The (ice-9 match) module provides a pattern matcher,
written by Alex Shinn, and compatible with Andrew K. Wright’s pattern
matcher found in many Scheme implementations.
A pattern matcher can match an object against several patterns and extract the elements that make it up. Patterns can represent any Scheme object: lists, strings, symbols, records, etc. They can optionally contain pattern variables. When a matching pattern is found, an expression associated with the pattern is evaluated, optionally with all pattern variables bound to the corresponding elements of the object:
(let ((l '(hello (world))))
(match l ;; <- the input object
(('hello (who)) ;; <- the pattern
who))) ;; <- the expression evaluated upon matching
⇒ world
In this example, list l matches the pattern ('hello (who)),
because it is a two-element list whose first element is the symbol
hello and whose second element is a one-element list. Here
who is a pattern variable. match, the pattern matcher,
locally binds who to the value contained in this one-element
list—i.e., the symbol world. An error would be raised if
l did not match the pattern.
The same object can be matched against a simpler pattern:
(let ((l '(hello (world))))
(match l
((x y)
(values x y))))
⇒ hello
⇒ (world)
Here pattern (x y) matches any two-element list, regardless of
the types of these elements. Pattern variables x and y are
bound to, respectively, the first and second element of l.
Patterns can be composed, and nested. For instance, ...
(ellipsis) means that the previous pattern may be matched zero or more
times in a list:
(match lst (((heads tails ...) ...) heads))
This expression returns the first element of each list within lst.
For proper lists of proper lists, it is equivalent to (map car
lst). However, it performs additional checks to make sure that
lst and the lists therein are proper lists, as prescribed by the
pattern, raising an error if they are not.
Compared to hand-written code, pattern matching noticeably improves
clarity and conciseness—no need to resort to series of car and
cdr calls when matching lists, for instance. It also improves
robustness, by making sure the input completely matches the
pattern—conversely, hand-written code often trades robustness for
conciseness. And of course, match is a macro, and the code it
expands to is just as efficient as equivalent hand-written code.
The pattern matcher is defined as follows:
Match object exp against the patterns in clause1
clause2 … in the order in which they appear. Return the
value produced by the first matching clause. If no clause matches,
throw an exception with key match-error.
Each clause has the form (pattern body1 body2 …). Each
pattern must follow the syntax described below. Each body is an
arbitrary Scheme expression, possibly referring to pattern variables of
pattern.
The syntax and interpretation of patterns is as follows:
patterns: matches:
pat ::= identifier anything, and binds identifier
| _ anything
| () the empty list
| #t #t
| #f #f
| string a string
| number a number
| character a character
| 'sexp an s-expression
| 'symbol a symbol (special case of s-expr)
| (pat_1 ... pat_n) list of n elements
| (pat_1 ... pat_n . pat_{n+1}) list of n or more
| (pat_1 ... pat_n pat_n+1 ooo) list of n or more, each element
of remainder must match pat_n+1
| #(pat_1 ... pat_n) vector of n elements
| #(pat_1 ... pat_n pat_n+1 ooo) vector of n or more, each element
of remainder must match pat_n+1
| #&pat box
| ($ record-name pat_1 ... pat_n) a record
| (= field pat) a ``field'' of an object
| (and pat_1 ... pat_n) if all of pat_1 thru pat_n match
| (or pat_1 ... pat_n) if any of pat_1 thru pat_n match
| (not pat_1 ... pat_n) if all pat_1 thru pat_n don't match
| (? predicate pat_1 ... pat_n) if predicate true and all of
pat_1 thru pat_n match
| (set! identifier) anything, and binds setter
| (get! identifier) anything, and binds getter
| `qp a quasi-pattern
| (identifier *** pat) matches pat in a tree and binds
identifier to the path leading
to the object that matches pat
ooo ::= ... zero or more
| ___ zero or more
| ..1 1 or more
quasi-patterns: matches:
qp ::= () the empty list
| #t #t
| #f #f
| string a string
| number a number
| character a character
| identifier a symbol
| (qp_1 ... qp_n) list of n elements
| (qp_1 ... qp_n . qp_{n+1}) list of n or more
| (qp_1 ... qp_n qp_n+1 ooo) list of n or more, each element
of remainder must match qp_n+1
| #(qp_1 ... qp_n) vector of n elements
| #(qp_1 ... qp_n qp_n+1 ooo) vector of n or more, each element
of remainder must match qp_n+1
| #&qp box
| ,pat a pattern
| ,@pat a pattern
The names quote, quasiquote, unquote,
unquote-splicing, ?, _, $, and,
or, not, set!, get!, ..., and
___ cannot be used as pattern variables.
Here is a more complex example:
(use-modules (srfi srfi-9))
(let ()
(define-record-type person
(make-person name friends)
person?
(name person-name)
(friends person-friends))
(letrec ((alice (make-person "Alice" (delay (list bob))))
(bob (make-person "Bob" (delay (list alice)))))
(match alice
(($ person name (= force (($ person "Bob"))))
(list 'friend-of-bob name))
(_ #f))))
⇒ (friend-of-bob "Alice")
Here the $ pattern is used to match a SRFI-9 record of type
person containing two or more slots. The value of the first slot
is bound to name. The = pattern is used to apply
force on the second slot, and then checking that the result
matches the given pattern. In other words, the complete pattern matches
any person whose second slot is a promise that evaluates to a
one-element list containing a person whose first slot is
"Bob".
The (ice-9 match) module also provides the following convenient
syntactic sugar macros wrapping around match.
Create a procedure of one argument that matches its argument against each clause, and returns the result of evaluating the corresponding expressions.
(match-lambda clause1 clause2 ...) ≡ (lambda (arg) (match arg clause1 clause2 ...))
((match-lambda
(('hello (who))
who))
'(hello (world)))
⇒ world
Create a procedure of any number of arguments that matches its argument list against each clause, and returns the result of evaluating the corresponding expressions.
(match-lambda* clause1 clause2 ...) ≡ (lambda args (match args clause1 clause2 ...))
((match-lambda*
(('hello (who))
who))
'hello '(world))
⇒ world
Match each pattern to the corresponding expression, and evaluate the
body with all matched variables in scope. Raise an error if any of the
expressions fail to match. match-let is analogous to named let
and can also be used for recursive functions which match on their
arguments as in match-lambda*.
(match-let (((x y) (list 1 2))
((a b) (list 3 4)))
(list a b x y))
⇒
(3 4 1 2)
Similar to match-let, but analogously to named let, locally
bind VARIABLE to a new procedure which accepts as many arguments as
there are INIT expressions. The procedure is initially applied to the
results of evaluating the INIT expressions. When called, the procedure
matches each argument against the corresponding PATTERN, and returns the
result(s) of evaluating the BODY expressions. See Iteration, for more on named let.
Similar to match-let, but analogously to let*, match and
bind the variables in sequence, with preceding match variables in scope.
(match-let* (((x y) (list 1 2))
((a b) (list x 4)))
(list a b x y))
≡
(match-let (((x y) (list 1 2)))
(match-let (((a b) (list x 4)))
(list a b x y)))
⇒
(1 4 1 2)
Similar to match-let, but analogously to letrec, match and
bind the variables with all match variables in scope.
Guile also comes with a pattern matcher specifically tailored to SXML
trees, See sxml-match: Pattern Matching of SXML.