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pmatch.scm
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pmatch.scm
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;; This is a new version of pmatch (August 8, 2012).
;; It has two important new features:
;; 1. It allows for a name to be given to the pmatch if an error ensues.
;; 2. A line from the specification has been removed. (see below). Without
;; that line removed, it was impossible for a pattern to be (quote ,x),
;; which might be worth having especially when we write an interpreter
;; for Scheme, which includes quote as a language form.
;;; Code written by Oleg Kiselyov
;; (http://pobox.com/~oleg/ftp/)
;;;
;;; Taken from leanTAP.scm
;;; http://kanren.cvs.sourceforge.net/kanren/kanren/mini/leanTAP.scm?view=log
; A simple linear pattern matcher
; It is efficient (generates code at macro-expansion time) and simple:
; it should work on any R5RS (and R6RS) Scheme system.
; (pmatch exp <clause> ...[<else-clause>])
; <clause> ::= (<pattern> <guard> exp ...)
; <else-clause> ::= (else exp ...)
; <guard> ::= boolean exp | ()
; <pattern> :: =
; ,var -- matches always and binds the var
; pattern must be linear! No check is done
; _ -- matches always
; 'exp -- comparison with exp (using equal?) REMOVED (August 8, 2012)
; exp -- comparison with exp (using equal?)
; (<pattern1> <pattern2> ...) -- matches the list of patterns
; (<pattern1> . <pattern2>) -- ditto
; () -- matches the empty list
(define-syntax pmatch
(syntax-rules (else guard)
((_ v (e ...) ...)
(pmatch-aux #f v (e ...) ...))
((_ v name (e ...) ...)
(pmatch-aux name v (e ...) ...))))
(define-syntax pmatch-aux
(syntax-rules (else guard)
((_ name (rator rand ...) cs ...)
(let ((v (rator rand ...)))
(pmatch-aux name v cs ...)))
((_ name v)
(begin
(if 'name
(printf "pmatch ~s failed\n~s\n" 'name v)
(printf "pmatch failed\n ~s\n" v))
(error 'pmatch "match failed")))
((_ name v (else e0 e ...)) (begin e0 e ...))
((_ name v (pat (guard g ...) e0 e ...) cs ...)
(let ((fk (lambda () (pmatch-aux name v cs ...))))
(ppat v pat (if (and g ...) (begin e0 e ...) (fk)) (fk))))
((_ name v (pat e0 e ...) cs ...)
(let ((fk (lambda () (pmatch-aux name v cs ...))))
(ppat v pat (begin e0 e ...) (fk))))))
(define-syntax ppat
(syntax-rules (? comma unquote)
((_ v ? kt kf) kt)
((_ v () kt kf) (if (null? v) kt kf))
; ((_ v (quote lit) kt kf) (if (equal? v (quote lit)) kt kf))
((_ v (unquote var) kt kf) (let ((var v)) kt))
((_ v (x . y) kt kf)
(if (pair? v)
(let ((vx (car v)) (vy (cdr v)))
(ppat vx x (ppat vy y kt kf) kf))
kf))
((_ v lit kt kf) (if (equal? v (quote lit)) kt kf))))