Bovine parser development

The bovine parser is the original Semantic parser, and is an implementation of an LL parser. It is good for simple languages. It has many conveniences making grammar writing easy. The conveniences make it less powerful than a Bison-like LALR parser. For more information, see (wisent)Wisent Parser Development.

Bovine LL grammars are stored in files with a .by extension. When compiled, the contents is converted into a file of the form NAME-by.el. This, in turn is byte compiled. See (grammar-fw)Grammar Framework Manual.

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, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License”.

(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual.”

1 Starting Rules
2 Bovine Grammar Rules
3 Optional Lambda Expressions
4 Examples
Appendix A GNU Free Documentation License

1 Starting Rules

In Bison, one and only one nonterminal is designated as the “start” symbol. In Semantic, one or more nonterminals can be designated as the “start” symbol. They are declared following the %start keyword separated by spaces. See (grammar-fw)Grammar Framework Manual.

If no %start keyword is used in a grammar, then the very first is used. Internally the first start nonterminal is targeted by the reserved symbol bovine-toplevel, so it can be found by the parser harness.

To find locally defined variables, the local context handler needs to parse the body of functional code. The scopestart declaration specifies the name of a nonterminal used as the goal to parse a local context, see (grammar-fw)Grammar Framework Manual. Internally the scopestart nonterminal is targeted by the reserved symbol bovine-inner-scope, so it can be found by the parser harness.

2 Bovine Grammar Rules

The rules are what allow the compiler to create tags from a language file. Once the setup is done in the prologue, you can start writing rules. See (grammar-fw)Grammar Framework Manual.

result : components1 optional-semantic-action1)
       | components2 optional-semantic-action2
       ;

result is a nonterminal, that is a symbol synthesized in your grammar. components is a list of elements that are to be matched if result is to be made. optional-semantic-action is an optional sequence of simplified Emacs Lisp expressions for concocting the parse tree.

In bison, each time an element of components is found, it is shifted onto the parser stack. (The stack of matched elements.) When all components’ elements have been matched, it is reduced to result. See Algorithm in The GNU Bison Manual.

A particular result written into your grammar becomes the parser’s goal. It is designated by a %start statement (see Starting Rules). The value returned by the associated optional-semantic-action is the parser’s result. It should be a tree of Semantic tags, see (semantic-appdev)Semantic Application Development.

components is made up of symbols. A symbol such as FOO means that a syntactic token of class FOO must be matched.

How Lexical Tokens Match
Grammar-to-Lisp Details
Order of components in rules

2.1 How Lexical Tokens Match

A lexical rule must be used to define how to match a lexical token.

For instance:

%keyword FOO "foo"

Means that FOO is a reserved language keyword, matched as such by looking up into a keyword table, see (grammar-fw)Grammar Framework Manual. This is because "foo" will be converted to FOO in the lexical analysis stage. Thus the symbol FOO won’t be available any other way.

If we specify our token in this way:

%token <symbol> FOO "foo"

then FOO will match the string "foo" explicitly, but it won’t do so at the lexical level, allowing use of the text "foo" in other forms of regular expressions.

In that case, FOO is a symbol-type token. To match, a symbol must first be encountered, and then it must string-match "foo".

Caution:: Be especially careful to remember that "foo", and more generally the %token’s match-value string, is a regular expression!

Non symbol tokens are also allowed. For example:

%token <punctuation> PERIOD "[.]"

filename : symbol PERIOD symbol
         ;

PERIOD is a punctuation-type token that will explicitly match one period when used in the above rule.

Please Note:: symbol, punctuation, etc., are predefined lexical token types, based on the syntax class-character associations currently in effect.

2.2 Grammar-to-Lisp Details

For the bovinator, lexical token matching patterns are inlined. When the grammar-to-lisp converter encounters a lexical token declaration of the form:

%token <type> token-name match-value

It substitutes every occurrences of token-name in rules, by its expanded form:

type match-value

For example:

%token <symbol> MOOSE "moose"

find_a_moose: MOOSE
            ;

Will generate this pseudo equivalent-rule:

find_a_moose: symbol "moose"   ;; invalid syntax!
            ;

Thus, from the bovinator point of view, the components part of a rule is made up of symbols and strings. A string in the mix means that the previous symbol must have the additional constraint of exactly matching it, as described in How Lexical Tokens Match.

Please Note:: For the bovinator, this task was mixed into the language definition to simplify implementation, though Bison’s technique is more efficient.

2.3 Order of components in rules

If a rule has multiple components, order is important, for example

headerfile : symbol PERIOD symbol
           | symbol
           ;

would match ‘foo.h’ or the C++ header ‘foo’. The bovine parser will first attempt to match the long form, and then the short form. If they were in reverse order, then the long form would never be tested.

3 Optional Lambda Expressions

The OLE (Optional Lambda Expression) is converted into a bovine lambda. This lambda has special short-cuts to simplify reading the semantic action definition. An OLE like this:

( $1 )

results in a lambda return which consists entirely of the string or object found by matching the first (zeroth) element of match. An OLE like this:

( ,(foo $1) )

executes foo on the first argument, and then splices its return into the return list whereas:

( (foo $1) )

executes foo, and that is placed in the return list.

Here are other things that can appear inline:

$1

The first object matched.

,$1

The first object spliced into the list (assuming it is a list from a non-terminal).

'$1

The first object matched, placed in a list. I.e., ( $1 ).

foo

The symbol foo (exactly as displayed).

(foo)

A function call to foo which is stuck into the return list.

,(foo)

A function call to foo which is spliced into the return list.

'(foo)

A function call to foo which is stuck into the return list in a list.

(EXPAND $1 nonterminal depth)

A list starting with EXPAND performs a recursive parse on the token passed to it (represented by ‘$1’ above.) The semantic list is a common token to expand, as there are often interesting things in the list. The nonterminal is a symbol in your table which the bovinator will start with when parsing. nonterminal’s definition is the same as any other nonterminal. depth should be at least ‘1’ when descending into a semantic list.

(EXPANDFULL $1 nonterminal depth)

Is like EXPAND, except that the parser will iterate over nonterminal until there are no more matches. (The same way the parser iterates over the starting rule (see Starting Rules). This lets you have much simpler rules in this specific case, and also lets you have positional information in the returned tokens, and error skipping.

(ASSOC symbol1 value1 symbol2 value2 …)

This is used for creating an association list. Each symbol is included in the list if the associated value is non-nil. While the items are all listed explicitly, the created structure is an association list of the form:

((symbol1 . value1) (symbol2 . value2) …)

(TAG name class [attributes])

This creates one tag in the current buffer.

name: Is a string that represents the tag in the language.
class: Is the kind of tag being create, such as function, or variable, though any symbol will work.
attributes: Is an optional set of labeled values such as :constant-flag t :parent "parenttype".

(TAG-VARIABLE name type default-value [attributes])

(TAG-FUNCTION name type arg-list [attributes])

(TAG-TYPE name type members parents [attributes])

(TAG-INCLUDE name system-flag [attributes])

(TAG-PACKAGE name detail [attributes])

(TAG-CODE name detail [attributes])

Create a tag with name of respectively the class variable, function, type, include, package, and code. See (semantic-appdev)Semantic Application Development, for the lisp functions these translate into.

If the symbol %quotemode backquote is specified, then use ,@ to splice a list in, and , to evaluate the expression. This lets you send $1 as a symbol into a list instead of having it expanded inline.

4 Examples

The rule:

any-symbol: symbol
          ;

is equivalent to

any-symbol: symbol
            ( $1 )
          ;

which, if it matched the string ‘"A"’, would return

( "A" )

If this rule were used like this:

%token <punctuation> EQUAL "="
…
assign: any-symbol EQUAL any-symbol
        ( $1 $3 )
      ;

it would match ‘"A=B"’, and return

( ("A") ("B") )

The letters ‘A’ and ‘B’ come back in lists because ‘any-symbol’ is a nonterminal, not an actual lexical element.

To get a better result with nonterminals, use , to splice lists in like this:

%token <punctuation> EQUAL "="
…
assign: any-symbol EQUAL any-symbol
        ( ,$1 ,$3 )
      ;

which would return

( "A" "B" )

Appendix A GNU Free Documentation License

Version 1.3, 3 November 2008

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:
1. 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.
2. 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.
3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
4. Preserve all the copyright notices of the Document.
5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
6. 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.
7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
8. Include an unaltered copy of this License.
9. 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.
10. 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.
11. 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.
12. 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.
13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
15. 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.