including literate docs

lib/chibi/match/match.scm

@@ -1,20 +1,203 @@
 ;;;; match.scm -- portable hygienic pattern matcher
+;;;; -*- coding: utf-8 -*-
 ;;
 ;; This code is written by Alex Shinn and placed in the
 ;; Public Domain.  All warranties are disclaimed.
 
-;; This is a full superset of the popular MATCH package by Andrew
+;;> @example-import[(srfi 9)]
-;; Wright, written in fully portable SYNTAX-RULES (R5RS only, breaks
-;; in R6RS SYNTAX-RULES), and thus preserving hygiene.
 
-;; This is a simple generative pattern matcher - each pattern is
+;;> This is a full superset of the popular @hyperlink[
-;; expanded into the required tests, calling a failure continuation if
+;;> "http://www.cs.indiana.edu/scheme-repository/code.match.html"]{match}
-;; the tests fail.  This makes the logic easy to follow and extend,
+;;> package by Andrew Wright, written in fully portable @scheme{syntax-rules}
-;; but produces sub-optimal code in cases where you have many similar
+;;> and thus preserving hygiene.
-;; clauses due to repeating the same tests.  Nonetheless a smart
+
-;; compiler should be able to remove the redundant tests.  For
+;;> The most notable extensions are the ability to use @emph{non-linear}
-;; MATCH-LET and DESTRUCTURING-BIND type uses there is no performance
+;;> patterns - patterns in which the same identifier occurs multiple
-;; hit.
+;;> times, tail patterns after ellipsis, and the experimental tree patterns.
+
+;;> @subsubsection{Patterns}
+
+;;> Patterns are written to look like the printed representation of
+;;> the objects they match.  The basic usage is
+
+;;> @scheme{(match expr (pat body ...) ...)}
+
+;;> where the result of @var{expr} is matched against each pattern in
+;;> turn, and the corresponding body is evaluated for the first to
+;;> succeed.  Thus, a list of three elements matches a list of three
+;;> elements.
+
+;;> @example{(let ((ls (list 1 2 3))) (match ls ((1 2 3) #t)))}
+
+;;> If no patterns match an error is signalled.
+
+;;> Identifiers will match anything, and make the corresponding
+;;> binding available in the body.
+
+;;> @example{(match (list 1 2 3) ((a b c) b))}
+
+;;> If the same identifier occurs multiple times, the first instance
+;;> will match anything, but subsequent instances must match a value
+;;> which is @scheme{equal?} to the first.
+
+;;> @example{(match (list 1 2 1) ((a a b) 1) ((a b a) 2))}
+
+;;> The special identifier @scheme{_} matches anything, no matter how
+;;> many times it is used, and does not bind the result in the body.
+
+;;> @example{(match (list 1 2 1) ((_ _ b) 1) ((a b a) 2))}
+
+;;> To match a literal identifier (or list or any other literal), use
+;;> @scheme{quote}.
+
+;;> @example{(match 'a ('b 1) ('a 2))}
+
+;;> Analogous to its normal usage in scheme, @scheme{quasiquote} can
+;;> be used to quote a mostly literally matching object with selected
+;;> parts unquoted.
+
+;;> @example|{(match (list 1 2 3) (`(1 ,b ,c) (list b c)))}|
+
+;;> Often you want to match any number of a repeated pattern.  Inside
+;;> a list pattern you can append @scheme{...} after an element to
+;;> match zero or more of that pattern (like a regexp Kleene star).
+
+;;> @example{(match (list 1 2) ((1 2 3 ...) #t))}
+;;> @example{(match (list 1 2 3) ((1 2 3 ...) #t))}
+;;> @example{(match (list 1 2 3 3 3) ((1 2 3 ...) #t))}
+
+;;> Pattern variables matched inside the repeated pattern are bound to
+;;> a list of each matching instance in the body.
+
+;;> @example{(match (list 1 2) ((a b c ...) c))}
+;;> @example{(match (list 1 2 3) ((a b c ...) c))}
+;;> @example{(match (list 1 2 3 4 5) ((a b c ...) c))}
+
+;;> More than one @scheme{...} may not be used in the same list, since
+;;> this would require exponential backtracking in the general case.
+;;> However, @scheme{...} need not be the final element in the list,
+;;> and may be succeeded by a fixed number of patterns.
+
+;;> @example{(match (list 1 2 3 4) ((a b c ... d e) c))}
+;;> @example{(match (list 1 2 3 4 5) ((a b c ... d e) c))}
+;;> @example{(match (list 1 2 3 4 5 6 7) ((a b c ... d e) c))}
+
+;;> @scheme{___} is provided as an alias for @scheme{...} when it is
+;;> inconvenient to use the ellipsis (as in a syntax-rules template).
+
+;;> The @scheme{..1} syntax is exactly like the @scheme{...} except
+;;> that it matches one or more repetitions (like a regexp "+").
+
+;;> @example{(match (list 1 2) ((a b c ..1) c))}
+;;> @example{(match (list 1 2 3) ((a b c ..1) c))}
+
+;;> The boolean operators @scheme{and}, @scheme{or} and @scheme{not}
+;;> can be used to group and negate patterns analogously to their
+;;> Scheme counterparts.
+
+;;> The @scheme{and} operator ensures that all subpatterns match.
+;;> This operator is often used with the idiom @scheme{(and x pat)} to
+;;> bind @var{x} to the entire value that matches @var{pat}
+;;> (c.f. "as-patterns" in ML or Haskell).  Another common use is in
+;;> conjunction with @scheme{not} patterns to match a general case
+;;> with certain exceptions.
+
+;;> @example{(match 1 ((and) #t))}
+;;> @example{(match 1 ((and x) x))}
+;;> @example{(match 1 ((and x 1) x))}
+
+;;> The @scheme{or} operator ensures that at least one subpattern
+;;> matches.  If the same identifier occurs in different subpatterns,
+;;> it is matched independently.  All identifiers from all subpatterns
+;;> are bound if the @scheme{or} operator matches, but the binding is
+;;> only defined for identifiers from the subpattern which matched.
+
+;;> @example{(match 1 ((or) #t) (else #f))}
+;;> @example{(match 1 ((or x) x))}
+;;> @example{(match 1 ((or x 2) x))}
+
+;;> The @scheme{not} operator succeeds if the given pattern doesn't
+;;> match.  None of the identifiers used are available in the body.
+
+;;> @example{(match 1 ((not 2) #t))}
+
+;;> The more general operator @scheme{?} can be used to provide a
+;;> predicate.  The usage is @scheme{(? predicate pat ...)} where
+;;> @var{predicate} is a Scheme expression evaluating to a predicate
+;;> called on the value to match, and any optional patterns after the
+;;> predicate are then matched as in an @scheme{and} pattern.
+
+;;> @example{(match 1 ((? odd? x) x))}
+
+;;> The field operator @scheme{=} is used to extract an arbitrary
+;;> field and match against it.  It is useful for more complex or
+;;> conditional destructuring that can't be more directly expressed in
+;;> the pattern syntax.  The usage is @scheme{(= field pat)}, where
+;;> @var{field} can be any expression, and should result in a
+;;> procedure of one argument, which is applied to the value to match
+;;> to generate a new value to match against @var{pat}.
+
+;;> Thus the pattern @scheme{(and (= car x) (= cdr y))} is equivalent
+;;> to @scheme{(x . y)}, except it will result in an immediate error
+;;> if the value isn't a pair.
+
+;;> @example{(match '(1 . 2) ((= car x) x))}
+;;> @example{(match 4 ((= sqrt x) x))}
+
+;;> The record operator @scheme{$} is used as a concise way to match
+;;> records defined by SRFI-9 (or SRFI-99).  The usage is
+;;> @scheme{($ rtd field ...)}, where @var{rtd} should be the record
+;;> type descriptor specified as the first argument to
+;;> @scheme{define-record-type}, and each @var{field} is a subpattern
+;;> matched against the fields of the record in order.  Not all fields
+;;> must be present.
+
+;;> @example{
+;;> (let ()
+;;>   (define-record-type employee
+;;>     (make-employee name title)
+;;>     employee?
+;;>     (name get-name)
+;;>     (title get-title))
+;;>   (match (make-employee "Bob" "Doctor")
+;;>     (($ employee n t) (list t n))))
+;;> }
+
+;;> The @scheme{set!} and @scheme{get!} operators are used to bind an
+;;> identifier to the setter and getter of a field, respectively.  The
+;;> setter is a procedure of one argument, which mutates the field to
+;;> that argument.  The getter is a procedure of no arguments which
+;;> returns the current value of the field.
+
+;;> @example{(let ((x (cons 1 2))) (match x ((1 . (set! s)) (s 3) x)))}
+;;> @example{(match '(1 . 2) ((1 . (get! g)) (g)))}
+
+;;> The new operator @scheme{***} can be used to search a tree for
+;;> subpatterns.  A pattern of the form @scheme{(x *** y)} represents
+;;> the subpattern @var{y} located somewhere in a tree where the path
+;;> from the current object to @var{y} can be seen as a list of the
+;;> form @scheme{(x ...)}.  @var{y} can immediately match the current
+;;> object in which case the path is the empty list.  In a sense it's
+;;> a 2-dimensional version of the @scheme{...} pattern.
+
+;;> As a common case the pattern @scheme{(_ *** y)} can be used to
+;;> search for @var{y} anywhere in a tree, regardless of the path
+;;> used.
+
+;;> @example{(match '(a (a (a b))) ((x *** 'b) x))}
+;;> @example{(match '(a (b) (c (d e) (f g))) ((x *** 'g) x))}
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Notes
+
+;; The implementation is a simple generative pattern matcher - each
+;; pattern is expanded into the required tests, calling a failure
+;; continuation if the tests fail.  This makes the logic easy to
+;; follow and extend, but produces sub-optimal code in cases where you
+;; have many similar clauses due to repeating the same tests.
+;; Nonetheless a smart compiler should be able to remove the redundant
+;; tests.  For MATCH-LET and DESTRUCTURING-BIND type uses there is no
+;; performance hit.
 
 ;; The original version was written on 2006/11/29 and described in the
 ;; following Usenet post:
@@ -52,6 +235,21 @@
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+;;> @subsubsection{Syntax}
+
+;;> @subsubsubsection{(match expr (pattern . body) ...)@br{}
+;;> (match expr (pattern (=> failure) . body) ...)}
+
+;;> The result of @var{expr} is matched against each @var{pattern} in
+;;> turn, according to the pattern rules described in the previous
+;;> section, until the the first @var{pattern} matches.  When a match is
+;;> found, the corresponding @var{body}s are evaluated in order,
+;;> and the result of the last expression is returned as the result
+;;> of the entire @scheme{match}.  If a @var{failure} is provided,
+;;> then it is bound to a procedure of no arguments which continues,
+;;> processing at the next @var{pattern}.  If no @var{pattern} matches,
+;;> an error is signalled.
+
 ;; The basic interface.  MATCH just performs some basic syntax
 ;; validation, binds the match expression to a temporary variable `v',
 ;; and passes it on to MATCH-NEXT.  It's a constant throughout the
@@ -361,21 +559,7 @@
     ((_ x sk)
      (match-syntax-error "dotted tail not allowed after ellipse" x))))
 
-;; Matching a tree search pattern is only slightly more complicated.
+;; To implement the tree search, we use two recursive procedures.  TRY
-;; Here we allow patterns of the form
-;;
-;;     (x *** y)
-;;
-;; to represent the pattern y located somewhere in a tree where the
-;; path from the current object to y can be seen as a list of the form
-;; (X ...).  Y can immediately match the current object in which case
-;; the path is the empty list.  In a sense it's a 2-dimensional
-;; version of the ... pattern.
-;;
-;; As a common case the pattern (_ *** y) can be used to search for Y
-;; anywhere in a tree, regardless of the path used.
-;;
-;; To implement the search, we use two recursive procedures.  TRY
 ;; attempts to match Y once, and on success it calls the normal SK on
 ;; the accumulated list ids as in MATCH-GEN-ELLIPSES.  On failure, we
 ;; call NEXT which first checks if the current value is a list
@@ -593,24 +777,42 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Gimme some sugar baby.
 
+;;> Shortcut for @scheme{lambda} + @scheme{match}.  Creates a
+;;> procedure of one argument, and matches that argument against each
+;;> clause.
+
 (define-syntax match-lambda
   (syntax-rules ()
-    ((_ clause ...) (lambda (expr) (match expr clause ...)))))
+    ((_ (pattern . body) ...) (lambda (expr) (match expr (pattern . body) ...)))))
+
+;;> Similar to @scheme{match-lambda}.  Creates a procedure of any
+;;> number of arguments, and matches the argument list against each
+;;> clause.
 
 (define-syntax match-lambda*
   (syntax-rules ()
-    ((_ clause ...) (lambda expr (match expr clause ...)))))
+    ((_ (pattern . body) ...) (lambda expr (match expr (pattern . body) ...)))))
+
+;;> Matches each var to the corresponding expression, and evaluates
+;;> the body with all match variables in scope.  Raises an error if
+;;> any of the expressions fail to match.  Syntax analogous to named
+;;> let can also be used for recursive functions which match on their
+;;> arguments as in @scheme{match-lambda*}.
 
 (define-syntax match-let
   (syntax-rules ()
-    ((_ (vars ...) . body)
+    ((_ ((var value) ...) . body)
-     (match-let/helper let () () (vars ...) . body))
+     (match-let/helper let () () ((var value) ...) . body))
-    ((_ loop . rest)
+    ((_ loop ((var init) ...) . body)
-     (match-named-let loop () . rest))))
+     (match-named-let loop ((var init) ...) . body))))
+
+;;> Similar to @scheme{match-let}, but analogously to @scheme{letrec}
+;;> matches and binds the variables with all match variables in scope.
 
 (define-syntax match-letrec
   (syntax-rules ()
-    ((_ vars . body) (match-let/helper letrec () () vars . body))))
+    ((_ ((var value) ...) . body)
+     (match-let/helper letrec () () ((var value) ...) . body))))
 
 (define-syntax match-let/helper
   (syntax-rules ()
@@ -638,6 +840,12 @@
     ((_ loop (v ...) ((pat expr) . rest) . body)
      (match-named-let loop (v ... (pat expr tmp)) rest . body))))
 
+;;> @subsubsubsection{(match-let* ((var value) ...) body ...)}
+
+;;> Similar to @scheme{match-let}, but analogously to @scheme{let*}
+;;> matches and binds the variables in sequence, with preceding match
+;;> variables in scope.
+
 (define-syntax match-let*
   (syntax-rules ()
     ((_ () . body)