression
evaluates the <
expression, and returns that value as the value
of the and
expression.
This way, we avoid an error.
Here is a short test of the defuns-per-range
function. First,
evaluate the expression that binds (a shortened)
top-of-ranges
list to the list of values, then evaluate the
expression for binding the sorted-lengths
list, and then
evaluate the defuns-per-range
function.
;; (Shorter list than we will use later.)
(setq top-of-ranges
'(110 120 130 140 150
160 170 180 190 200))
(setq sorted-lengths
'(85 86 110 116 122 129 154 176 179 200 265 300 300))
(defuns-per-range sorted-lengths top-of-ranges)
The list returned looks like this:
(2 2 2 0 0 1 0 2 0 0 4)
Indeed, there are two elements of the sorted-lengths
list
smaller than 110, two elements between 110 and 119, two elements
between 120 and 129, and so on. There are four elements with a value
of 200 or larger.
Next: Your .emacs File, Previous: Counting Words in a defun
, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
15 Readying a Graph
Our goal is to construct a graph showing the numbers of function definitions of various lengths in the Emacs lisp sources.
As a practical matter, if you were creating a graph, you would
probably use a program such as gnuplot
to do the job.
(gnuplot
is nicely integrated into GNU Emacs.) In this case,
however, we create one from scratch, and in the process we will
re-acquaint ourselves with some of what we learned before and learn
more.
In this chapter, we will first write a simple graph printing function. This first definition will be a prototype, a rapidly written function that enables us to reconnoiter this unknown graph-making territory. We will discover dragons, or find that they are myth. After scouting the terrain, we will feel more confident and enhance the function to label the axes automatically.
- Printing the Columns of a Graph
- The
graph-body-print
Function - The
recursive-graph-body-print
Function - Need for Printed Axes
- Exercise
Next: The graph-body-print
Function, Up: Readying a Graph [Contents][Index]
Printing the Columns of a Graph
Since Emacs is designed to be flexible and work with all kinds of terminals, including character-only terminals, the graph will need to be made from one of the typewriter symbols. An asterisk will do; as we enhance the graph-printing function, we can make the choice of symbol a user option.
We can call this function graph-body-print
; it will take a
numbers-list
as its only argument. At this stage, we will not
label the graph, but only print its body.
The graph-body-print
function inserts a vertical column of
asterisks for each element in the numbers-list
. The height of
each line is determined by the value of that element of the
numbers-list
.
Inserting columns is a repetitive act; that means that this function can
be written either with a while
loop or recursively.
Our first challenge is to discover how to print a column of asterisks. Usually, in Emacs, we print characters onto a screen horizontally, line by line, by typing. We have two routes we can follow: write our own column-insertion function or discover whether one exists in Emacs.
To see whether there is one in Emacs, we can use the M-x apropos
command. This command is like the C-h a (command-apropos
)
command, except that the latter finds only those functions that are
commands. The M-x apropos command lists all symbols that match
a regular expression, including functions that are not interactive.
What we want to look for is some command that prints or inserts
columns. Very likely, the name of the function will contain either
the word “print” or the word “insert” or the word “column”.
Therefore, we can simply type M-x apropos RET
print\|insert\|column RET and look at the result. On my system, this
command once took quite some time, and then produced a list of 79
functions and variables. Now it does not take much time at all and
produces a list of 211 functions and variables. Scanning down the
list, the only function that looks as if it might do the job is
insert-rectangle
.
Indeed, this is the function we want; its documentation says:
insert-rectangle: Insert text of RECTANGLE with upper left corner at point. RECTANGLE's first line is inserted at point, its second line is inserted at a point vertically under point, etc. RECTANGLE should be a list of strings. After this command, the mark is at the upper left corner and point is at the lower right corner.
We can run a quick test, to make sure it does what we expect of it.
Here is the result of placing the cursor after the
insert-rectangle
expression and typing C-u C-x C-e
(eval-last-sexp
). The function inserts the strings
‘"first"’, ‘"second"’, and ‘"third"’ at and below
point. Also the function returns nil
.
(insert-rectangle '("first" "second" "third"))first second thirdnil
Of course, we won’t be inserting the text of the
insert-rectangle
expression itself into the buffer in which we
are making the graph, but will call the function from our program. We
shall, however, have to make sure that point is in the buffer at the
place where the insert-rectangle
function will insert its
column of strings.
If you are reading this in Info, you can see how this works by
switching to another buffer, such as the *scratch* buffer,
placing point somewhere in the buffer, typing M-:, typing the
insert-rectangle
expression into the minibuffer at the prompt,
and then typing RET. This causes Emacs to evaluate the
expression in the minibuffer, but to use as the value of point the
position of point in the *scratch* buffer. (M-: is the
key binding for eval-expression
. Also, nil
does not
appear in the *scratch* buffer since the expression is
evaluated in the minibuffer.)
We find when we do this that point ends up at the end of the last inserted line—that is to say, this function moves point as a side-effect. If we were to repeat the command, with point at this position, the next insertion would be below and to the right of the previous insertion. We don’t want this! If we are going to make a bar graph, the columns need to be beside each other.
So we discover that each cycle of the column-inserting while
loop must reposition point to the place we want it, and that place
will be at the top, not the bottom, of the column. Moreover, we
remember that when we print a graph, we do not expect all the columns
to be the same height. This means that the top of each column may be
at a different height from the previous one. We cannot simply
reposition point to the same line each time, but moved over to the
right—or perhaps we can…
We are planning to make the columns of the bar graph out of asterisks.
The number of asterisks in the column is the number specified by the
current element of the numbers-list
. We need to construct a
list of asterisks of the right length for each call to
insert-rectangle
. If this list consists solely of the requisite
number of asterisks, then we will have to position point the right number
of lines above the base for the graph to print correctly. This could
be difficult.
Alternatively, if we can figure out some way to pass
insert-rectangle
a list of the same length each time, then we
can place point on the same line each time, but move it over one
column to the right for each new column. If we do this, however, some
of the entries in the list passed to insert-rectangle
must be
blanks rather than asterisks. For example, if the maximum height of
the graph is 5, but the height of the column is 3, then
insert-rectangle
requires an argument that looks like this:
(" " " " "*" "*" "*")
This last proposal is not so difficult, so long as we can determine
the column height. There are two ways for us to specify the column
height: we can arbitrarily state what it will be, which would work
fine for graphs of that height; or we can search through the list of
numbers and use the maximum height of the list as the maximum height
of the graph. If the latter operation were difficult, then the former
procedure would be easiest, but there is a function built into Emacs
that determines the maximum of its arguments. We can use that
function. The function is called max
and it returns the
largest of all its arguments, which must be numbers. Thus, for
example,
(max 3 4 6 5 7 3)
returns 7. (A corresponding function called min
returns the
smallest of all its arguments.)
However, we cannot simply call max
on the numbers-list
;
the max
function expects numbers as its argument, not a list of
numbers. Thus, the following expression,
(max '(3 4 6 5 7 3))
produces the following error message;
Wrong type of argument: number-or-marker-p, (3 4 6 5 7 3)
We need a function that passes a list of arguments to a function.
This function is apply
. This function applies its first
argument (a function) to its remaining arguments, the last of which
may be a list.
For example,
(apply 'max 3 4 7 3 '(4 8 5))
returns 8.
(Incidentally, I don’t know how you would learn of this function
without a book such as this. It is possible to discover other
functions, like search-forward
or insert-rectangle
, by
guessing at a part of their names and then using apropos
. Even
though its base in metaphor is clear—apply its first argument to
the rest—I doubt a novice would come up with that particular word
when using apropos
or other aid. Of course, I could be wrong;
after all, the function was first named by someone who had to invent
it.)
The second and subsequent arguments to apply
are optional, so
we can use apply
to call a function and pass the elements of a
list to it, like this, which also returns 8:
(apply 'max '(4 8 5))
This latter way is how we will use apply
. The
recursive-lengths-list-many-files
function returns a numbers’
list to which we can apply max
(we could also apply max
to
the sorted numbers’ list; it does not matter whether the list is
sorted or not.)
Hence, the operation for finding the maximum height of the graph is this:
(setq max-graph-height (apply 'max numbers-list))
Now we can return to the question of how to create a list of strings
for a column of the graph. Told the maximum height of the graph
and the number of asterisks that should appear in the column, the
function should return a list of strings for the
insert-rectangle
command to insert.
Each column is made up of asterisks or blanks. Since the function is
passed the value of the height of the column and the number of
asterisks in the column, the number of blanks can be found by
subtracting the number of asterisks from the height of the column.
Given the number of blanks and the number of asterisks, two
while
loops can be used to construct the list:
;;; First version.
(defun column-of-graph (max-graph-height actual-height)
"Return list of strings that is one column of a graph."
(let ((insert-list nil)
(number-of-top-blanks
(- max-graph-height actual-height)))
;; Fill in asterisks.
(while (> actual-height 0)
(setq insert-list (cons "*" insert-list))
(setq actual-height (1- actual-height)))
;; Fill in blanks.
(while (> number-of-top-blanks 0)
(setq insert-list (cons " " insert-list))
(setq number-of-top-blanks
(1- number-of-top-blanks)))
;; Return whole list.
insert-list))
If you install this function and then evaluate the following expression you will see that it returns the list as desired:
(column-of-graph 5 3)
returns
(" " " " "*" "*" "*")
As written, column-of-graph
contains a major flaw: the symbols
used for the blank and for the marked entries in the column are
hard-coded as a space and asterisk. This is fine for a prototype,
but you, or another user, may wish to use other symbols. For example,
in testing the graph function, you may want to use a period in place
of the space, to make sure the point is being repositioned properly
each time the insert-rectangle
function is called; or you might
want to substitute a ‘+’ sign or other symbol for the asterisk.
You might even want to make a graph-column that is more than one
display column wide. The program should be more flexible. The way to
do that is to replace the blank and the asterisk with two variables
that we can call graph-blank
and graph-symbol
and define
those variables separately.
Also, the documentation is not well written. These considerations lead us to the second version of the function:
(defvar graph-symbol "*" "String used as symbol in graph, usually an asterisk.")
(defvar graph-blank " " "String used as blank in graph, usually a blank space. graph-blank must be the same number of columns wide as graph-symbol.")
(For an explanation of defvar
, see
Initializing a Variable with defvar
.)
;;; Second version.
(defun column-of-graph (max-graph-height actual-height)
"Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols.
The graph-symbols are contiguous entries at the end of the list. The list will be inserted as one column of a graph. The strings are either graph-blank or graph-symbol."
(let ((insert-list nil) (number-of-top-blanks (- max-graph-height actual-height)))
;; Fill in graph-symbols
.
(while (> actual-height 0)
(setq insert-list (cons graph-symbol insert-list))
(setq actual-height (1- actual-height)))
;; Fill in graph-blanks
.
(while (> number-of-top-blanks 0)
(setq insert-list (cons graph-blank insert-list))
(setq number-of-top-blanks
(1- number-of-top-blanks)))
;; Return whole list.
insert-list))
If we wished, we could rewrite column-of-graph
a third time to
provide optionally for a line graph as well as for a bar graph. This
would not be hard to do. One way to think of a line graph is that it
is no more than a bar graph in which the part of each bar that is
below the top is blank. To construct a column for a line graph, the
function first constructs a list of blanks that is one shorter than
the value, then it uses cons
to attach a graph symbol to the
list; then it uses cons
again to attach the top blanks to
the list.
It is easy to see how to write such a function, but since we don’t
need it, we will not do it. But the job could be done, and if it were
done, it would be done with column-of-graph
. Even more
important, it is worth noting that few changes would have to be made
anywhere else. The enhancement, if we ever wish to make it, is
simple.
Now, finally, we come to our first actual graph printing function.
This prints the body of a graph, not the labels for the vertical and
horizontal axes, so we can call this graph-body-print
.
Next: The recursive-graph-body-print
Function, Previous: Printing the Columns of a Graph, Up: Readying a Graph [Contents][Index]
15.1 The graph-body-print
Function
After our preparation in the preceding section, the
graph-body-print
function is straightforward. The function
will print column after column of asterisks and blanks, using the
elements of a numbers’ list to specify the number of asterisks in each
column. This is a repetitive act, which means we can use a
decrementing while
loop or recursive function for the job. In
this section, we will write the definition using a while
loop.
The column-of-graph
function requires the height of the graph
as an argument, so we should determine and record that as a local variable.
This leads us to the following template for the while
loop
version of this function:
(defun graph-body-print (numbers-list) "documentation…" (let ((height … …))
(while numbers-list insert-columns-and-reposition-point (setq numbers-list (cdr numbers-list)))))
We need to fill in the slots of the template.
Clearly, we can use the (apply 'max numbers-list)
expression to
determine the height of the graph.
The while
loop will cycle through the numbers-list
one
element at a time. As it is shortened by the (setq numbers-list
(cdr numbers-list))
expression, the CAR of each instance of the
list is the value of the argument for column-of-graph
.
At each cycle of the while
loop, the insert-rectangle
function inserts the list returned by column-of-graph
. Since
the insert-rectangle
function moves point to the lower right of
the inserted rectangle, we need to save the location of point at the
time the rectangle is inserted, move back to that position after the
rectangle is inserted, and then move horizontally to the next place
from which insert-rectangle
is called.
If the inserted columns are one character wide, as they will be if
single blanks and asterisks are used, the repositioning command is
simply (forward-char 1)
; however, the width of a column may be
greater than one. This means that the repositioning command should be
written (forward-char symbol-width)
. The symbol-width
itself is the length of a graph-blank
and can be found using
the expression (length graph-blank)
. The best place to bind
the symbol-width
variable to the value of the width of graph
column is in the varlist of the let
expression.
These considerations lead to the following function definition:
(defun graph-body-print (numbers-list) "Print a bar graph of the NUMBERS-LIST. The numbers-list consists of the Y-axis values." (let ((height (apply 'max numbers-list)) (symbol-width (length graph-blank)) from-position)
(while numbers-list (setq from-position (point)) (insert-rectangle (column-of-graph height (car numbers-list))) (goto-char from-position) (forward-char symbol-width)
;; Draw graph column by column.
(sit-for 0)
(setq numbers-list (cdr numbers-list)))
;; Place point for X axis labels.
(forward-line height)
(insert "\n")
))
The one unexpected expression in this function is the
(sit-for 0)
expression in the while
loop. This
expression makes the graph printing operation more interesting to
watch than it would be otherwise. The expression causes Emacs to
sit or do nothing for a zero length of time and then redraw the
screen. Placed here, it causes Emacs to redraw the screen column by
column. Without it, Emacs would not redraw the screen until the
function exits.
We can test graph-body-print
with a short list of numbers.
- Install
graph-symbol
,graph-blank
,column-of-graph
, which are in Printing the Columns of a Graph, andgraph-body-print
. - Copy the following expression:
(graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3))
- Switch to the *scratch* buffer and place the cursor where you want the graph to start.
- Type M-: (
eval-expression
). - Yank the
graph-body-print
expression into the minibuffer with C-y (yank)
. - Press RET to evaluate the
graph-body-print
expression.
Emacs will print a graph like this:
* * ** * **** *** **** ********* * ************ *************
Next: Need for Printed Axes, Previous: The graph-body-print
Function, Up: Readying a Graph [Contents][Index]
15.2 The recursive-graph-body-print
Function
The graph-body-print
function may also be written recursively.
The recursive solution is divided into two parts: an outside wrapper
that uses a let
expression to determine the values of several
variables that need only be found once, such as the maximum height of
the graph, and an inside function that is called recursively to print
the graph.
The wrapper is uncomplicated:
(defun recursive-graph-body-print (numbers-list) "Print a bar graph of the NUMBERS-LIST. The numbers-list consists of the Y-axis values." (let ((height (apply 'max numbers-list)) (symbol-width (length graph-blank)) from-position) (recursive-graph-body-print-internal numbers-list height symbol-width)))
The recursive function is a little more difficult. It has four parts:
the do-again-test, the printing code, the recursive call, and the
next-step-expression. The do-again-test is a when
expression that determines whether the numbers-list
contains
any remaining elements; if it does, the function prints one column of
the graph using the printing code and calls itself again. The
function calls itself again according to the value produced by the
next-step-expression which causes the call to act on a shorter
version of the numbers-list
.
(defun recursive-graph-body-print-internal (numbers-list height symbol-width) "Print a bar graph. Used within recursive-graph-body-print function."
(when numbers-list (setq from-position (point)) (insert-rectangle (column-of-graph height (car numbers-list)))
(goto-char from-position)
(forward-char symbol-width)
(sit-for 0) ; Draw graph column by column.
(recursive-graph-body-print-internal
(cdr numbers-list) height symbol-width)))
After installation, this expression can be tested; here is a sample:
(recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1))
Here is what recursive-graph-body-print
produces:
* ** * **** * **** *** * ********* ************ *************
Either of these two functions, graph-body-print
or
recursive-graph-body-print
, create the body of a graph.
Next: Exercise, Previous: The recursive-graph-body-print
Function, Up: Readying a Graph [Contents][Index]
15.3 Need for Printed Axes
A graph needs printed axes, so you can orient yourself. For a do-once project, it may be reasonable to draw the axes by hand using Emacs’s Picture mode; but a graph drawing function may be used more than once.
For this reason, I have written enhancements to the basic
print-graph-body
function that automatically print labels for
the horizontal and vertical axes. Since the label printing functions
do not contain much new material, I have placed their description in
an appendix. See A Graph with Labeled Axes.
Previous: Need for Printed Axes, Up: Readying a Graph [Contents][Index]
15.4 Exercise
Write a line graph version of the graph printing functions.
Next: Debugging, Previous: Readying a Graph, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
16 Your .emacs File
“You don’t have to like Emacs to like it”—this seemingly paradoxical statement is the secret of GNU Emacs. The plain, out-of-the-box Emacs is a generic tool. Most people who use it customize it to suit themselves.
GNU Emacs is mostly written in Emacs Lisp; this means that by writing expressions in Emacs Lisp you can change or extend Emacs.
- Emacs’s Default Configuration
- Site-wide Initialization Files
- Specifying Variables using
defcustom
- Beginning a .emacs File
- Text and Auto Fill Mode
- Mail Aliases
- Indent Tabs Mode
- Some Key Bindings
- Keymaps
- Loading Files
- Autoloading
- A Simple Extension:
line-to-top-of-window
- X11 Colors
- Miscellaneous Settings for a .emacs File
- A Modified Mode Line
Next: Site-wide Initialization Files, Up: Your .emacs File [Contents][Index]
Emacs’s Default Configuration
There are those who appreciate Emacs’s default configuration. After all, Emacs starts you in C mode when you edit a C file, starts you in Fortran mode when you edit a Fortran file, and starts you in Fundamental mode when you edit an unadorned file. This all makes sense, if you do not know who is going to use Emacs. Who knows what a person hopes to do with an unadorned file? Fundamental mode is the right default for such a file, just as C mode is the right default for editing C code. (Enough programming languages have syntaxes that enable them to share or nearly share features, so C mode is now provided by CC mode, the C Collection.)
But when you do know who is going to use Emacs—you, yourself—then it makes sense to customize Emacs.
For example, I seldom want Fundamental mode when I edit an otherwise undistinguished file; I want Text mode. This is why I customize Emacs: so it suits me.
You can customize and extend Emacs by writing or adapting a ~/.emacs file. This is your personal initialization file; its contents, written in Emacs Lisp, tell Emacs what to do.18
A ~/.emacs file contains Emacs Lisp code. You can write this
code yourself; or you can use Emacs’s customize
feature to write
the code for you. You can combine your own expressions and
auto-written Customize expressions in your .emacs file.
(I myself prefer to write my own expressions, except for those,
particularly fonts, that I find easier to manipulate using the
customize
command. I combine the two methods.)
Most of this chapter is about writing expressions yourself. It describes a simple .emacs file; for more information, see The Init File in The GNU Emacs Manual, and The Init File in The GNU Emacs Lisp Reference Manual.
Next: Specifying Variables using defcustom
, Previous: Emacs’s Default Configuration, Up: Your .emacs File [Contents][Index]
16.1 Site-wide Initialization Files
In addition to your personal initialization file, Emacs automatically loads various site-wide initialization files, if they exist. These have the same form as your .emacs file, but are loaded by everyone.
Two site-wide initialization files, site-load.el and site-init.el, are loaded into Emacs and then dumped if a dumped version of Emacs is created, as is most common. (Dumped copies of Emacs load more quickly. However, once a file is loaded and dumped, a change to it does not lead to a change in Emacs unless you load it yourself or re-dump Emacs. See Building Emacs in The GNU Emacs Lisp Reference Manual, and the INSTALL file.)
Three other site-wide initialization files are loaded automatically each time you start Emacs, if they exist. These are site-start.el, which is loaded before your .emacs file, and default.el, and the terminal type file, which are both loaded after your .emacs file.
Settings and definitions in your .emacs file will overwrite
conflicting settings and definitions in a site-start.el file,
if it exists; but the settings and definitions in a default.el
or terminal type file will overwrite those in your .emacs file.
(You can prevent interference from a terminal type file by setting
term-file-prefix
to nil
. See A
Simple Extension.)
The INSTALL file that comes in the distribution contains descriptions of the site-init.el and site-load.el files.
The loadup.el, startup.el, and loaddefs.el files control loading. These files are in the lisp directory of the Emacs distribution and are worth perusing.
The loaddefs.el file contains a good many suggestions as to what to put into your own .emacs file, or into a site-wide initialization file.
Next: Beginning a .emacs File, Previous: Site-wide Initialization Files, Up: Your .emacs File [Contents][Index]
16.2 Specifying Variables using defcustom
You can specify variables using defcustom
so that you and
others can then use Emacs’s customize
feature to set their
values. (You cannot use customize
to write function
definitions; but you can write defuns
in your .emacs
file. Indeed, you can write any Lisp expression in your .emacs
file.)
The customize
feature depends on the defcustom
macro.
Although you can use defvar
or setq
for variables that
users set, the defcustom
macro is designed for the job.
You can use your knowledge of defvar
for writing the
first three arguments for defcustom
. The first argument to
defcustom
is the name of the variable. The second argument is
the variable’s initial value, if any; and this value is set only if
the value has not already been set. The third argument is the
documentation.
The fourth and subsequent arguments to defcustom
specify types
and options; these are not featured in defvar
. (These
arguments are optional.)
Each of these arguments consists of a keyword followed by a value. Each keyword starts with the colon character ‘:’.
For example, the customizable user option variable
text-mode-hook
looks like this:
(defcustom text-mode-hook nil "Normal hook run when entering Text mode and many related modes." :type 'hook :options '(turn-on-auto-fill flyspell-mode) :group 'wp)
The name of the variable is text-mode-hook
; it has no default
value; and its documentation string tells you what it does.
The :type
keyword tells Emacs the kind of data to which
text-mode-hook
should be set and how to display the value in a
Customization buffer.
The :options
keyword specifies a suggested list of values for
the variable. Usually, :options
applies to a hook.
The list is only a suggestion; it is not exclusive; a person who sets
the variable may set it to other values; the list shown following the
:options
keyword is intended to offer convenient choices to a
user.
Finally, the :group
keyword tells the Emacs Customization
command in which group the variable is located. This tells where to
find it.
The defcustom
macro recognizes more than a dozen keywords.
For more information, see Writing Customization
Definitions in The GNU Emacs Lisp Reference Manual.
Consider text-mode-hook
as an example.
There are two ways to customize this variable. You can use the customization command or write the appropriate expressions yourself.
Using the customization command, you can type:
M-x customize
and find that the group for editing files of text is called “Text”.
Enter that group. Text Mode Hook is the first member. You can click
on its various options, such as turn-on-auto-fill
, to set the
values. After you click on the button to
Save for Future Sessions
Emacs will write an expression into your .emacs file. It will look like this:
(custom-set-variables ;; custom-set-variables was added by Custom. ;; If you edit it by hand, you could mess it up, so be careful. ;; Your init file should contain only one such instance. ;; If there is more than one, they won't work right. '(text-mode-hook '(turn-on-auto-fill text-mode-hook-identify)))
(The text-mode-hook-identify
function tells
toggle-text-mode-auto-fill
which buffers are in Text mode.
It comes on automatically.)
The custom-set-variables
function works somewhat differently
than a setq
. While I have never learned the differences, I
modify the custom-set-variables
expressions in my .emacs
file by hand: I make the changes in what appears to me to be a
reasonable manner and have not had any problems. Others prefer to use
the Customization command and let Emacs do the work for them.
Another custom-set-…
function is custom-set-faces
.
This function sets the various font faces. Over time, I have set a
considerable number of faces. Some of the time, I re-set them using
customize
; other times, I simply edit the
custom-set-faces
expression in my .emacs file itself.
The second way to customize your text-mode-hook
is to set it
yourself in your .emacs file using code that has nothing to do
with the custom-set-…
functions.
When you do this, and later use customize
, you will see a
message that says
CHANGED outside Customize; operating on it here may be unreliable.
This message is only a warning. If you click on the button to
Save for Future Sessions
Emacs will write a custom-set-…
expression near the end
of your .emacs file that will be evaluated after your
hand-written expression. It will, therefore, overrule your
hand-written expression. No harm will be done. When you do this,
however, be careful to remember which expression is active; if you
forget, you may confuse yourself.
So long as you remember where the values are set, you will have no trouble. In any event, the values are always set in your initialization file, which is usually called .emacs.
I myself use customize
for hardly anything. Mostly, I write
expressions myself.
Incidentally, to be more complete concerning defines: defsubst
defines an inline function. The syntax is just like that of
defun
. defconst
defines a symbol as a constant. The
intent is that neither programs nor users should ever change a value
set by defconst
. (You can change it; the value set is a
variable; but please do not.)
Next: Text and Auto Fill Mode, Previous: Specifying Variables using defcustom
, Up: Your .emacs File [Contents][Index]
16.3 Beginning a .emacs File
When you start Emacs, it loads your .emacs file unless you tell
it not to by specifying ‘-q’ on the command line. (The
emacs -q
command gives you a plain, out-of-the-box Emacs.)
A .emacs file contains Lisp expressions. Often, these are no more than expressions to set values; sometimes they are function definitions.
See The Init File ~/.emacs in The GNU Emacs Manual, for a short description of initialization files.
This chapter goes over some of the same ground, but is a walk among extracts from a complete, long-used .emacs file—my own.
The first part of the file consists of comments: reminders to myself. By now, of course, I remember these things, but when I started, I did not.
;;;; Bob's .emacs file ; Robert J. Chassell ; 26 September 1985
Look at that date! I started this file a long time ago. I have been adding to it ever since.
; Each section in this file is introduced by a ; line beginning with four semicolons; and each ; entry is introduced by a line beginning with ; three semicolons.
This describes the usual conventions for comments in Emacs Lisp. Everything on a line that follows a semicolon is a comment. Two, three, and four semicolons are used as subsection and section markers. (See Comments in The GNU Emacs Lisp Reference Manual, for more about comments.)
;;;; The Help Key ; Control-h is the help key; ; after typing control-h, type a letter to ; indicate the subject about which you want help. ; For an explanation of the help facility, ; type control-h two times in a row.
Just remember: type C-h two times for help.
; To find out about any mode, type control-h m ; while in that mode. For example, to find out ; about mail mode, enter mail mode and then type ; control-h m.
“Mode help”, as I call this, is very helpful. Usually, it tells you all you need to know.
Of course, you don’t need to include comments like these in your .emacs file. I included them in mine because I kept forgetting about Mode help or the conventions for comments—but I was able to remember to look here to remind myself.
Next: Mail Aliases, Previous: Beginning a .emacs File, Up: Your .emacs File [Contents][Index]
16.4 Text and Auto Fill Mode
Now we come to the part that turns on Text mode and Auto Fill mode19.
;;; Text mode and Auto Fill mode ;; The next two lines put Emacs into Text mode ;; and Auto Fill mode, and are for writers who ;; want to start writing prose rather than code. (setq-default major-mode 'text-mode) (add-hook 'text-mode-hook 'turn-on-auto-fill)
Here is the first part of this .emacs file that does something besides remind a forgetful human!
The first of the two lines in parentheses tells Emacs to turn on Text mode when you find a file, unless that file should go into some other mode, such as C mode.
When Emacs reads a file, it looks at the extension to the file name, if any. (The extension is the part that comes after a ‘.’.) If the file ends with a ‘.c’ or ‘.h’ extension then Emacs turns on C mode. Also, Emacs looks at first nonblank line of the file; if the line says ‘-*- C -*-’, Emacs turns on C mode. Emacs possesses a list of extensions and specifications that it uses automatically. In addition, Emacs looks near the last page for a per-buffer, local variables list, if any.
Now, back to the .emacs file.
Here is the line again; how does it work?
(setq-default major-mode 'text-mode)
This line is a short, but complete Emacs Lisp expression.
We are already familiar with setq
. We use a similar macro
setq-default
to set the following variable,
major-mode
20, to the subsequent value, which is text-mode
. The
single-quote before text-mode
tells Emacs to deal directly with
the text-mode
symbol, not with whatever it might stand for.
See Setting the Value of a Variable, for a reminder of how
setq
works. The main point is that there is no difference
between the procedure you use to set a value in your .emacs
file and the procedure you use anywhere else in Emacs.
Here is the next line:
(add-hook 'text-mode-hook 'turn-on-auto-fill)
In this line, the add-hook
command adds
turn-on-auto-fill
to the variable.
turn-on-auto-fill
is the name of a program, that, you guessed
it!, turns on Auto Fill mode.
Every time Emacs turns on Text mode, Emacs runs the commands hooked onto Text mode. So every time Emacs turns on Text mode, Emacs also turns on Auto Fill mode.
In brief, the first line causes Emacs to enter Text mode when you edit a file, unless the file name extension, a first non-blank line, or local variables to tell Emacs otherwise.
Text mode among other actions, sets the syntax table to work conveniently for writers. In Text mode, Emacs considers an apostrophe as part of a word like a letter; but Emacs does not consider a period or a space as part of a word. Thus, M-f moves you over ‘it's’. On the other hand, in C mode, M-f stops just after the ‘t’ of ‘it's’.
The second line causes Emacs to turn on Auto Fill mode when it turns on Text mode. In Auto Fill mode, Emacs automatically breaks a line that is too wide and brings the excessively wide part of the line down to the next line. Emacs breaks lines between words, not within them.
When Auto Fill mode is turned off, lines continue to the right as you
type them. Depending on how you set the value of
truncate-lines
, the words you type either disappear off the
right side of the screen, or else are shown, in a rather ugly and
unreadable manner, as a continuation line on the screen.
In addition, in this part of my .emacs file, I tell the Emacs fill commands to insert two spaces after a colon:
(setq colon-double-space t)
Next: Indent Tabs Mode, Previous: Text and Auto Fill Mode, Up: Your .emacs File [Contents][Index]
16.5 Mail Aliases
Here is a setq
that turns on mail aliases, along with more
reminders.
;;; Message mode ; To enter message mode, type 'C-x m' ; To enter RMAIL (for reading mail), ; type 'M-x rmail' (setq mail-aliases t)
This setq
sets the value of the variable
mail-aliases
to t
. Since t
means true, the line
says, in effect, “Yes, use mail aliases.”
Mail aliases are convenient short names for long email addresses or for lists of email addresses. The file where you keep your aliases is ~/.mailrc. You write an alias like this:
alias geo george@foobar.wiz.edu
When you write a message to George, address it to ‘geo’; the mailer will automatically expand ‘geo’ to the full address.
Next: Some Key Bindings, Previous: Mail Aliases, Up: Your .emacs File [Contents][Index]
16.6 Indent Tabs Mode
By default, Emacs inserts tabs in place of multiple spaces when it
formats a region. (For example, you might indent many lines of text
all at once with the indent-region
command.) Tabs look fine on
a terminal or with ordinary printing, but they produce badly indented
output when you use TeX or Texinfo since TeX ignores tabs.
The following turns off Indent Tabs mode:
;;; Prevent Extraneous Tabs (setq-default indent-tabs-mode nil)
Note that this line uses setq-default
rather than the
setq
that we have seen before; setq-default
sets values only in buffers that do not have their own local
values for the variable.
Next: Keymaps, Previous: Indent Tabs Mode, Up: Your .emacs File [Contents][Index]
16.7 Some Key Bindings
Now for some personal key bindings:
;;; Compare windows (global-set-key "\C-cw" 'compare-windows)
compare-windows
is a nifty command that compares the text in
your current window with text in the next window. It makes the
comparison by starting at point in each window, moving over text in
each window as far as they match. I use this command all the time.
This also shows how to set a key globally, for all modes.
The command is global-set-key
. It is followed by the
key binding. In a .emacs file, the keybinding is written as
shown: \C-c
stands for Control-C, which means to press the
control key and the c key at the same time. The w
means
to press the w key. The key binding is surrounded by double
quotation marks. In documentation, you would write this as
C-c w. (If you were binding a META key, such as
M-c, rather than a CTRL key, you would write
\M-c
in your .emacs file. See Rebinding Keys in Your Init File in The GNU Emacs Manual, for
details.)
The command invoked by the keys is compare-windows
. Note that
compare-windows
is preceded by a single-quote; otherwise, Emacs
would first try to evaluate the symbol to determine its value.
These three things, the double quotation marks, the backslash before the ‘C’, and the single-quote are necessary parts of key binding that I tend to forget. Fortunately, I have come to remember that I should look at my existing .emacs file, and adapt what is there.
As for the key binding itself: C-c w. This combines the prefix key, C-c, with a single character, in this case, w. This set of keys, C-c followed by a single character, is strictly reserved for individuals’ own use. (I call these own keys, since these are for my own use.) You should always be able to create such a key binding for your own use without stomping on someone else’s key binding. If you ever write an extension to Emacs, please avoid taking any of these keys for public use. Create a key like C-c C-w instead. Otherwise, we will run out of own keys.
Here is another key binding, with a comment:
;;; Key binding for 'occur' ; I use occur a lot, so let's bind it to a key: (global-set-key "\C-co" 'occur)
The occur
command shows all the lines in the current buffer
that contain a match for a regular expression. When the region is
active, occur
restricts matches to such region. Otherwise it
uses the entire buffer.
Matching lines are shown in a buffer called *Occur*.
That buffer serves as a menu to jump to occurrences.
Here is how to unbind a key, so it does not work:
;;; Unbind 'C-x f' (global-unset-key "\C-xf")
There is a reason for this unbinding: I found I inadvertently typed C-x f when I meant to type C-x C-f. Rather than find a file, as I intended, I accidentally set the width for filled text, almost always to a width I did not want. Since I hardly ever reset my default width, I simply unbound the key.
The following rebinds an existing key:
;;; Rebind 'C-x C-b' for 'buffer-menu' (global-set-key "\C-x\C-b" 'buffer-menu)
By default, C-x C-b runs the
list-buffers
command. This command lists
your buffers in another window. Since I
almost always want to do something in that
window, I prefer the buffer-menu
command, which not only lists the buffers,
but moves point into that window.
Next: Loading Files, Previous: Some Key Bindings, Up: Your .emacs File [Contents][Index]
16.8 Keymaps
Emacs uses keymaps to record which keys call which commands.
When you use global-set-key
to set the key binding for a single
command in all parts of Emacs, you are specifying the key binding in
current-global-map
.
Specific modes, such as C mode or Text mode, have their own keymaps; the mode-specific keymaps override the global map that is shared by all buffers.
The global-set-key
function binds, or rebinds, the global
keymap. For example, the following binds the key C-x C-b to the
function buffer-menu
:
(global-set-key "\C-x\C-b" 'buffer-menu)
Mode-specific keymaps are bound using the define-key
function,
which takes a specific keymap as an argument, as well as the key and
the command. For example, my .emacs file contains the
following expression to bind the texinfo-insert-@group
command
to C-c C-c g:
(define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group)
The texinfo-insert-@group
function itself is a little extension
to Texinfo mode that inserts ‘@group’ into a Texinfo file. I
use this command all the time and prefer to type the three strokes
C-c C-c g rather than the six strokes @ g r o u p.
(‘@group’ and its matching ‘@end group’ are commands that
keep all enclosed text together on one page; many multi-line examples
in this book are surrounded by ‘@group … @end group’.)
Here is the texinfo-insert-@group
function definition:
(defun texinfo-insert-@group () "Insert the string @group in a Texinfo buffer." (interactive) (beginning-of-line) (insert "@group\n"))
(Of course, I could have used Abbrev mode to save typing, rather than write a function to insert a word; but I prefer key strokes consistent with other Texinfo mode key bindings.)
You will see numerous define-key
expressions in
loaddefs.el as well as in the various mode libraries, such as
cc-mode.el and lisp-mode.el.
See Customizing Key Bindings in The GNU Emacs Manual, and Keymaps in The GNU Emacs Lisp Reference Manual, for more information about keymaps.
Next: Autoloading, Previous: Keymaps, Up: Your .emacs File [Contents][Index]
16.9 Loading Files
Many people in the GNU Emacs community have written extensions to Emacs. As time goes by, these extensions are often included in new releases. For example, the Calendar and Diary packages are now part of the standard GNU Emacs, as is Calc.
You can use a load
command to evaluate a complete file and
thereby install all the functions and variables in the file into Emacs.
For example:
(load "~/emacs/slowsplit")
This evaluates, i.e., loads, the slowsplit.el file or if it
exists, the faster, byte compiled slowsplit.elc file from the
emacs sub-directory of your home directory. The file contains
the function split-window-quietly
, which John Robinson wrote in
1989.
The split-window-quietly
function splits a window with the
minimum of redisplay. I installed it in 1989 because it worked well
with the slow 1200 baud terminals I was then using. Nowadays, I only
occasionally come across such a slow connection, but I continue to use
the function because I like the way it leaves the bottom half of a
buffer in the lower of the new windows and the top half in the upper
window.
To replace the key binding for the default
split-window-vertically
, you must also unset that key and bind
the keys to split-window-quietly
, like this:
(global-unset-key "\C-x2") (global-set-key "\C-x2" 'split-window-quietly)
If you load many extensions, as I do, then instead of specifying the
exact location of the extension file, as shown above, you can specify
that directory as part of Emacs’s load-path
. Then, when Emacs
loads a file, it will search that directory as well as its default
list of directories. (The default list is specified in paths.h
when Emacs is built.)
The following command adds your ~/emacs directory to the existing load path:
;;; Emacs Load Path (setq load-path (cons "~/emacs" load-path))
Incidentally, load-library
is an interactive interface to the
load
function. The complete function looks like this:
(defun load-library (library) "Load the Emacs Lisp library named LIBRARY. This is an interface to the function `load'. LIBRARY is searched for in `load-path', both with and without `load-suffixes' (as well as `load-file-rep-suffixes'). See Info node `(emacs)Lisp Libraries' for more details. See `load-file' for a different interface to `load'." (interactive (list (completing-read "Load library: " (apply-partially 'locate-file-completion-table load-path (get-load-suffixes))))) (load library))
The name of the function, load-library
, comes from the use of
“library” as a conventional synonym for “file”. The source for the
load-library
command is in the files.el library.
Another interactive command that does a slightly different job is
load-file
. See Libraries of Lisp Code for
Emacs in The GNU Emacs Manual, for information on the
distinction between load-library
and this command.
Next: A Simple Extension: line-to-top-of-window
, Previous: Loading Files, Up: Your .emacs File [Contents][Index]
16.10 Autoloading
Instead of installing a function by loading the file that contains it, or by evaluating the function definition, you can make the function available but not actually install it until it is first called. This is called autoloading.
When you execute an autoloaded function, Emacs automatically evaluates the file that contains the definition, and then calls the function.
Emacs starts quicker with autoloaded functions, since their libraries are not loaded right away; but you need to wait a moment when you first use such a function, while its containing file is evaluated.
Rarely used functions are frequently autoloaded. The
loaddefs.el library contains thousands of autoloaded functions,
from 5x5
to zone
. Of course, you may
come to use a rare function frequently. When you do, you should
load that function’s file with a load
expression in your
.emacs file.
In my .emacs file, I load 14 libraries that contain functions that would otherwise be autoloaded. (Actually, it would have been better to include these files in my dumped Emacs, but I forgot. See Building Emacs in The GNU Emacs Lisp Reference Manual, and the INSTALL file for more about dumping.)
You may also want to include autoloaded expressions in your .emacs
file. autoload
is a built-in function that takes up to five
arguments, the final three of which are optional. The first argument
is the name of the function to be autoloaded; the second is the name
of the file to be loaded. The third argument is documentation for the
function, and the fourth tells whether the function can be called
interactively. The fifth argument tells what type of
object—autoload
can handle a keymap or macro as well as a
function (the default is a function).
Here is a typical example:
(autoload 'html-helper-mode "html-helper-mode" "Edit HTML documents" t)
(html-helper-mode
is an older alternative to html-mode
,
which is a standard part of the distribution.)
This expression autoloads the html-helper-mode
function. It
takes it from the html-helper-mode.el file (or from the byte
compiled version html-helper-mode.elc, if that exists.) The
file must be located in a directory specified by load-path
.
The documentation says that this is a mode to help you edit documents
written in the HyperText Markup Language. You can call this mode
interactively by typing M-x html-helper-mode. (You need to
duplicate the function’s regular documentation in the autoload
expression because the regular function is not yet loaded, so its
documentation is not available.)
See Autoload in The GNU Emacs Lisp Reference Manual, for more information.
Next: X11 Colors, Previous: Autoloading, Up: Your .emacs File [Contents][Index]
16.11 A Simple Extension: line-to-top-of-window
Here is a simple extension to Emacs that moves the line that point is on to the top of the window. I use this all the time, to make text easier to read.
You can put the following code into a separate file and then load it from your .emacs file, or you can include it within your .emacs file.
Here is the definition:
;;; Line to top of window; ;;; replace three keystroke sequence C-u 0 C-l (defun line-to-top-of-window () "Move the line that point is on to top of window." (interactive) (recenter 0))
Now for the key binding.
Function keys as well as mouse button events and non-ASCII characters are written within square brackets, without quotation marks.
I bind line-to-top-of-window
to my F6 function key like
this:
(global-set-key [f6] 'line-to-top-of-window)
For more information, see Rebinding Keys in Your Init File in The GNU Emacs Manual.
If you run two versions of GNU Emacs, such as versions 27 and 28, and use one .emacs file, you can select which code to evaluate with the following conditional:
(cond ((= 27 emacs-major-version) ;; evaluate version 27 code ( … )) ((= 28 emacs-major-version) ;; evaluate version 28 code ( … )))
For example, recent versions blink their cursors by default. I hate such blinking, as well as other features, so I placed the following in my .emacs file21:
(when (>= emacs-major-version 21) (blink-cursor-mode 0) ;; Insert newline when you press 'C-n' (next-line) ;; at the end of the buffer (setq next-line-add-newlines t)
;; Turn on image viewing (auto-image-file-mode t)
;; Turn on menu bar (this bar has text) ;; (Use numeric argument to turn on) (menu-bar-mode 1)
;; Turn off tool bar (this bar has icons) ;; (Use numeric argument to turn on) (tool-bar-mode nil)
;; Turn off tooltip mode for tool bar ;; (This mode causes icon explanations to pop up) ;; (Use numeric argument to turn on) (tooltip-mode nil) ;; If tooltips turned on, make tips appear promptly (setq tooltip-delay 0.1) ; default is 0.7 second )
Next: Miscellaneous Settings for a .emacs File, Previous: A Simple Extension: line-to-top-of-window
, Up: Your .emacs File [Contents][Index]
16.12 X11 Colors
You can specify colors when you use Emacs with the MIT X Windowing system.
I dislike the default colors and specify my own.
Here are the expressions in my .emacs file that set values:
;; Set cursor color (set-cursor-color "white") ;; Set mouse color (set-mouse-color "white") ;; Set foreground and background (set-foreground-color "white") (set-background-color "darkblue")
;;; Set highlighting colors for isearch and drag (set-face-foreground 'highlight "white") (set-face-background 'highlight "blue")
(set-face-foreground 'region "cyan") (set-face-background 'region "blue")
(set-face-foreground 'secondary-selection "skyblue") (set-face-background 'secondary-selection "darkblue")
;; Set calendar highlighting colors (with-eval-after-load 'calendar (set-face-foreground 'diary "skyblue") (set-face-background 'holiday "slate blue") (set-face-foreground 'holiday "white"))
The various shades of blue soothe my eye and prevent me from seeing the screen flicker.
Alternatively, I could have set my specifications in various X initialization files. For example, I could set the foreground, background, cursor, and pointer (i.e., mouse) colors in my ~/.Xresources file like this:
Emacs*foreground: white Emacs*background: darkblue Emacs*cursorColor: white Emacs*pointerColor: white
In any event, since it is not part of Emacs, I set the root color of my X window in my ~/.xinitrc file, like this22:
xsetroot -solid Navy -fg white &
Next: A Modified Mode Line, Previous: X11 Colors, Up: Your .emacs File [Contents][Index]
16.13 Miscellaneous Settings for a .emacs File
Here are a few miscellaneous settings:
- - Set the shape and color of the mouse cursor:
; Cursor shapes are defined in ; '/usr/include/X11/cursorfont.h'; ; for example, the 'target' cursor is number 128; ; the 'top_left_arrow' cursor is number 132.
(let ((mpointer (x-get-resource "*mpointer" "*emacs*mpointer"))) ;; If you have not set your mouse pointer ;; then set it, otherwise leave as is: (if (eq mpointer nil) (setq mpointer "132")) ; top_left_arrow
(setq x-pointer-shape (string-to-number mpointer)) (set-mouse-color "white"))
- - Or you can set the values of a variety of features in an alist, like
this:
(setq-default default-frame-alist '((cursor-color . "white") (mouse-color . "white") (foreground-color . "white") (background-color . "DodgerBlue4") ;; (cursor-type . bar) (cursor-type . box)
(tool-bar-lines . 0) (menu-bar-lines . 1) (width . 80) (height . 58) (font . "-Misc-Fixed-Medium-R-Normal--20-200-75-75-C-100-ISO8859-1") ))
- - Convert CTRL-h into DEL and DEL
into CTRL-h.
(Some older keyboards needed this, although I have not seen the problem recently.);; Translate 'C-h' to <DEL>. ; (keyboard-translate ?\C-h ?\C-?) ;; Translate <DEL> to 'C-h'. (keyboard-translate ?\C-? ?\C-h)
- - Turn off a blinking cursor!
(if (fboundp 'blink-cursor-mode) (blink-cursor-mode -1))
or start GNU Emacs with the command
emacs -nbc
. - - When using
grep
‘-i’ Ignore case distinctions
‘-n’ Prefix each line of output with line number
‘-H’ Print the filename for each match.
‘-e’ Protect patterns beginning with a hyphen character, ‘-’(setq grep-command "grep -i -nH -e ")
- - Find an existing buffer, even if it has a different name
This avoids problems with symbolic links.(setq find-file-existing-other-name t)
- - Set your language environment and default input method
(set-language-environment "latin-1") ;; Remember you can enable or disable multilingual text input ;; with the
toggle-input-method'
(C-\) command (setq default-input-method "latin-1-prefix")If you want to write with Chinese GB characters, set this instead:
(set-language-environment "Chinese-GB") (setq default-input-method "chinese-tonepy")
Fixing Unpleasant Key Bindings
Some systems bind keys unpleasantly. Sometimes, for example, the CTRL key appears in an awkward spot rather than at the far left of the home row.
Usually, when people fix these sorts of key bindings, they do not
change their ~/.emacs file. Instead, they bind the proper keys
on their consoles with the loadkeys
or install-keymap
commands in their boot script and then include xmodmap
commands
in their .xinitrc or .Xsession file for X Windows.
For a boot script:
loadkeys /usr/share/keymaps/i386/qwerty/emacs2.kmap.gz
or
install-keymap emacs2
For a .xinitrc or .Xsession file when the Caps Lock key is at the far left of the home row:
# Bind the key labeled 'Caps Lock' to 'Control' # (Such a broken user interface suggests that keyboard manufacturers # think that computers are typewriters from 1885.) xmodmap -e "clear Lock" xmodmap -e "add Control = Caps_Lock"
In a .xinitrc or .Xsession file, to convert an ALT key to a META key:
# Some ill designed keyboards have a key labeled ALT and no Meta xmodmap -e "keysym Alt_L = Meta_L Alt_L"
Previous: Miscellaneous Settings for a .emacs File, Up: Your .emacs File [Contents][Index]
16.14 A Modified Mode Line
Finally, a feature I really like: a modified mode line.
When I work over a network, I forget which machine I am using. Also, I tend to lose track of where I am, and which line point is on.
So I reset my mode line to look like this:
-:-- foo.texi rattlesnake:/home/bob/ Line 1 (Texinfo Fill) Top
I am visiting a file called foo.texi, on my machine rattlesnake in my /home/bob buffer. I am on line 1, in Texinfo mode, and am at the top of the buffer.
My .emacs file has a section that looks like this:
;; Set a Mode Line that tells me which machine, which directory, ;; and which line I am on, plus the other customary information. (setq-default mode-line-format (quote (#("-" 0 1 (help-echo "mouse-1: select window, mouse-2: delete others ...")) mode-line-mule-info mode-line-modified mode-line-frame-identification " "
mode-line-buffer-identification " " (:eval (substring (system-name) 0 (string-match "\\..+" (system-name)))) ":" default-directory #(" " 0 1 (help-echo "mouse-1: select window, mouse-2: delete others ...")) (line-number-mode " Line %l ") global-mode-string
#(" %[(" 0 6 (help-echo "mouse-1: select window, mouse-2: delete others ...")) (:eval (format-time-string "%F")) mode-line-process minor-mode-alist #("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...))) ")%] " (-3 . "%P") ;; "-%-" )))
Here, I redefine the default mode line. Most of the parts are from the original; but I make a few changes. I set the default mode line format so as to permit various modes, such as Info, to override it.
Many elements in the list are self-explanatory:
mode-line-modified
is a variable that tells whether the buffer
has been modified, mode-name
tells the name of the mode, and so
on. However, the format looks complicated because of two features we
have not discussed.
The first string in the mode line is a dash or hyphen, ‘-’. In
the old days, it would have been specified simply as "-"
. But
nowadays, Emacs can add properties to a string, such as highlighting
or, as in this case, a help feature. If you place your mouse cursor
over the hyphen, some help information appears (By default, you must
wait seven-tenths of a second before the information appears. You can
change that timing by changing the value of tooltip-delay
.)
The new string format has a special syntax:
#("-" 0 1 (help-echo "mouse-1: select window, ..."))
The #(
begins a list. The first element of the list is the
string itself, just one ‘-’. The second and third
elements specify the range over which the fourth element applies. A
range starts after a character, so a zero means the range
starts just before the first character; a 1 means that the range ends
just after the first character. The third element is the property for
the range. It consists of a property list, a
property name, in this case, ‘help-echo’, followed by a value, in this
case, a string. The second, third, and fourth elements of this new
string format can be repeated.
See Text Properties in The GNU Emacs Lisp Reference Manual, and see Mode Line Format in The GNU Emacs Lisp Reference Manual, for more information.
mode-line-buffer-identification
displays the current buffer name. It is a list
beginning (#("%12b" 0 4 …
.
The #(
begins the list.
The ‘"%12b"’ displays the current buffer name, using the
buffer-name
function with which we are familiar; the ‘12’
specifies the maximum number of characters that will be displayed.
When a name has fewer characters, whitespace is added to fill out to
this number. (Buffer names can and often should be longer than 12
characters; this length works well in a typical 80 column wide
window.)
:eval
says to evaluate the following form and use the result as
a string to display. In this case, the expression displays the first
component of the full system name. The end of the first component is
a ‘.’ (period), so I use the string-match
function to
tell me the length of the first component. The substring from the
zeroth character to that length is the name of the machine.
This is the expression:
(:eval (substring (system-name) 0 (string-match "\\..+" (system-name))))
‘%[’ and ‘%]’ cause a pair of square brackets to appear for each recursive editing level. ‘%n’ says “Narrow” when narrowing is in effect. ‘%P’ tells you the percentage of the buffer that is above the bottom of the window, or “Top”, “Bottom”, or “All”. (A lower case ‘p’ tell you the percentage above the top of the window.) ‘%-’ inserts enough dashes to fill out the line.
Remember, you don’t have to like Emacs to like it—your own Emacs can have different colors, different commands, and different keys than a default Emacs.
On the other hand, if you want to bring up a plain out-of-the-box Emacs, with no customization, type:
emacs -q
This will start an Emacs that does not load your ~/.emacs initialization file. A plain, default Emacs. Nothing more.
Next: Conclusion, Previous: Your .emacs File, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
17 Debugging
GNU Emacs has two debuggers, debug
and edebug
. The
first is built into the internals of Emacs and is always with you;
the second requires that you instrument a function before you can use it.
Both debuggers are described extensively in Debugging Lisp Programs in The GNU Emacs Lisp Reference Manual. In this chapter, I will walk through a short example of each.
Next: debug-on-entry
, Up: Debugging [Contents][Index]
17.1 debug
Suppose you have written a function definition that is intended to
return the sum of the numbers 1 through a given number. (This is the
triangle
function discussed earlier. See Example with Decrementing Counter, for a discussion.)
However, your function definition has a bug. You have mistyped ‘1=’ for ‘1-’. Here is the broken definition:
(defun triangle-bugged (number)
"Return sum of numbers 1 through NUMBER inclusive."
(let ((total 0))
(while (> number 0)
(setq total (+ total number))
(setq number (1= number))) ; Error here.
total))
If you are reading this in Info, you can evaluate this definition in
the normal fashion. You will see triangle-bugged
appear in the
echo area.
Now evaluate the triangle-bugged
function with an
argument of 4:
(triangle-bugged 4)
This will create and enter a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) (1= number) (setq number (1= number)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) triangle-bugged(4)
eval((triangle-bugged 4) nil) eval-expression((triangle-bugged 4) nil nil 127) funcall-interactively(eval-expression (triangle-bugged 4) nil nil 127) call-interactively(eval-expression nil nil) command-execute(eval-expression) ---------- Buffer: *Backtrace* ----------
(I have reformatted this example slightly; the debugger does not fold long lines. As usual, you can quit the debugger by typing q in the *Backtrace* buffer.)
In practice, for a bug as simple as this, the Lisp error line will
tell you what you need to know to correct the definition. The
function 1=
is void.
However, suppose you are not quite certain what is going on? You can read the complete backtrace.
Emacs automatically starts the debugger that puts you in the *Backtrace* buffer. You can also start the debugger manually as described below.
Read the *Backtrace* buffer from the bottom up; it tells you
what Emacs did that led to the error. Emacs made an interactive call
to C-x C-e (eval-last-sexp
), which led to the evaluation
of the triangle-bugged
expression. Each line above tells you
what the Lisp interpreter evaluated next.
The third line from the top of the buffer is
(setq number (1= number))
Emacs tried to evaluate this expression; in order to do so, it tried to evaluate the inner expression shown on the second line from the top:
(1= number)
This is where the error occurred; as the top line says:
Debugger entered--Lisp error: (void-function 1=)
You can correct the mistake, re-evaluate the function definition, and then run your test again.
Next: debug-on-quit
and (debug)
, Previous: debug
, Up: Debugging [Contents][Index]
17.2 debug-on-entry
Emacs starts the debugger automatically when your function has an error.
Incidentally, you can start the debugger manually for all versions of Emacs; the advantage is that the debugger runs even if you do not have a bug in your code. Sometimes your code will be free of bugs!
You can enter the debugger when you call the function by calling
debug-on-entry
.
Type:
M-x debug-on-entry RET triangle-bugged RET
Now, evaluate the following:
(triangle-bugged 5)
All versions of Emacs will create a *Backtrace* buffer and tell
you that it is beginning to evaluate the triangle-bugged
function:
---------- Buffer: *Backtrace* ---------- Debugger entered--entering a function: * triangle-bugged(5) eval((triangle-bugged 5) nil)
eval-expression((triangle-bugged 5) nil nil 127) funcall-interactively(eval-expression (triangle-bugged 5) nil nil 127) call-interactively(eval-expression nil nil) command-execute(eval-expression) ---------- Buffer: *Backtrace* ----------
In the *Backtrace* buffer, type d. Emacs will evaluate
the first expression in triangle-bugged
; the buffer will look
like this:
---------- Buffer: *Backtrace* ---------- Debugger entered--beginning evaluation of function call form: * (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) * triangle-bugged(5) eval((triangle-bugged 5))
eval((triangle-bugged 5) nil) eval-expression((triangle-bugged 5) nil nil 127) funcall-interactively(eval-expression (triangle-bugged 5) nil nil 127) call-interactively(eval-expression nil nil) command-execute(eval-expression) ---------- Buffer: *Backtrace* ----------
Now, type d again, eight times, slowly. Each time you type d, Emacs will evaluate another expression in the function definition.
Eventually, the buffer will look like this:
---------- Buffer: *Backtrace* ---------- Debugger entered--beginning evaluation of function call form: * (setq number (1= number)) * (while (> number 0) (setq total (+ total number)) (setq number (1= number)))
* (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) * triangle-bugged(5) eval((triangle-bugged 5) nil)
eval-expression((triangle-bugged 5) nil nil 127) funcall-interactively(eval-expression (triangle-bugged 5) nil nil 127) call-interactively(eval-expression nil nil) command-execute(eval-expression) ---------- Buffer: *Backtrace* ----------
Finally, after you type d two more times, Emacs will reach the error, and the top two lines of the *Backtrace* buffer will look like this:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) * (1= number) … ---------- Buffer: *Backtrace* ----------
By typing d, you were able to step through the function.
You can quit a *Backtrace* buffer by typing q in it; this
quits the trace, but does not cancel debug-on-entry
.
To cancel the effect of debug-on-entry
, call
cancel-debug-on-entry
and the name of the function, like this:
M-x cancel-debug-on-entry RET triangle-bugged RET
(If you are reading this in Info, cancel debug-on-entry
now.)
Next: The edebug
Source Level Debugger, Previous: debug-on-entry
, Up: Debugging [Contents][Index]
17.3 debug-on-quit
and (debug)
In addition to setting debug-on-error
or calling debug-on-entry
,
there are two other ways to start debug
.
You can start debug
whenever you type C-g
(keyboard-quit
) by setting the variable debug-on-quit
to
t
. This is useful for debugging infinite loops.
Or, you can insert a line that says (debug)
into your code
where you want the debugger to start, like this:
(defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (debug) ; Start debugger. (setq number (1= number))) ; Error here. total))
The debug
function is described in detail in The Lisp Debugger in The GNU Emacs Lisp Reference Manual.
Next: Debugging Exercises, Previous: debug-on-quit
and (debug)
, Up: Debugging [Contents][Index]
17.4 The edebug
Source Level Debugger
Edebug is a source level debugger. Edebug normally displays the source of the code you are debugging, with an arrow at the left that shows which line you are currently executing.
You can walk through the execution of a function, line by line, or run quickly until reaching a breakpoint where execution stops.
Edebug is described in Edebug in The GNU Emacs Lisp Reference Manual.
Here is a bugged function definition for triangle-recursively
.
See Recursion in place of a counter,
for a review of it.
(defun triangle-recursively-bugged (number)
"Return sum of numbers 1 through NUMBER inclusive.
Uses recursion."
(if (= number 1)
1
(+ number
(triangle-recursively-bugged
(1= number))))) ; Error here.
Normally, you would install this definition by positioning your cursor
after the function’s closing parenthesis and typing C-x C-e
(eval-last-sexp
) or else by positioning your cursor within the
definition and typing C-M-x (eval-defun
). (By default,
the eval-defun
command works only in Emacs Lisp mode or in Lisp
Interaction mode.)
However, to prepare this function definition for Edebug, you must first instrument the code using a different command. You can do this by positioning your cursor within or just after the definition and typing
M-x edebug-defun RET
This will cause Emacs to load Edebug automatically if it is not already loaded, and properly instrument the function.
After instrumenting the function, place your cursor after the
following expression and type C-x C-e (eval-last-sexp
):
(triangle-recursively-bugged 3)
You will be jumped back to the source for
triangle-recursively-bugged
and the cursor positioned at the
beginning of the if
line of the function. Also, you will see
an arrowhead at the left hand side of that line. The arrowhead marks
the line where the function is executing. (In the following examples,
we show the arrowhead with ‘=>’; in a windowing system, you may
see the arrowhead as a solid triangle in the window fringe.)
=>∗(if (= number 1)
In the example, the location of point is displayed as ‘∗’ (in a printed book, it is displayed with a five pointed star).
If you now press SPC, point will move to the next expression to be executed; the line will look like this:
=>(if ∗(= number 1)
As you continue to press SPC, point will move from expression to
expression. At the same time, whenever an expression returns a value,
that value will be displayed in the echo area. For example, after you
move point past number
, you will see the following:
Result: 3 (#o3, #x3, ?\C-c)
This means the value of number
is 3, which is octal three,
hexadecimal three, and ASCII Control-C (the third letter of the
alphabet, in case you need to know this information).
You can continue moving through the code until you reach the line with the error. Before evaluation, that line looks like this:
=> ∗(1= number))))) ; Error here.
When you press SPC once again, you will produce an error message that says:
Symbol's function definition is void: 1=
This is the bug.
Press q to quit Edebug.
To remove instrumentation from a function definition, simply re-evaluate it with a command that does not instrument it. For example, you could place your cursor after the definition’s closing parenthesis and type C-x C-e.
Edebug does a great deal more than walk with you through a function. You can set it so it races through on its own, stopping only at an error or at specified stopping points; you can cause it to display the changing values of various expressions; you can find out how many times a function is called, and more.
Edebug is described in Edebug in The GNU Emacs Lisp Reference Manual.
Previous: The edebug
Source Level Debugger, Up: Debugging [Contents][Index]
17.5 Debugging Exercises
- Install the
count-words-example
function and then cause it to enter the built-in debugger when you call it. Run the command on a region containing two words. You will need to press d a remarkable number of times. On your system, is a hook called after the command finishes? (For information on hooks, see Command Loop Overview in The GNU Emacs Lisp Reference Manual.) - Copy
count-words-example
into the *scratch* buffer, instrument the function for Edebug, and walk through its execution. The function does not need to have a bug, although you can introduce one if you wish. If the function lacks a bug, the walk-through completes without problems. - While running Edebug, type ? to see a list of all the Edebug commands.
(The
global-edebug-prefix
is usually C-x X, i.e., CTRL-x followed by an upper case X; use this prefix for commands made outside of the Edebug debugging buffer.) - In the Edebug debugging buffer, use the p
(
edebug-bounce-point
) command to see where in the region thecount-words-example
is working. - Move point to some spot further down the function and then type the
h (
edebug-goto-here
) command to jump to that location. - Use the t (
edebug-trace-mode
) command to cause Edebug to walk through the function on its own; use an upper case T foredebug-Trace-fast-mode
. - Set a breakpoint, then run Edebug in Trace mode until it reaches the stopping point.
Next: The the-the
Function, Previous: Debugging, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
18 Conclusion
We have now reached the end of this Introduction. You have now learned enough about programming in Emacs Lisp to set values, to write simple .emacs files for yourself and your friends, and write simple customizations and extensions to Emacs.
This is a place to stop. Or, if you wish, you can now go onward, and teach yourself.
You have learned some of the basic nuts and bolts of programming. But only some. There are a great many more brackets and hinges that are easy to use that we have not touched.
A path you can follow right now lies among the sources to GNU Emacs and in The GNU Emacs Lisp Reference Manual.
The Emacs Lisp sources are an adventure. When you read the sources and come across a function or expression that is unfamiliar, you need to figure out or find out what it does.
Go to the Reference Manual. It is a thorough, complete, and fairly easy-to-read description of Emacs Lisp. It is written not only for experts, but for people who know what you know. (The Reference Manual comes with the standard GNU Emacs distribution. Like this introduction, it comes as a Texinfo source file, so you can read it on your computer and as a typeset, printed book.)
Go to the other built-in help that is part of GNU Emacs: the built-in
documentation for all functions and variables, and
xref-find-definitions
, the program that takes you to sources.
Here is an example of how I explore the sources. Because of its name,
simple.el is the file I looked at first, a long time ago. As
it happens some of the functions in simple.el are complicated,
or at least look complicated at first sight. The open-line
function, for example, looks complicated.
You may want to walk through this function slowly, as we did with the
forward-sentence
function. (See The
forward-sentence
function.) Or you may want to skip that
function and look at another, such as split-line
. You don’t
need to read all the functions. According to
count-words-in-defun
, the split-line
function contains
102 words and symbols.
Even though it is short, split-line
contains expressions
we have not studied: skip-chars-forward
, indent-to
,
current-column
and insert-and-inherit
.
Consider the skip-chars-forward
function.
In GNU Emacs, you can find out more about skip-chars-forward
by
typing C-h f (describe-function
) and the name of the
function. This gives you the function documentation.
You may be able to guess what is done by a well named function such as
indent-to
; or you can look it up, too. Incidentally, the
describe-function
function itself is in help.el; it is
one of those long, but decipherable functions. You can look up
describe-function
using the C-h f command!
In this instance, since the code is Lisp, the *Help* buffer
contains the name of the library containing the function’s source.
You can put point over the name of the library and press the RET key,
which in this situation is bound to help-follow
, and be taken
directly to the source, in the same way as M-.
(xref-find-definitions
).
The definition for describe-function
illustrates how to
customize the interactive
expression without using the standard
character codes; and it shows how to create a temporary buffer.
(The indent-to
function is written in C rather than Emacs Lisp;
it is a built-in function. help-follow
takes you to its
source as does xref-find-definitions
, when properly set up.)
You can look at a function’s source using
xref-find-definitions
, which is bound to M-. Finally,
you can find out what the Reference Manual has to say by visiting the
manual in Info, and typing i (Info-index
) and the name of
the function, or by looking up the function in the index to a printed
copy of the manual.
Similarly, you can find out what is meant by
insert-and-inherit
.
Other interesting source files include paragraphs.el, loaddefs.el, and loadup.el. The paragraphs.el file includes short, easily understood functions as well as longer ones. The loaddefs.el file contains the many standard autoloads and many keymaps. I have never looked at it all; only at parts. loadup.el is the file that loads the standard parts of Emacs; it tells you a great deal about how Emacs is built. (See Building Emacs in The GNU Emacs Lisp Reference Manual, for more about building.)
As I said, you have learned some nuts and bolts; however, and very
importantly, we have hardly touched major aspects of programming; I
have said nothing about how to sort information, except to use the
predefined sort
function; I have said nothing about how to store
information, except to use variables and lists; I have said nothing
about how to write programs that write programs. These are topics for
another, and different kind of book, a different kind of learning.
What you have done is learn enough for much practical work with GNU Emacs. What you have done is get started. This is the end of a beginning.
Next: Handling the Kill Ring, Previous: Conclusion, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
Appendix A The the-the
Function
Sometimes when you you write text, you duplicate words—as with “you
you” near the beginning of this sentence. I find that most
frequently, I duplicate “the”; hence, I call the function for
detecting duplicated words, the-the
.
As a first step, you could use the following regular expression to search for duplicates:
\\(\\w+[ \t\n]+\\)\\1
This regexp matches one or more word-constituent characters followed by one or more spaces, tabs, or newlines. However, it does not detect duplicated words on different lines, since the ending of the first word, the end of the line, is different from the ending of the second word, a space. (For more information about regular expressions, see Regular Expression Searches, as well as Syntax of Regular Expressions in The GNU Emacs Manual, and Regular Expressions in The GNU Emacs Lisp Reference Manual.)
You might try searching just for duplicated word-constituent characters but that does not work since the pattern detects doubles such as the two occurrences of “th” in “with the”.
Another possible regexp searches for word-constituent characters followed by non-word-constituent characters, reduplicated. Here, ‘\\w+’ matches one or more word-constituent characters and ‘\\W*’ matches zero or more non-word-constituent characters.
\\(\\(\\w+\\)\\W*\\)\\1
Again, not useful.
Here is the pattern that I use. It is not perfect, but good enough. ‘\\b’ matches the empty string, provided it is at the beginning or end of a word; ‘[^@ \n\t]+’ matches one or more occurrences of any characters that are not an @-sign, space, newline, or tab.
\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b
One can write more complicated expressions, but I found that this expression is good enough, so I use it.
Here is the the-the
function, as I include it in my
.emacs file, along with a handy global key binding:
(defun the-the () "Search forward for for a duplicated word." (interactive) (message "Searching for for duplicated words ...") (push-mark)
;; This regexp is not perfect ;; but is fairly good over all: (if (re-search-forward "\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move) (message "Found duplicated word.") (message "End of buffer")))
;; Bind 'the-the' to C-c \ (global-set-key "\C-c\\" 'the-the)
Here is test text:
one two two three four five five six seven
You can substitute the other regular expressions shown above in the function definition and try each of them on this list.
Next: A Graph with Labeled Axes, Previous: The the-the
Function, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
Appendix B Handling the Kill Ring
The kill ring is a list that is transformed into a ring by the
workings of the current-kill
function. The yank
and
yank-pop
commands use the current-kill
function.
This appendix describes the current-kill
function as well as
both the yank
and the yank-pop
commands, but first,
consider the workings of the kill ring.
Next: The current-kill
Function, Up: Handling the Kill Ring [Contents][Index]
What the Kill Ring Does
The kill ring has a default maximum length of sixty items; this number is too large for an explanation. Instead, set it to four. Please evaluate the following:
(setq old-kill-ring-max kill-ring-max) (setq kill-ring-max 4)
Then, please copy each line of the following indented example into the kill ring. You may kill each line with C-k or mark it and copy it with M-w.
(In a read-only buffer, such as the *info* buffer, the kill
command, C-k (kill-line
), will not remove the text,
merely copy it to the kill ring. However, your machine may beep at
you. Alternatively, for silence, you may copy the region of each line
with the M-w (kill-ring-save
) command. You must mark
each line for this command to succeed, but it does not matter at which
end you put point or mark.)
Please invoke the calls in order, so that five elements attempt to fill the kill ring:
first some text second piece of text third line fourth line of text fifth bit of text
Then find the value of kill-ring
by evaluating
kill-ring
It is:
("fifth bit of text" "fourth line of text" "third line" "second piece of text")
The first element, ‘first some text’, was dropped.
To return to the old value for the length of the kill ring, evaluate:
(setq kill-ring-max old-kill-ring-max)
Next: yank
, Previous: What the Kill Ring Does, Up: Handling the Kill Ring [Contents][Index]
B.1 The current-kill
Function
The current-kill
function changes the element in the kill ring
to which kill-ring-yank-pointer
points. (Also, the
kill-new
function sets kill-ring-yank-pointer
to point
to the latest element of the kill ring. The kill-new
function is used directly or indirectly by kill-append
,
copy-region-as-kill
, kill-ring-save
, kill-line
,
and kill-region
.)
Next: current-kill
in Outline, Up: The current-kill
Function [Contents][Index]
The code for current-kill
The current-kill
function is used by yank
and by
yank-pop
. Here is the code for current-kill
:
(defun current-kill (n &optional do-not-move) "Rotate the yanking point by N places, and then return that kill. If N is zero and `interprogram-paste-function' is set to a function that returns a string or a list of strings, and if that function doesn't return nil, then that string (or list) is added to the front of the kill ring and the string (or first string in the list) is returned as the latest kill.
If N is not zero, and if `yank-pop-change-selection' is non-nil, use `interprogram-cut-function' to transfer the kill at the new yank point into the window system selection.
If optional arg DO-NOT-MOVE is non-nil, then don't actually move the yanking point; just return the Nth kill forward." (let ((interprogram-paste (and (= n 0) interprogram-paste-function (funcall interprogram-paste-function))))
(if interprogram-paste (progn ;; Disable the interprogram cut function when we add the new ;; text to the kill ring, so Emacs doesn't try to own the ;; selection, with identical text. (let ((interprogram-cut-function nil)) (if (listp interprogram-paste) (mapc 'kill-new (nreverse interprogram-paste)) (kill-new interprogram-paste))) (car kill-ring))
(or kill-ring (error "Kill ring is empty")) (let ((ARGth-kill-element (nthcdr (mod (- n (length kill-ring-yank-pointer)) (length kill-ring)) kill-ring))) (unless do-not-move (setq kill-ring-yank-pointer ARGth-kill-element) (when (and yank-pop-change-selection (> n 0) interprogram-cut-function) (funcall interprogram-cut-function (car ARGth-kill-element)))) (car ARGth-kill-element)))))
Remember also that the kill-new
function sets
kill-ring-yank-pointer
to the latest element of the kill
ring, which means that all the functions that call it set the value
indirectly: kill-append
, copy-region-as-kill
,
kill-ring-save
, kill-line
, and kill-region
.
Here is the line in kill-new
, which is explained in
The kill-new
function.
(setq kill-ring-yank-pointer kill-ring)
Previous: The code for current-kill
, Up: The current-kill
Function [Contents][Index]
current-kill
in Outline
The current-kill
function looks complex, but as usual, it can
be understood by taking it apart piece by piece. First look at it in
skeletal form:
(defun current-kill (n &optional do-not-move) "Rotate the yanking point by N places, and then return that kill." (let varlist body…)
This function takes two arguments, one of which is optional. It has a documentation string. It is not interactive.
The Body of current-kill
The body of the function definition is a let
expression, which
itself has a body as well as a varlist.
The let
expression declares a variable that will be only usable
within the bounds of this function. This variable is called
interprogram-paste
and is for copying to another program. It
is not for copying within this instance of GNU Emacs. Most window
systems provide a facility for interprogram pasting. Sadly, that
facility usually provides only for the last element. Most windowing
systems have not adopted a ring of many possibilities, even though
Emacs has provided it for decades.
The if
expression has two parts, one if there exists
interprogram-paste
and one if not.
Let us consider the else-part of the current-kill
function. (The then-part uses the kill-new
function, which
we have already described. See The
kill-new
function.)
(or kill-ring (error "Kill ring is empty")) (let ((ARGth-kill-element (nthcdr (mod (- n (length kill-ring-yank-pointer)) (length kill-ring)) kill-ring))) (or do-not-move (setq kill-ring-yank-pointer ARGth-kill-element)) (car ARGth-kill-element))
The code first checks whether the kill ring has content; otherwise it signals an error.
Note that the or
expression is very similar to testing length
with an if
:
(if (zerop (length kill-ring)) ; if-part (error "Kill ring is empty")) ; then-part ;; No else-part
If there is not anything in the kill ring, its length must be zero and
an error message sent to the user: ‘Kill ring is empty’. The
current-kill
function uses an or
expression which is
simpler. But an if
expression reminds us what goes on.
This if
expression uses the function zerop
which returns
true if the value it is testing is zero. When zerop
tests
true, the then-part of the if
is evaluated. The then-part is a
list starting with the function error
, which is a function that
is similar to the message
function
(see The message
Function) in that
it prints a one-line message in the echo area. However, in addition
to printing a message, error
also stops evaluation of the
function within which it is embedded. This means that the rest of the
function will not be evaluated if the length of the kill ring is zero.
Then the current-kill
function selects the element to return.
The selection depends on the number of places that current-kill
rotates and on where kill-ring-yank-pointer
points.
Next, either the optional do-not-move
argument is true or the
current value of kill-ring-yank-pointer
is set to point to the
list. Finally, another expression returns the first element of the
list even if the do-not-move
argument is true.
Next: Determining the Element, Previous: The Body of current-kill
, Up: current-kill
in Outline [Contents][Index]
Digression about the word “error”
In my opinion, it is slightly misleading, at least to humans, to use
the term “error” as the name of the error
function. A better
term would be “cancel”. Strictly speaking, of course, you cannot
point to, much less rotate a pointer to a list that has no length, so
from the point of view of the computer, the word “error” is correct.
But a human expects to attempt this sort of thing, if only to find out
whether the kill ring is full or empty. This is an act of
exploration.
From the human point of view, the act of exploration and discovery is not necessarily an error, and therefore should not be labeled as one, even in the bowels of a computer. As it is, the code in Emacs implies that a human who is acting virtuously, by exploring his or her environment, is making an error. This is bad. Even though the computer takes the same steps as it does when there is an error, a term such as “cancel” would have a clearer connotation.
Previous: Digression about the word “error”, Up: current-kill
in Outline [Contents][Index]
Determining the Element
Among other actions, the else-part of the if
expression sets
the value of kill-ring-yank-pointer
to
ARGth-kill-element
when the kill ring has something in it and
the value of do-not-move
is nil
.
The code looks like this:
(nthcdr (mod (- n (length kill-ring-yank-pointer)) (length kill-ring)) kill-ring)))
This needs some examination. Unless it is not supposed to move the
pointer, the current-kill
function changes where
kill-ring-yank-pointer
points.
That is what the
(setq kill-ring-yank-pointer ARGth-kill-element))
expression does. Also, clearly, ARGth-kill-element
is being
set to be equal to some CDR of the kill ring, using the
nthcdr
function that is described in an earlier section.
(See copy-region-as-kill
.) How does it do this?
As we have seen before (see nthcdr
), the nthcdr
function
works by repeatedly taking the CDR of a list—it takes the
CDR of the CDR of the CDR …
The two following expressions produce the same result:
(setq kill-ring-yank-pointer (cdr kill-ring)) (setq kill-ring-yank-pointer (nthcdr 1 kill-ring))
However, the nthcdr
expression is more complicated. It uses
the mod
function to determine which CDR to select.
(You will remember to look at inner functions first; indeed, we will
have to go inside the mod
.)
The mod
function returns the value of its first argument modulo
the second; that is to say, it returns the remainder after dividing
the first argument by the second. The value returned has the same
sign as the second argument.
Thus,
(mod 12 4)
⇒ 0 ;; because there is no remainder
(mod 13 4)
⇒ 1
In this case, the first argument is often smaller than the second. That is fine.
(mod 0 4) ⇒ 0 (mod 1 4) ⇒ 1
We can guess what the -
function does. It is like +
but
subtracts instead of adds; the -
function subtracts its second
argument from its first. Also, we already know what the length
function does (see Find the Length of a List: length
). It returns the length of a list.
And n
is the name of the required argument to the
current-kill
function.
So when the first argument to nthcdr
is zero, the nthcdr
expression returns the whole list, as you can see by evaluating the
following:
;; kill-ring-yank-pointer and kill-ring have a length of four ;; and (mod (- 0 4) 4) ⇒ 0 (nthcdr (mod (- 0 4) 4) '("fourth line of text" "third line" "second piece of text" "first some text"))
When the first argument to the current-kill
function is one,
the nthcdr
expression returns the list without its first
element.
(nthcdr (mod (- 1 4) 4) '("fourth line of text" "third line" "second piece of text" "first some text"))
Incidentally, both kill-ring
and kill-ring-yank-pointer
are global variables. That means that any expression in Emacs
Lisp can access them. They are not like the local variables set by
let
or like the symbols in an argument list.
Local variables can only be accessed
within the let
that defines them or the function that specifies
them in an argument list (and within expressions called by them).
(See let
Prevents Confusion, and
The defun
Macro.)
Next: yank-pop
, Previous: The current-kill
Function, Up: Handling the Kill Ring [Contents][Index]
B.2 yank
After learning about current-kill
, the code for the
yank
function is almost easy.
The yank
function does not use the
kill-ring-yank-pointer
variable directly. It calls
insert-for-yank
which calls current-kill
which sets the
kill-ring-yank-pointer
variable.
The code looks like this:
(defun yank (&optional arg) "Reinsert (\"paste\") the last stretch of killed text. More precisely, reinsert the stretch of killed text most recently killed OR yanked. Put point at end, and set mark at beginning. With just \\[universal-argument] as argument, same but put point at beginning (and mark at end). With argument N, reinsert the Nth most recently killed stretch of killed text. When this command inserts killed text into the buffer, it honors `yank-excluded-properties' and `yank-handler' as described in the doc string for `insert-for-yank-1', which see. See also the command `yank-pop' (\\[yank-pop])."
(interactive "*P") (setq yank-window-start (window-start)) ;; If we don't get all the way thru, make last-command indicate that ;; for the following command. (setq this-command t) (push-mark (point))
(insert-for-yank (current-kill (cond ((listp arg) 0) ((eq arg '-) -2) (t (1- arg))))) (if (consp arg) ;; This is like exchange-point-and-mark, but doesn't activate the mark. ;; It is cleaner to avoid activation, even though the command ;; loop would deactivate the mark because we inserted text. (goto-char (prog1 (mark t) (set-marker (mark-marker) (point) (current-buffer)))))
;; If we do get all the way thru, make this-command indicate that. (if (eq this-command t) (setq this-command 'yank)) nil)
The key expression is insert-for-yank
, which inserts the string
returned by current-kill
, but removes some text properties from
it.
However, before getting to that expression, the function sets the value
of yank-window-start
to the position returned by the
(window-start)
expression, the position at which the display
currently starts. The yank
function also sets
this-command
and pushes the mark.
After it yanks the appropriate element, if the optional argument is a CONS rather than a number or nothing, it puts point at beginning of the yanked text and mark at its end.
(The prog1
function is like progn
but returns the value
of its first argument rather than the value of its last argument. Its
first argument is forced to return the buffer’s mark as an integer.
You can see the documentation for these functions by placing point
over them in this buffer and then typing C-h f
(describe-function
) followed by a RET; the default is the
function.)
The last part of the function tells what to do when it succeeds.
Next: The ring.el File, Previous: yank
, Up: Handling the Kill Ring [Contents][Index]
B.3 yank-pop
After understanding yank
and current-kill
, you know how
to approach the yank-pop
function. Leaving out the
documentation to save space, it looks like this:
(defun yank-pop (&optional arg) "…" (interactive "*p") (if (not (eq last-command 'yank)) (error "Previous command was not a yank"))
(setq this-command 'yank) (unless arg (setq arg 1)) (let ((inhibit-read-only t) (before (< (point) (mark t))))
(if before (funcall (or yank-undo-function 'delete-region) (point) (mark t)) (funcall (or yank-undo-function 'delete-region) (mark t) (point))) (setq yank-undo-function nil)
(set-marker (mark-marker) (point) (current-buffer)) (insert-for-yank (current-kill arg)) ;; Set the window start back where it was in the yank command, ;; if possible. (set-window-start (selected-window) yank-window-start t)
(if before ;; This is like exchange-point-and-mark, ;; but doesn't activate the mark. ;; It is cleaner to avoid activation, even though the command ;; loop would deactivate the mark because we inserted text. (goto-char (prog1 (mark t) (set-marker (mark-marker) (point) (current-buffer)))))) nil)
The function is interactive with a small ‘p’ so the prefix
argument is processed and passed to the function. The command can
only be used after a previous yank; otherwise an error message is
sent. This check uses the variable last-command
which is set
by yank
and is discussed elsewhere.
(See copy-region-as-kill
.)
The let
clause sets the variable before
to true or false
depending whether point is before or after mark and then the region
between point and mark is deleted. This is the region that was just
inserted by the previous yank and it is this text that will be
replaced.
funcall
calls its first argument as a function, passing
remaining arguments to it. The first argument is whatever the
or
expression returns. The two remaining arguments are the
positions of point and mark set by the preceding yank
command.
There is more, but that is the hardest part.
Previous: yank-pop
, Up: Handling the Kill Ring [Contents][Index]
B.4 The ring.el File
Interestingly, GNU Emacs possesses a file called ring.el that
provides many of the features we just discussed. But functions such
as kill-ring-yank-pointer
do not use this library, possibly
because they were written earlier.
Next: Free Software and Free Manuals, Previous: Handling the Kill Ring, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
Appendix C A Graph with Labeled Axes
Printed axes help you understand a graph. They convey scale. In an earlier chapter (see Readying a Graph), we wrote the code to print the body of a graph. Here we write the code for printing and labeling vertical and horizontal axes, along with the body itself.
- Labeled Example Graph
- The
print-graph
Varlist - The
print-Y-axis
Function - The
print-X-axis
Function - Printing the Whole Graph
Next: The print-graph
Varlist, Up: A Graph with Labeled Axes [Contents][Index]
Labeled Example Graph
Since insertions fill a buffer to the right and below point, the new graph printing function should first print the Y or vertical axis, then the body of the graph, and finally the X or horizontal axis. This sequence lays out for us the contents of the function:
- Set up code.
- Print Y axis.
- Print body of graph.
- Print X axis.
Here is an example of how a finished graph should look:
10 - * * * * ** * *** 5 - * ******* * *** ******* ************* *************** 1 - **************** | | | | 1 5 10 15
In this graph, both the vertical and the horizontal axes are labeled with numbers. However, in some graphs, the horizontal axis is time and would be better labeled with months, like this:
5 - * * ** * ******* ********** ** 1 - ************** | ^ | Jan June Jan
Indeed, with a little thought, we can easily come up with a variety of vertical and horizontal labeling schemes. Our task could become complicated. But complications breed confusion. Rather than permit this, it is better choose a simple labeling scheme for our first effort, and to modify or replace it later.
These considerations suggest the following outline for the
print-graph
function:
(defun print-graph (numbers-list) "documentation…" (let ((height … …))
(print-Y-axis height … ) (graph-body-print numbers-list) (print-X-axis … )))
We can work on each part of the print-graph
function definition
in turn.
Next: The print-Y-axis
Function, Previous: Labeled Example Graph, Up: A Graph with Labeled Axes [Contents][Index]
C.1 The print-graph
Varlist
In writing the print-graph
function, the first task is to write
the varlist in the let
expression. (We will leave aside for the
moment any thoughts about making the function interactive or about the
contents of its documentation string.)
The varlist should set several values. Clearly, the top of the label
for the vertical axis must be at least the height of the graph, which
means that we must obtain this information here. Note that the
print-graph-body
function also requires this information. There
is no reason to calculate the height of the graph in two different
places, so we should change print-graph-body
from the way we
defined it earlier to take advantage of the calculation.
Similarly, both the function for printing the X axis labels and the
print-graph-body
function need to learn the value of the width of
each symbol. We can perform the calculation here and change the
definition for print-graph-body
from the way we defined it in the
previous chapter.
The length of the label for the horizontal axis must be at least as long as the graph. However, this information is used only in the function that prints the horizontal axis, so it does not need to be calculated here.
These thoughts lead us directly to the following form for the varlist
in the let
for print-graph
:
(let ((height (apply 'max numbers-list)) ; First version.
(symbol-width (length graph-blank)))
As we shall see, this expression is not quite right.
Next: The print-X-axis
Function, Previous: The print-graph
Varlist, Up: A Graph with Labeled Axes [Contents][Index]
C.2 The print-Y-axis
Function
The job of the print-Y-axis
function is to print a label for
the vertical axis that looks like this:
10 - 5 - 1 -
The function should be passed the height of the graph, and then should construct and insert the appropriate numbers and marks.
- The
print-Y-axis
Function in Detail - What height should the label be?
- Side Trip: Compute a Remainder
- Construct a Y Axis Element
- Create a Y Axis Column
- The Not Quite Final Version of
print-Y-axis
The print-Y-axis
Function in Detail
It is easy enough to see in the figure what the Y axis label should look like; but to say in words, and then to write a function definition to do the job is another matter. It is not quite true to say that we want a number and a tic every five lines: there are only three lines between the ‘1’ and the ‘5’ (lines 2, 3, and 4), but four lines between the ‘5’ and the ‘10’ (lines 6, 7, 8, and 9). It is better to say that we want a number and a tic mark on the base line (number 1) and then that we want a number and a tic on the fifth line from the bottom and on every line that is a multiple of five.
Next: Side Trip: Compute a Remainder, Previous: The print-Y-axis
Function in Detail, Up: The print-Y-axis
Function [Contents][Index]
What height should the label be?
The next issue is what height the label should be? Suppose the maximum
height of tallest column of the graph is seven. Should the highest
label on the Y axis be ‘5 -’, and should the graph stick up above
the label? Or should the highest label be ‘7 -’, and mark the peak
of the graph? Or should the highest label be 10 -
, which is a
multiple of five, and be higher than the topmost value of the graph?
The latter form is preferred. Most graphs are drawn within rectangles
whose sides are an integral number of steps long—5, 10, 15, and so
on for a step distance of five. But as soon as we decide to use a
step height for the vertical axis, we discover that the simple
expression in the varlist for computing the height is wrong. The
expression is (apply 'max numbers-list)
. This returns the
precise height, not the maximum height plus whatever is necessary to
round up to the nearest multiple of five. A more complex expression
is required.
As usual in cases like this, a complex problem becomes simpler if it is divided into several smaller problems.
First, consider the case when the highest value of the graph is an integral multiple of five—when it is 5, 10, 15, or some higher multiple of five. We can use this value as the Y axis height.
A fairly simply way to determine whether a number is a multiple of five is to divide it by five and see if the division results in a remainder. If there is no remainder, the number is a multiple of five. Thus, seven divided by five has a remainder of two, and seven is not an integral multiple of five. Put in slightly different language, more reminiscent of the classroom, five goes into seven once, with a remainder of two. However, five goes into ten twice, with no remainder: ten is an integral multiple of five.
Next: Construct a Y Axis Element, Previous: What height should the label be?, Up: The print-Y-axis
Function [Contents][Index]
C.2.1 Side Trip: Compute a Remainder
In Lisp, the function for computing a remainder is %
. The
function returns the remainder of its first argument divided by its
second argument. As it happens, %
is a function in Emacs Lisp
that you cannot discover using apropos
: you find nothing if you
type M-x apropos RET remainder RET. The only way to
learn of the existence of %
is to read about it in a book such
as this or in the Emacs Lisp sources.
You can try the %
function by evaluating the following two
expressions:
(% 7 5) (% 10 5)
The first expression returns 2 and the second expression returns 0.
To test whether the returned value is zero or some other number, we
can use the zerop
function. This function returns t
if
its argument, which must be a number, is zero.
(zerop (% 7 5)) ⇒ nil (zerop (% 10 5)) ⇒ t
Thus, the following expression will return t
if the height
of the graph is evenly divisible by five:
(zerop (% height 5))
(The value of height
, of course, can be found from (apply
'max numbers-list)
.)
On the other hand, if the value of height
is not a multiple of
five, we want to reset the value to the next higher multiple of five.
This is straightforward arithmetic using functions with which we are
already familiar. First, we divide the value of height
by five
to determine how many times five goes into the number. Thus, five
goes into twelve twice. If we add one to this quotient and multiply by
five, we will obtain the value of the next multiple of five that is
larger than the height. Five goes into twelve twice. Add one to two,
and multiply by five; the result is fifteen, which is the next multiple
of five that is higher than twelve. The Lisp expression for this is:
(* (1+ (/ height 5)) 5)
For example, if you evaluate the following, the result is 15:
(* (1+ (/ 12 5)) 5)
All through this discussion, we have been using 5 as the value
for spacing labels on the Y axis; but we may want to use some other
value. For generality, we should replace 5 with a variable to
which we can assign a value. The best name I can think of for this
variable is Y-axis-label-spacing
.
Using this term, and an if
expression, we produce the
following:
(if (zerop (% height Y-axis-label-spacing))
height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing))
This expression returns the value of height
itself if the height
is an even multiple of the value of the Y-axis-label-spacing
or
else it computes and returns a value of height
that is equal to
the next higher multiple of the value of the Y-axis-label-spacing
.
We can now include this expression in the let
expression of the
print-graph
function (after first setting the value of
Y-axis-label-spacing
):
(defvar Y-axis-label-spacing 5 "Number of lines from one Y axis label to next.")
… (let* ((height (apply 'max numbers-list)) (height-of-top-line (if (zerop (% height Y-axis-label-spacing)) height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))
(symbol-width (length graph-blank))))
…
(Note use of the let*
function: the initial value of height is
computed once by the (apply 'max numbers-list)
expression and
then the resulting value of height
is used to compute its
final value. See The let*
expression, for
more about let*
.)
Next: Create a Y Axis Column, Previous: Side Trip: Compute a Remainder, Up: The print-Y-axis
Function [Contents][Index]
C.2.2 Construct a Y Axis Element
When we print the vertical axis, we want to insert strings such as ‘5 -’ and ‘10 - ’ every five lines. Moreover, we want the numbers and dashes to line up, so shorter numbers must be padded with leading spaces. If some of the strings use two digit numbers, the strings with single digit numbers must include a leading blank space before the number.
To figure out the length of the number, the length
function is
used. But the length
function works only with a string, not with
a number. So the number has to be converted from being a number to
being a string. This is done with the number-to-string
function.
For example,
(length (number-to-string 35)) ⇒ 2 (length (number-to-string 100)) ⇒ 3
(number-to-string
is also called int-to-string
; you will
see this alternative name in various sources.)
In addition, in each label, each number is followed by a string such
as ‘ - ’, which we will call the Y-axis-tic
marker.
This variable is defined with defvar
:
(defvar Y-axis-tic " - " "String that follows number in a Y axis label.")
The length of the Y label is the sum of the length of the Y axis tic mark and the length of the number of the top of the graph.
(length (concat (number-to-string height) Y-axis-tic)))
This value will be calculated by the print-graph
function in
its varlist as full-Y-label-width
and passed on. (Note that we
did not think to include this in the varlist when we first proposed it.)
To make a complete vertical axis label, a tic mark is concatenated
with a number; and the two together may be preceded by one or more
spaces depending on how long the number is. The label consists of
three parts: the (optional) leading spaces, the number, and the tic
mark. The function is passed the value of the number for the specific
row, and the value of the width of the top line, which is calculated
(just once) by print-graph
.
(defun Y-axis-element (number full-Y-label-width) "Construct a NUMBERed label element. A numbered element looks like this ` 5 - ', and is padded as needed so all line up with the element for the largest number."
(let* ((leading-spaces (- full-Y-label-width (length (concat (number-to-string number) Y-axis-tic)))))
(concat (make-string leading-spaces ? ) (number-to-string number) Y-axis-tic)))
The Y-axis-element
function concatenates together the leading
spaces, if any; the number, as a string; and the tic mark.
To figure out how many leading spaces the label will need, the function subtracts the actual length of the label—the length of the number plus the length of the tic mark—from the desired label width.
Blank spaces are inserted using the make-string
function. This
function takes two arguments: the first tells it how long the string
will be and the second is a symbol for the character to insert, in a
special format. The format is a question mark followed by a blank
space, like this, ‘? ’. See Character Type in The GNU Emacs Lisp Reference Manual, for a description of the
syntax for characters. (Of course, you might want to replace the
blank space by some other character … You know what to do.)
The number-to-string
function is used in the concatenation
expression, to convert the number to a string that is concatenated
with the leading spaces and the tic mark.
Next: The Not Quite Final Version of print-Y-axis
, Previous: Construct a Y Axis Element, Up: The print-Y-axis
Function [Contents][Index]
C.2.3 Create a Y Axis Column
The preceding functions provide all the tools needed to construct a function that generates a list of numbered and blank strings to insert as the label for the vertical axis:
(defun Y-axis-column (height width-of-label) "Construct list of Y axis labels and blank strings. For HEIGHT of line above base and WIDTH-OF-LABEL." (let (Y-axis)
(while (> height 1)
(if (zerop (% height Y-axis-label-spacing))
;; Insert label.
(setq Y-axis
(cons
(Y-axis-element height width-of-label)
Y-axis))
;; Else, insert blanks. (setq Y-axis (cons (make-string width-of-label ? ) Y-axis))) (setq height (1- height))) ;; Insert base line. (setq Y-axis (cons (Y-axis-element 1 width-of-label) Y-axis)) (nreverse Y-axis)))
In this function, we start with the value of height
and
repetitively subtract one from its value. After each subtraction, we
test to see whether the value is an integral multiple of the
Y-axis-label-spacing
. If it is, we construct a numbered label
using the Y-axis-element
function; if not, we construct a
blank label using the make-string
function. The base line
consists of the number one followed by a tic mark.
Previous: Create a Y Axis Column, Up: The print-Y-axis
Function [Contents][Index]
C.2.4 The Not Quite Final Version of print-Y-axis
The list constructed by the Y-axis-column
function is passed to
the print-Y-axis
function, which inserts the list as a column.
(defun print-Y-axis (height full-Y-label-width) "Insert Y axis using HEIGHT and FULL-Y-LABEL-WIDTH. Height must be the maximum height of the graph. Full width is the width of the highest label element." ;; Value of height and full-Y-label-width ;; are passed by print-graph.
(let ((start (point))) (insert-rectangle (Y-axis-column height full-Y-label-width)) ;; Place point ready for inserting graph. (goto-char start) ;; Move point forward by value of full-Y-label-width (forward-char full-Y-label-width)))
The print-Y-axis
uses the insert-rectangle
function to
insert the Y axis labels created by the Y-axis-column
function.
In addition, it places point at the correct position for printing the body of
the graph.
You can test print-Y-axis
:
- Install
Y-axis-label-spacing Y-axis-tic Y-axis-element Y-axis-column print-Y-axis
- Copy the following expression:
(print-Y-axis 12 5)
- Switch to the *scratch* buffer and place the cursor where you want the axis labels to start.
- Type M-: (
eval-expression
). - Yank the
graph-body-print
expression into the minibuffer with C-y (yank)
. - Press RET to evaluate the expression.
Emacs will print labels vertically, the top one being ‘10 - ’. (The print-graph
function will pass the value of
height-of-top-line
, which in this case will end up as 15,
thereby getting rid of what might appear as a bug.)
Next: Printing the Whole Graph, Previous: The print-Y-axis
Function, Up: A Graph with Labeled Axes [Contents][Index]
C.3 The print-X-axis
Function
X axis labels are much like Y axis labels, except that the ticks are on a line above the numbers. Labels should look like this:
| | | | 1 5 10 15
The first tic is under the first column of the graph and is preceded by
several blank spaces. These spaces provide room in rows above for the Y
axis labels. The second, third, fourth, and subsequent ticks are all
spaced equally, according to the value of X-axis-label-spacing
.
The second row of the X axis consists of numbers, preceded by several
blank spaces and also separated according to the value of the variable
X-axis-label-spacing
.
The value of the variable X-axis-label-spacing
should itself be
measured in units of symbol-width
, since you may want to change
the width of the symbols that you are using to print the body of the
graph without changing the ways the graph is labeled.
Next: X Axis Tic Marks, Up: The print-X-axis
Function [Contents][Index]
Similarities and differences
The print-X-axis
function is constructed in more or less the
same fashion as the print-Y-axis
function except that it has
two lines: the line of tic marks and the numbers. We will write a
separate function to print each line and then combine them within the
print-X-axis
function.
This is a three step process:
- Write a function to print the X axis tic marks,
print-X-axis-tic-line
. - Write a function to print the X numbers,
print-X-axis-numbered-line
. - Write a function to print both lines, the
print-X-axis
function, usingprint-X-axis-tic-line
andprint-X-axis-numbered-line
.
Previous: Similarities and differences, Up: The print-X-axis
Function [Contents][Index]
C.3.1 X Axis Tic Marks
The first function should print the X axis tic marks. We must specify the tic marks themselves and their spacing:
(defvar X-axis-label-spacing (if (boundp 'graph-blank) (* 5 (length graph-blank)) 5) "Number of units from one X axis label to next.")
(Note that the value of graph-blank
is set by another
defvar
. The boundp
predicate checks whether it has
already been set; boundp
returns nil
if it has not. If
graph-blank
were unbound and we did not use this conditional
construction, we would enter the debugger and see an error message
saying ‘Debugger entered--Lisp error:
(void-variable graph-blank)’.)
Here is the defvar
for X-axis-tic-symbol
:
(defvar X-axis-tic-symbol "|" "String to insert to point to a column in X axis.")
The goal is to make a line that looks like this:
| | | |
The first tic is indented so that it is under the first column, which is indented to provide space for the Y axis labels.
A tic element consists of the blank spaces that stretch from one tic to
the next plus a tic symbol. The number of blanks is determined by the
width of the tic symbol and the X-axis-label-spacing
.
The code looks like this:
;;; X-axis-tic-element … (concat (make-string ;; Make a string of blanks. (- (* symbol-width X-axis-label-spacing) (length X-axis-tic-symbol)) ? ) ;; Concatenate blanks with tic symbol. X-axis-tic-symbol) …
Next, we determine how many blanks are needed to indent the first tic
mark to the first column of the graph. This uses the value of
full-Y-label-width
passed it by the print-graph
function.
The code to make X-axis-leading-spaces
looks like this:
;; X-axis-leading-spaces … (make-string full-Y-label-width ? ) …
We also need to determine the length of the horizontal axis, which is the length of the numbers list, and the number of ticks in the horizontal axis:
;; X-length … (length numbers-list)
;; tic-width … (* symbol-width X-axis-label-spacing)
;; number-of-X-ticks (if (zerop (% (X-length tic-width))) (/ (X-length tic-width)) (1+ (/ (X-length tic-width))))
All this leads us directly to the function for printing the X axis tic line:
(defun print-X-axis-tic-line
(number-of-X-tics X-axis-leading-spaces X-axis-tic-element)
"Print ticks for X axis."
(insert X-axis-leading-spaces)
(insert X-axis-tic-symbol) ; Under first column.
;; Insert second tic in the right spot. (insert (concat (make-string (- (* symbol-width X-axis-label-spacing) ;; Insert white space up to second tic symbol. (* 2 (length X-axis-tic-symbol))) ? ) X-axis-tic-symbol))
;; Insert remaining ticks.
(while (> number-of-X-tics 1)
(insert X-axis-tic-element)
(setq number-of-X-tics (1- number-of-X-tics))))
The line of numbers is equally straightforward:
First, we create a numbered element with blank spaces before each number:
(defun X-axis-element (number) "Construct a numbered X axis element." (let ((leading-spaces (- (* symbol-width X-axis-label-spacing) (length (number-to-string number))))) (concat (make-string leading-spaces ? ) (number-to-string number))))
Next, we create the function to print the numbered line, starting with the number 1 under the first column:
(defun print-X-axis-numbered-line (number-of-X-tics X-axis-leading-spaces) "Print line of X-axis numbers" (let ((number X-axis-label-spacing)) (insert X-axis-leading-spaces) (insert "1")
(insert (concat
(make-string
;; Insert white space up to next number.
(- (* symbol-width X-axis-label-spacing) 2)
? )
(number-to-string number)))
;; Insert remaining numbers.
(setq number (+ number X-axis-label-spacing))
(while (> number-of-X-tics 1)
(insert (X-axis-element number))
(setq number (+ number X-axis-label-spacing))
(setq number-of-X-tics (1- number-of-X-tics)))))
Finally, we need to write the print-X-axis
that uses
print-X-axis-tic-line
and
print-X-axis-numbered-line
.
The function must determine the local values of the variables used by both
print-X-axis-tic-line
and print-X-axis-numbered-line
, and
then it must call them. Also, it must print the carriage return that
separates the two lines.
The function consists of a varlist that specifies five local variables, and calls to each of the two line printing functions:
(defun print-X-axis (numbers-list) "Print X axis labels to length of NUMBERS-LIST." (let* ((leading-spaces (make-string full-Y-label-width ? ))
;; symbol-width is provided by graph-body-print
(tic-width (* symbol-width X-axis-label-spacing))
(X-length (length numbers-list))
(X-tic (concat (make-string
;; Make a string of blanks.
(- (* symbol-width X-axis-label-spacing)
(length X-axis-tic-symbol))
? )
;; Concatenate blanks with tic symbol.
X-axis-tic-symbol))
(tic-number (if (zerop (% X-length tic-width)) (/ X-length tic-width) (1+ (/ X-length tic-width)))))
(print-X-axis-tic-line tic-number leading-spaces X-tic) (insert "\n") (print-X-axis-numbered-line tic-number leading-spaces)))
You can test print-X-axis
:
- Install
X-axis-tic-symbol
,X-axis-label-spacing
,print-X-axis-tic-line
, as well asX-axis-element
,print-X-axis-numbered-line
, andprint-X-axis
. - Copy the following expression:
(progn (let ((full-Y-label-width 5) (symbol-width 1)) (print-X-axis '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16))))
- Switch to the *scratch* buffer and place the cursor where you want the axis labels to start.
- Type M-: (
eval-expression
). - Yank the test expression into the minibuffer
with C-y (
yank)
. - Press RET to evaluate the expression.
Emacs will print the horizontal axis like this:
| | | | | 1 5 10 15 20
Previous: The print-X-axis
Function, Up: A Graph with Labeled Axes [Contents][Index]
C.4 Printing the Whole Graph
Now we are nearly ready to print the whole graph.
The function to print the graph with the proper labels follows the outline we created earlier (see A Graph with Labeled Axes), but with additions.
Here is the outline:
(defun print-graph (numbers-list) "documentation…" (let ((height … …))
(print-Y-axis height … ) (graph-body-print numbers-list) (print-X-axis … )))
- Changes for the Final Version
- Testing
print-graph
- Graphing Numbers of Words and Symbols
- A
lambda
Expression: Useful Anonymity - The
mapcar
Function - Another Bug … Most Insidious
- The Printed Graph
Next: Testing print-graph
, Up: Printing the Whole Graph [Contents][Index]
Changes for the Final Version
The final version is different from what we planned in two ways: first, it contains additional values calculated once in the varlist; second, it carries an option to specify the labels’ increment per row. This latter feature turns out to be essential; otherwise, a graph may have more rows than fit on a display or on a sheet of paper.
This new feature requires a change to the Y-axis-column
function, to add vertical-step
to it. The function looks like
this:
;;; Final version.
(defun Y-axis-column
(height width-of-label &optional vertical-step)
"Construct list of labels for Y axis.
HEIGHT is maximum height of graph.
WIDTH-OF-LABEL is maximum width of label.
VERTICAL-STEP, an option, is a positive integer
that specifies how much a Y axis label increments
for each line. For example, a step of 5 means
that each line is five units of the graph."
(let (Y-axis (number-per-line (or vertical-step 1))) (while (> height 1) (if (zerop (% height Y-axis-label-spacing))
;; Insert label.
(setq Y-axis
(cons
(Y-axis-element
(* height number-per-line)
width-of-label)
Y-axis))
;; Else, insert blanks.
(setq Y-axis
(cons
(make-string width-of-label ? )
Y-axis)))
(setq height (1- height)))
;; Insert base line.
(setq Y-axis (cons (Y-axis-element
(or vertical-step 1)
width-of-label)
Y-axis))
(nreverse Y-axis)))
The values for the maximum height of graph and the width of a symbol
are computed by print-graph
in its let
expression; so
graph-body-print
must be changed to accept them.
;;; Final version.
(defun graph-body-print (numbers-list height symbol-width)
"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values.
HEIGHT is maximum height of graph.
SYMBOL-WIDTH is number of each column."
(let (from-position) (while numbers-list (setq from-position (point)) (insert-rectangle (column-of-graph height (car numbers-list))) (goto-char from-position) (forward-char symbol-width)
;; Draw graph column by column. (sit-for 0) (setq numbers-list (cdr numbers-list))) ;; Place point for X axis labels. (forward-line height) (insert "\n")))
Finally, the code for the print-graph
function:
;;; Final version.
(defun print-graph
(numbers-list &optional vertical-step)
"Print labeled bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values.
Optionally, VERTICAL-STEP, a positive integer, specifies how much a Y axis label increments for each line. For example, a step of 5 means that each row is five units."
(let* ((symbol-width (length graph-blank))
;; height
is both the largest number
;; and the number with the most digits.
(height (apply 'max numbers-list))
(height-of-top-line
(if (zerop (% height Y-axis-label-spacing))
height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))
(vertical-step (or vertical-step 1)) (full-Y-label-width (length
(concat (number-to-string (* height-of-top-line vertical-step)) Y-axis-tic))))
(print-Y-axis height-of-top-line full-Y-label-width vertical-step)
(graph-body-print numbers-list height-of-top-line symbol-width) (print-X-axis numbers-list)))
Next: Graphing Numbers of Words and Symbols, Previous: Changes for the Final Version, Up: Printing the Whole Graph [Contents][Index]
C.4.1 Testing print-graph
We can test the print-graph
function with a short list of numbers:
- Install the final versions of
Y-axis-column
,graph-body-print
, andprint-graph
(in addition to the rest of the code.) - Copy the following expression:
(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1))
- Switch to the *scratch* buffer and place the cursor where you want the axis labels to start.
- Type M-: (
eval-expression
). - Yank the test expression into the minibuffer
with C-y (
yank)
. - Press RET to evaluate the expression.
Emacs will print a graph that looks like this:
10 - * ** * 5 - **** * **** *** * ********* ************ 1 - ************* | | | | 1 5 10 15
On the other hand, if you pass print-graph
a
vertical-step
value of 2, by evaluating this expression:
(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1) 2)
The graph looks like this:
20 - * ** * 10 - **** * **** *** * ********* ************ 2 - ************* | | | | 1 5 10 15
(A question: is the ‘2’ on the bottom of the vertical axis a bug or a feature? If you think it is a bug, and should be a ‘1’ instead, (or even a ‘0’), you can modify the sources.)
Next: A lambda
Expression: Useful Anonymity, Previous: Testing print-graph
, Up: Printing the Whole Graph [Contents][Index]
C.4.2 Graphing Numbers of Words and Symbols
Now for the graph for which all this code was written: a graph that shows how many function definitions contain fewer than 10 words and symbols, how many contain between 10 and 19 words and symbols, how many contain between 20 and 29 words and symbols, and so on.
This is a multi-step process. First make sure you have loaded all the requisite code.
It is a good idea to reset the value of top-of-ranges
in case
you have set it to some different value. You can evaluate the
following:
(setq top-of-ranges '(10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300)
Next create a list of the number of words and symbols in each range.
Evaluate the following:
(setq list-for-graph (defuns-per-range (sort (recursive-lengths-list-many-files (directory-files "/usr/local/emacs/lisp" t ".+el$")) '<) top-of-ranges))
On my old machine, this took about an hour. It looked though 303 Lisp
files in my copy of Emacs version 19.23. After all that computing,
the list-for-graph
had this value:
(537 1027 955 785 594 483 349 292 224 199 166 120 116 99 90 80 67 48 52 45 41 33 28 26 25 20 12 28 11 13 220)
This means that my copy of Emacs had 537 function definitions with fewer than 10 words or symbols in them, 1,027 function definitions with 10 to 19 words or symbols in them, 955 function definitions with 20 to 29 words or symbols in them, and so on.
Clearly, just by looking at this list we can see that most function definitions contain ten to thirty words and symbols.
Now for printing. We do not want to print a graph that is 1,030 lines high … Instead, we should print a graph that is fewer than twenty-five lines high. A graph that height can be displayed on almost any monitor, and easily printed on a sheet of paper.
This means that each value in list-for-graph
must be reduced to
one-fiftieth its present value.
Here is a short function to do just that, using two functions we have
not yet seen, mapcar
and lambda
.
(defun one-fiftieth (full-range) "Return list, each number one-fiftieth of previous." (mapcar (lambda (arg) (/ arg 50)) full-range))
Next: The mapcar
Function, Previous: Graphing Numbers of Words and Symbols, Up: Printing the Whole Graph [Contents][Index]
C.4.3 A lambda
Expression: Useful Anonymity
lambda
is the symbol for an anonymous function, a function
without a name. Every time you use an anonymous function, you need to
include its whole body.
Thus,
(lambda (arg) (/ arg 50))
is a function that returns the value resulting from
dividing whatever is passed to it as arg
by 50.
Earlier, for example, we had a function multiply-by-seven
; it
multiplied its argument by 7. This function is similar, except it
divides its argument by 50; and, it has no name. The anonymous
equivalent of multiply-by-seven
is:
(lambda (number) (* 7 number))
(See The defun
Macro.)
If we want to multiply 3 by 7, we can write:
(multiply-by-seven 3) \_______________/ ^ | | function argument
This expression returns 21.
Similarly, we can write:
((lambda (number) (* 7 number)) 3) \____________________________/ ^ | | anonymous function argument
If we want to divide 100 by 50, we can write:
((lambda (arg) (/ arg 50)) 100) \______________________/ \_/ | | anonymous function argument
This expression returns 2. The 100 is passed to the function, which divides that number by 50.
See Lambda Expressions in The GNU Emacs
Lisp Reference Manual, for more about lambda
. Lisp and lambda
expressions derive from the Lambda Calculus.
Next: Another Bug … Most Insidious, Previous: A lambda
Expression: Useful Anonymity, Up: Printing the Whole Graph [Contents][Index]
C.4.4 The mapcar
Function
mapcar
is a function that calls its first argument with each
element of its second argument, in turn. The second argument must be
a sequence.
The ‘map’ part of the name comes from the mathematical phrase, “mapping over a domain”, meaning to apply a function to each of the elements in a domain. The mathematical phrase is based on the metaphor of a surveyor walking, one step at a time, over an area he is mapping. And ‘car’, of course, comes from the Lisp notion of the first of a list.
For example,
(mapcar '1+ '(2 4 6)) ⇒ (3 5 7)
The function 1+
which adds one to its argument, is executed on
each element of the list, and a new list is returned.
Contrast this with apply
, which applies its first argument to
all the remaining.
(See Readying a Graph, for an explanation of
apply
.)
In the definition of one-fiftieth
, the first argument is the
anonymous function:
(lambda (arg) (/ arg 50))
and the second argument is full-range
, which will be bound to
list-for-graph
.
The whole expression looks like this:
(mapcar (lambda (arg) (/ arg 50)) full-range))
See Mapping Functions in The GNU Emacs
Lisp Reference Manual, for more about mapcar
.
Using the one-fiftieth
function, we can generate a list in
which each element is one-fiftieth the size of the corresponding
element in list-for-graph
.
(setq fiftieth-list-for-graph (one-fiftieth list-for-graph))
The resulting list looks like this:
(10 20 19 15 11 9 6 5 4 3 3 2 2 1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4)
This, we are almost ready to print! (We also notice the loss of information: many of the higher ranges are 0, meaning that fewer than 50 defuns had that many words or symbols—but not necessarily meaning that none had that many words or symbols.)
Next: The Printed Graph, Previous: The mapcar
Function, Up: Printing the Whole Graph [Contents][Index]
C.4.5 Another Bug … Most Insidious
I said “almost ready to print”! Of course, there is a bug in the
print-graph
function … It has a vertical-step
option, but not a horizontal-step
option. The
top-of-range
scale goes from 10 to 300 by tens. But the
print-graph
function will print only by ones.
This is a classic example of what some consider the most insidious type of bug, the bug of omission. This is not the kind of bug you can find by studying the code, for it is not in the code; it is an omitted feature. Your best actions are to try your program early and often; and try to arrange, as much as you can, to write code that is easy to understand and easy to change. Try to be aware, whenever you can, that whatever you have written, will be rewritten, if not soon, eventually. A hard maxim to follow.
It is the print-X-axis-numbered-line
function that needs the
work; and then the print-X-axis
and the print-graph
functions need to be adapted. Not much needs to be done; there is one
nicety: the numbers ought to line up under the tic marks. This takes
a little thought.
Here is the corrected print-X-axis-numbered-line
:
(defun print-X-axis-numbered-line (number-of-X-tics X-axis-leading-spaces &optional horizontal-step) "Print line of X-axis numbers" (let ((number X-axis-label-spacing) (horizontal-step (or horizontal-step 1)))
(insert X-axis-leading-spaces)
;; Delete extra leading spaces.
(delete-char
(- (1-
(length (number-to-string horizontal-step)))))
(insert (concat
(make-string
;; Insert white space.
(- (* symbol-width
X-axis-label-spacing)
(1-
(length
(number-to-string horizontal-step)))
2)
? )
(number-to-string
(* number horizontal-step))))
;; Insert remaining numbers.
(setq number (+ number X-axis-label-spacing))
(while (> number-of-X-tics 1)
(insert (X-axis-element
(* number horizontal-step)))
(setq number (+ number X-axis-label-spacing))
(setq number-of-X-tics (1- number-of-X-tics)))))
If you are reading this in Info, you can see the new versions of
print-X-axis
print-graph
and evaluate them. If you are
reading this in a printed book, you can see the changed lines here
(the full text is too much to print).
(defun print-X-axis (numbers-list horizontal-step) "Print X axis labels to length of NUMBERS-LIST. Optionally, HORIZONTAL-STEP, a positive integer, specifies how much an X axis label increments for each column."
;; Value of symbol-width and full-Y-label-width
;; are passed by print-graph.
(let* ((leading-spaces
(make-string full-Y-label-width ? ))
;; symbol-width is provided by graph-body-print
(tic-width (* symbol-width X-axis-label-spacing))
(X-length (length numbers-list))
(X-tic
(concat
(make-string
;; Make a string of blanks.
(- (* symbol-width X-axis-label-spacing)
(length X-axis-tic-symbol))
? )
;; Concatenate blanks with tic symbol.
X-axis-tic-symbol))
(tic-number
(if (zerop (% X-length tic-width))
(/ X-length tic-width)
(1+ (/ X-length tic-width)))))
(print-X-axis-tic-line tic-number leading-spaces X-tic) (insert "\n") (print-X-axis-numbered-line tic-number leading-spaces horizontal-step)))
(defun print-graph (numbers-list &optional vertical-step horizontal-step) "Print labeled bar graph of the NUMBERS-LIST. The numbers-list consists of the Y-axis values.
Optionally, VERTICAL-STEP, a positive integer, specifies how much a Y axis label increments for each line. For example, a step of 5 means that each row is five units.
Optionally, HORIZONTAL-STEP, a positive integer,
specifies how much an X axis label increments for
each column."
(let* ((symbol-width (length graph-blank))
;; height
is both the largest number
;; and the number with the most digits.
(height (apply 'max numbers-list))
(height-of-top-line
(if (zerop (% height Y-axis-label-spacing))
height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))
(vertical-step (or vertical-step 1)) (full-Y-label-width (length (concat (number-to-string (* height-of-top-line vertical-step)) Y-axis-tic))))
(print-Y-axis height-of-top-line full-Y-label-width vertical-step) (graph-body-print numbers-list height-of-top-line symbol-width) (print-X-axis numbers-list horizontal-step)))
Previous: Another Bug … Most Insidious, Up: Printing the Whole Graph [Contents][Index]
C.4.6 The Printed Graph
When made and installed, you can call the print-graph
command
like this:
(print-graph fiftieth-list-for-graph 50 10)
Here is the graph:
1000 - * ** ** ** ** 750 - *** *** *** *** **** 500 - ***** ****** ****** ****** ******* 250 - ******** ********* * *********** * ************* * 50 - ***************** * * | | | | | | | | 10 50 100 150 200 250 300 350
The largest group of functions contain 10–19 words and symbols each.
Next: GNU Free Documentation License, Previous: A Graph with Labeled Axes, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
Appendix D Free Software and Free Manuals
by Richard M. Stallman
The biggest deficiency in free operating systems is not in the software—it is the lack of good free manuals that we can include in these systems. Many of our most important programs do not come with full manuals. Documentation is an essential part of any software package; when an important free software package does not come with a free manual, that is a major gap. We have many such gaps today.
Once upon a time, many years ago, I thought I would learn Perl. I got a copy of a free manual, but I found it hard to read. When I asked Perl users about alternatives, they told me that there were better introductory manuals—but those were not free.
Why was this? The authors of the good manuals had written them for O’Reilly Associates, which published them with restrictive terms—no copying, no modification, source files not available—which exclude them from the free software community.
That wasn’t the first time this sort of thing has happened, and (to our community’s great loss) it was far from the last. Proprietary manual publishers have enticed a great many authors to restrict their manuals since then. Many times I have heard a GNU user eagerly tell me about a manual that he is writing, with which he expects to help the GNU project—and then had my hopes dashed, as he proceeded to explain that he had signed a contract with a publisher that would restrict it so that we cannot use it.
Given that writing good English is a rare skill among programmers, we can ill afford to lose manuals this way.
Free documentation, like free software, is a matter of freedom, not price. The problem with these manuals was not that O’Reilly Associates charged a price for printed copies—that in itself is fine. The Free Software Foundation sells printed copies of free GNU manuals, too. But GNU manuals are available in source code form, while these manuals are available only on paper. GNU manuals come with permission to copy and modify; the Perl manuals do not. These restrictions are the problems.
The criterion for a free manual is pretty much the same as for free software: it is a matter of giving all users certain freedoms. Redistribution (including commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, on-line or on paper. Permission for modification is crucial too.
As a general rule, I don’t believe that it is essential for people to have permission to modify all sorts of articles and books. The issues for writings are not necessarily the same as those for software. For example, I don’t think you or I are obliged to give permission to modify articles like this one, which describe our actions and our views.
But there is a particular reason why the freedom to modify is crucial for documentation for free software. When people exercise their right to modify the software, and add or change its features, if they are conscientious they will change the manual too—so they can provide accurate and usable documentation with the modified program. A manual which forbids programmers to be conscientious and finish the job, or more precisely requires them to write a new manual from scratch if they change the program, does not fill our community’s needs.
While a blanket prohibition on modification is unacceptable, some kinds of limits on the method of modification pose no problem. For example, requirements to preserve the original author’s copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified, even to have entire sections that may not be deleted or changed, as long as these sections deal with nontechnical topics. (Some GNU manuals have them.)
These kinds of restrictions are not a problem because, as a practical matter, they don’t stop the conscientious programmer from adapting the manual to fit the modified program. In other words, they don’t block the free software community from making full use of the manual.
However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels; otherwise, the restrictions do block the community, the manual is not free, and so we need another manual.
Unfortunately, it is often hard to find someone to write another manual when a proprietary manual exists. The obstacle is that many users think that a proprietary manual is good enough—so they don’t see the need to write a free manual. They do not see that the free operating system has a gap that needs filling.
Why do users think that proprietary manuals are good enough? Some have not considered the issue. I hope this article will do something to change that.
Other users consider proprietary manuals acceptable for the same reason so many people consider proprietary software acceptable: they judge in purely practical terms, not using freedom as a criterion. These people are entitled to their opinions, but since those opinions spring from values which do not include freedom, they are no guide for those of us who do value freedom.
Please spread the word about this issue. We continue to lose manuals to proprietary publishing. If we spread the word that proprietary manuals are not sufficient, perhaps the next person who wants to help GNU by writing documentation will realize, before it is too late, that he must above all make it free.
We can also encourage commercial publishers to sell free, copylefted manuals instead of proprietary ones. One way you can help this is to check the distribution terms of a manual before you buy it, and prefer copylefted manuals to non-copylefted ones.
Note: The Free Software Foundation maintains a page on its Web site
that lists free books available from other publishers:
https://www.gnu.org/doc/other-free-books.html
Next: Index, Previous: Free Software and Free Manuals, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
Appendix E GNU Free Documentation License
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. https://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
- PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
- APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
- VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
- COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
- MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
- Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
- List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
- State on the Title page the name of the publisher of the Modified Version, as the publisher.
- Preserve all the copyright notices of the Document.
- Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
- Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
- Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
- Include an unaltered copy of this License.
- Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
- Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
- For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
- Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
- Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
- Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
- Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
- COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
- COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
- AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
- TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
- TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
- FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/licenses/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
- RELICENSING
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
with the Invariant Sections being list their titles, with the Front-Cover Texts being list, and with the Back-Cover Texts being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
Next: About the Author, Previous: GNU Free Documentation License, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
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Previous: Index, Up: An Introduction to Programming in Emacs Lisp [Contents][Index]
About the Author
Robert J. Chassell (1946–2017) started working with GNU Emacs in 1985. He wrote and edited, taught Emacs and Emacs Lisp, and spoke throughout the world on software freedom. Chassell was a founding Director and Treasurer of the Free Software Foundation, Inc. He was co-author of the Texinfo manual, and edited more than a dozen other books. He graduated from Cambridge University, in England. He had an abiding interest in social and economic history and flew his own airplane.