guile/doc/ref/match.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C) 2010, 2011, 2012  Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.
@c

@c The pattern syntax is taken from the documentation available in
@c Andrew K. Wright's implementation of `match.scm', which is in the
@c public domain.  See Guile before commit
@c d967913f05301a35573c5d3f7217d0994bbb1016 (Thu Jun 17 2010) or
@c <http://www.cs.indiana.edu/scheme-repository/code.match.html>.

@c FIXME: This section is a bit rough on the edges.  The introduction
@c could be improved, e.g., by adding examples.

@node Pattern Matching
@section Pattern Matching

@cindex pattern matching
@cindex (ice-9 match)

The @code{(ice-9 match)} module provides a @dfn{pattern matcher},
written by Alex Shinn, and compatible with Andrew K. Wright's pattern
matcher found in many Scheme implementations.

@cindex pattern variable
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
@dfn{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:

@example
(let ((l '(hello (world))))
  (match l           ;; <- the input object
    (('hello (who))  ;; <- the pattern
     who)))          ;; <- the expression evaluated upon matching
@result{} world
@end example

In this example, list @var{l} matches the pattern @code{('hello (who))},
because it is a two-element list whose first element is the symbol
@code{hello} and whose second element is a one-element list.  Here
@var{who} is a pattern variable.  @code{match}, the pattern matcher,
locally binds @var{who} to the value contained in this one-element
list---i.e., the symbol @code{world}.  An error would be raised if
@var{l} did not match the pattern.

The same object can be matched against a simpler pattern:

@example
(let ((l '(hello (world))))
  (match l
    ((x y)
     (values x y))))
@result{} hello
@result{} (world)
@end example

Here pattern @code{(x y)} matches any two-element list, regardless of
the types of these elements.  Pattern variables @var{x} and @var{y} are
bound to, respectively, the first and second element of @var{l}.

Patterns can be composed, and nested.  For instance, @code{...}
(ellipsis) means that the previous pattern may be matched zero or more
times in a list:

@example
(match lst
  (((heads tails ...) ...)
   heads))
@end example

@noindent
This expression returns the first element of each list within @var{lst}.
For proper lists of proper lists, it is equivalent to @code{(map car
lst)}.  However, it performs additional checks to make sure that
@var{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 @code{car} and
@code{cdr} calls when matching lists, for instance.  It also improves
robustness, by making sure the input @emph{completely} matches the
pattern---conversely, hand-written code often trades robustness for
conciseness.  And of course, @code{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:

@deffn {Scheme Syntax} match exp clause1 clause2 @dots{}
Match object @var{exp} against the patterns in @var{clause1}
@var{clause2} @dots{}  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 @code{match-error}.

Each clause has the form @code{(pattern body1 body2 @dots{})}.  Each
@var{pattern} must follow the syntax described below.  Each body is an
arbitrary Scheme expression, possibly referring to pattern variables of
@var{pattern}.
@end deffn

@c FIXME: Document other forms:
@c
@c exp ::= ...
@c       | (match exp clause ...)
@c       | (match-lambda clause ...)
@c       | (match-lambda* clause ...)
@c       | (match-let ((pat exp) ...) body)
@c       | (match-let* ((pat exp) ...) body)
@c       | (match-letrec ((pat exp) ...) body)
@c       | (match-define pat exp)
@c
@c clause ::= (pat body) | (pat => exp)

The syntax and interpretation of patterns is as follows:

@verbatim
        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
@end verbatim

The names @code{quote}, @code{quasiquote}, @code{unquote},
@code{unquote-splicing}, @code{?}, @code{_}, @code{$}, @code{and},
@code{or}, @code{not}, @code{set!}, @code{get!}, @code{...}, and
@code{___} cannot be used as pattern variables.

Here is a more complex example:

@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))))

@result{} (friend-of-bob "Alice")
@end example

@noindent
Here the @code{$} pattern is used to match a SRFI-9 record of type
@var{person} containing two or more slots.  The value of the first slot
is bound to @var{name}.  The @code{=} pattern is used to apply
@code{force} on the second slot, and then checking that the result
matches the given pattern.  In other words, the complete pattern matches
any @var{person} whose second slot is a promise that evaluates to a
one-element list containing a @var{person} whose first slot is
@code{"Bob"}.

The @code{(ice-9 match)} module also provides the following convenient
syntactic sugar macros wrapping around @code{match}.

@deffn {Scheme Syntax} match-lambda clause1 clause2 @dots{}
Create a procedure of one argument that matches its argument against
each clause, and returns the result of evaluating the corresponding
expressions.

@example
(match-lambda clause1 clause2 @dots{})
@equiv{}
(lambda (arg) (match arg clause1 clause2 @dots{}))
@end example
@end deffn

@example
((match-lambda
   (('hello (who))
    who))
 '(hello (world)))
@result{} world
@end example

@deffn {Scheme Syntax} match-lambda* clause1 clause2 @dots{}
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.

@example
(match-lambda* clause1 clause2 @dots{})
@equiv{}
(lambda args (match args clause1 clause2 @dots{}))
@end example
@end deffn

@example
((match-lambda*
   (('hello (who))
    who))
 'hello '(world))
@result{} world
@end example

@deffn {Scheme Syntax} match-let ((pattern expression) @dots{}) body
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.  @code{match-let} is analogous to named let
and can also be used for recursive functions which match on their
arguments as in @code{match-lambda*}.

@example
(match-let (((x y) (list 1 2))
            ((a b) (list 3 4)))
  (list a b x y))
@result{}
(3 4 1 2)
@end example
@end deffn

@deffn {Scheme Syntax} match-let variable ((pattern init) @dots{}) body
Similar to @code{match-let}, but analogously to @dfn{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.  @xref{while do,
Iteration}, for more on @dfn{named let}.
@end deffn

@deffn {Scheme Syntax} match-let* ((variable expression) @dots{}) body
Similar to @code{match-let}, but analogously to @code{let*}, match and
bind the variables in sequence, with preceding match variables in scope.

@example
(match-let* (((x y) (list 1 2))
             ((a b) (list x 4)))
  (list a b x y))
@equiv{}
(match-let (((x y) (list 1 2)))
  (match-let (((a b) (list x 4)))
    (list a b x y)))
@result{}
(1 4 1 2)
@end example
@end deffn

@deffn {Scheme Syntax} match-letrec ((variable expression) @dots{}) body
Similar to @code{match-let}, but analogously to @code{letrec}, match and
bind the variables with all match variables in scope.
@end deffn

Guile also comes with a pattern matcher specifically tailored to SXML
trees, @xref{sxml-match}.