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; Part of Scheme 48 1.9. See file COPYING for notices and license.
; Authors: Richard Kelsey, Jonathan Rees
; Rewrite-rule compiler (a.k.a. "extend-syntax")
; Example:
;
; (define-syntax or
; (syntax-rules ()
; ((or) #f)
; ((or e) e)
; ((or e1 e ...) (let ((temp e1))
; (if temp temp (or e ...))))))
(define-syntax syntax-rules
(let ()
(define name? symbol?)
(define (segment-pattern? pattern)
(and (segment-template? pattern)
(or (null? (cddr pattern))
(syntax-error "segment matching not implemented" pattern))))
(define (segment-template? pattern)
(and (pair? pattern)
(pair? (cdr pattern))
(memq (cadr pattern) indicators-for-zero-or-more)))
(define indicators-for-zero-or-more (list (string->symbol "...") '---))
(lambda (exp r c)
(define %input (r '%input)) ;Gensym these, if you like.
(define %compare (r '%compare))
(define %rename (r '%rename))
(define %tail (r '%tail))
(define %temp (r '%temp))
(define rules (cddr exp))
(define subkeywords (cadr exp))
(define (make-transformer rules)
`(lambda (,%input ,%rename ,%compare)
(let ((,%tail (cdr ,%input)))
(cond ,@(map process-rule rules)
(else
(syntax-error
"use of macro doesn't match definition"
,%input))))))
(define (process-rule rule)
(if (and (pair? rule)
(pair? (cdr rule))
(null? (cddr rule)))
(let ((pattern (cdar rule))
(template (cadr rule)))
`((and ,@(process-match %tail pattern))
(let* ,(process-pattern pattern
%tail
(lambda (x) x))
,(process-template template
0
(meta-variables pattern 0 '())))))
(syntax-error "ill-formed syntax rule" rule)))
; Generate code to test whether input expression matches pattern
(define (process-match input pattern)
(cond ((name? pattern)
(if (member pattern subkeywords)
`((,%compare ,input (,%rename ',pattern)))
`()))
((segment-pattern? pattern)
(process-segment-match input (car pattern)))
((pair? pattern)
`((let ((,%temp ,input))
(and (pair? ,%temp)
,@(process-match `(car ,%temp) (car pattern))
,@(process-match `(cdr ,%temp) (cdr pattern))))))
((or (null? pattern) (boolean? pattern) (char? pattern))
`((eq? ,input ',pattern)))
(else
`((equal? ,input ',pattern)))))
(define (process-segment-match input pattern)
(let ((conjuncts (process-match '(car l) pattern)))
(if (null? conjuncts)
`((list? ,input)) ;+++
`((let loop ((l ,input))
(or (null? l)
(and (pair? l)
,@conjuncts
(loop (cdr l)))))))))
; Generate code to take apart the input expression
; This is pretty bad, but it seems to work (can't say why).
(define (process-pattern pattern path mapit)
(cond ((name? pattern)
(if (memq pattern subkeywords)
'()
(list (list pattern (mapit path)))))
((segment-pattern? pattern)
(process-pattern (car pattern)
%temp
(lambda (x) ;temp is free in x
(mapit (if (eq? %temp x)
path ;+++
`(map (lambda (,%temp) ,x)
,path))))))
((pair? pattern)
(append (process-pattern (car pattern) `(car ,path) mapit)
(process-pattern (cdr pattern) `(cdr ,path) mapit)))
(else '())))
; Generate code to compose the output expression according to template
(define (process-template template rank env)
(cond ((name? template)
(let ((probe (assq template env)))
(if probe
(if (<= (cdr probe) rank)
template
(syntax-error "template rank error (too few ...'s?)"
template))
`(,%rename ',template))))
((segment-template? template)
(let ((vars
(free-meta-variables (car template) (+ rank 1) env '())))
(if (null? vars)
(syntax-error "too many ...'s" template)
(let* ((x (process-template (car template)
(+ rank 1)
env))
(gen (if (equal? (list x) vars)
x ;+++
`(map (lambda ,vars ,x)
,@vars))))
(if (null? (cddr template))
gen ;+++
`(append ,gen ,(process-template (cddr template)
rank env)))))))
((pair? template)
`(cons ,(process-template (car template) rank env)
,(process-template (cdr template) rank env)))
(else `(quote ,template))))
; Return an association list of (var . rank)
(define (meta-variables pattern rank vars)
(cond ((name? pattern)
(if (memq pattern subkeywords)
vars
(cons (cons pattern rank) vars)))
((segment-pattern? pattern)
(meta-variables (car pattern) (+ rank 1) vars))
((pair? pattern)
(meta-variables (car pattern) rank
(meta-variables (cdr pattern) rank vars)))
(else vars)))
; Return a list of meta-variables of given higher rank
(define (free-meta-variables template rank env free)
(cond ((name? template)
(if (and (not (memq template free))
(let ((probe (assq template env)))
(and probe (>= (cdr probe) rank))))
(cons template free)
free))
((segment-template? template)
(free-meta-variables (car template)
rank env
(free-meta-variables (cddr template)
rank env free)))
((pair? template)
(free-meta-variables (car template)
rank env
(free-meta-variables (cdr template)
rank env free)))
(else free)))
c ;ignored
;; Kludge for Scheme 48 static linker.
;; `(cons ,(make-transformer rules)
;; ',(find-free-names-in-syntax-rules subkeywords rules))
(make-transformer rules))))
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