From: Ben <ben@srctxt.com>
To: kawa@sourceware.org
Subject: questions on libraries, pattern matching etc
Date: Sun, 15 Dec 2019 23:47:00 -0000 [thread overview]
Message-ID: <04256eab-fe24-417b-9f0d-fc554c9add96@www.fastmail.com> (raw)
hi
I'd like to test how I can use pattern match in Kawa. First I did try to use Kawas pattern matching function, but from what I saw it is a bit limited, for example there is no matching of lists. Thats why I did try to use the famous match.scm code from Alex Shinn. In order to exclude potential collitions with Kawas 'match', I renamed all 'match' strings in to 'pmatch' and renamed also the file to 'pmatch.scm'
kawa -Dkawa.import.path=".:libs/kawa/*.scm" t.scm
---- t.scm ---
(import pmatch)
(pmatch (list 11 99 )
(( a b )
(display a)
(display b))
(_ (display "gaga")))
=> 11 99
-------
But I also get the following warning :
/libs/kawa/pmatch.scm:88:34: warning - no use of failure
Do you know how I can prevent that warning?
Ben
----- match.scm ----
1 ;;;; match.scm -- portable hygienic pattern matcher
2 ;;
3 ;; This code is written by Alex Shinn and placed in the
4 ;; Public Domain. All warranties are disclaimed.
5
6 ;; This is a full superset of the popular MATCH package by Andrew
7 ;; Wright, written in fully portable SYNTAX-RULES (R5RS only, breaks
8 ;; in R6RS SYNTAX-RULES), and thus preserving hygiene.
9
10 ;; This is a simple generative pattern matcher - each pattern is
11 ;; expanded into the required tests, calling a failure continuation if
12 ;; the tests fail. This makes the logic easy to follow and extend,
13 ;; but produces sub-optimal code in cases where you have many similar
14 ;; clauses due to repeating the same tests. Nonetheless a smart
15 ;; compiler should be able to remove the redundant tests. For
16 ;; MATCH-LET and DESTRUCTURING-BIND type uses there is no performance
17 ;; hit.
18
19 ;; The original version was written on 2006/11/29 and described in the
20 ;; following Usenet post:
21 ;; http://groups.google.com/group/comp.lang.scheme/msg/0941234de7112ffd
22 ;; and is still available at
23 ;; http://synthcode.com/scheme/match-simple.scm
24 ;; A variant of this file which uses COND-EXPAND in a few places can
25 ;; be found at
26 ;; http://synthcode.com/scheme/match-cond-expand.scm
27 ;;
28 ;; 2008/03/20 - fixing bug where (a ...) matched non-lists
29 ;; 2008/03/15 - removing redundant check in vector patterns
30 ;; 2008/03/06 - you can use `...' portably now (thanks to Taylor Campbell)
31 ;; 2007/09/04 - fixing quasiquote patterns
32 ;; 2007/07/21 - allowing ellipse patterns in non-final list positions
33 ;; 2007/04/10 - fixing potential hygiene issue in match-check-ellipse
34 ;; (thanks to Taylor Campbell)
35 ;; 2007/04/08 - clean up, commenting
36 ;; 2006/12/24 - bugfixes
37 ;; 2006/12/01 - non-linear patterns, shared variables in OR, get!/set!
38
39 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
40 ;; force compile-time syntax errors with useful messages
41
42 (define-syntax match-syntax-error
43 (syntax-rules ()
44 ((_)
45 (match-syntax-error "invalid match-syntax-error usage"))))
46
47 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
48
49 ;; The basic interface. MATCH just performs some basic syntax
50 ;; validation, binds the match expression to a temporary variable `v',
51 ;; and passes it on to MATCH-NEXT. It's a constant throughout the
52 ;; code below that the binding `v' is a direct variable reference, not
53 ;; an expression.
54
55 (define-syntax match
56 (syntax-rules ()
57 ((match)
58 (match-syntax-error "missing match expression"))
59 ((match atom)
60 (match-syntax-error "missing match clause"))
61 ((match (app ...) (pat . body) ...)
62 (let ((v (app ...)))
63 (match-next v (app ...) (set! (app ...)) (pat . body) ...)))
64 ((match #(vec ...) (pat . body) ...)
65 (let ((v #(vec ...)))
66 (match-next v v (set! v) (pat . body) ...)))
67 ((match atom (pat . body) ...)
68 (match-next atom atom (set! atom) (pat . body) ...))
69 ))
70
71 ;; MATCH-NEXT passes each clause to MATCH-ONE in turn with its failure
72 ;; thunk, which is expanded by recursing MATCH-NEXT on the remaining
73 ;; clauses. `g' and `s' are the get! and set! expressions
74 ;; respectively.
75
76 (define-syntax match-next
77 (syntax-rules (=>)
78 ;; no more clauses, the match failed
79 ((match-next v g s)
80 (error 'match "no matching pattern"))
81 ;; named failure continuation
82 ((match-next v g s (pat (=> failure) . body) . rest)
83 (let ((failure (lambda () (match-next v g s . rest))))
84 ;; match-one analyzes the pattern for us
85 (match-one v pat g s (match-drop-ids (begin . body)) (failure) ())))
86 ;; anonymous failure continuation, give it a dummy name
87 ((match-next v g s (pat . body) . rest)
88 (match-next v g s (pat (=> failure) . body) . rest))))
89
90 ;; MATCH-ONE first checks for ellipse patterns, otherwise passes on to
91 ;; MATCH-TWO.
92
93 (define-syntax match-one
94 (syntax-rules ()
95 ;; If it's a list of two values, check to see if the second one is
96 ;; an ellipse and handle accordingly, otherwise go to MATCH-TWO.
97 ((match-one v (p q . r) g s sk fk i)
98 (match-check-ellipse
99 q
100 (match-extract-vars p (match-gen-ellipses v p r g s sk fk i) i ())
101 (match-two v (p q . r) g s sk fk i)))
102 ;; Otherwise, go directly to MATCH-TWO.
103 ((match-one . x)
104 (match-two . x))))
105
106 ;; This is the guts of the pattern matcher. We are passed a lot of
107 ;; information in the form:
108 ;;
109 ;; (match-two var pattern getter setter success-k fail-k (ids ...))
110 ;;
111 ;; usually abbreviated
112 ;;
113 ;; (match-two v p g s sk fk i)
114 ;;
115 ;; where VAR is the symbol name of the current variable we are
116 ;; matching, PATTERN is the current pattern, getter and setter are the
117 ;; corresponding accessors (e.g. CAR and SET-CAR! of the pair holding
118 ;; VAR), SUCCESS-K is the success continuation, FAIL-K is the failure
119 ;; continuation (which is just a thunk call and is thus safe to expand
120 ;; multiple times) and IDS are the list of identifiers bound in the
121 ;; pattern so far.
122
123 (define-syntax match-two
124 (syntax-rules (_ ___ quote quasiquote ? $ = and or not set! get!)
125 ((match-two v () g s (sk ...) fk i)
126 (if (null? v) (sk ... i) fk))
127 ((match-two v (quote p) g s (sk ...) fk i)
128 (if (equal? v 'p) (sk ... i) fk))
129 ((match-two v (quasiquote p) g s sk fk i)
130 (match-quasiquote v p g s sk fk i))
131 ((match-two v (and) g s (sk ...) fk i) (sk ... i))
132 ((match-two v (and p q ...) g s sk fk i)
133 (match-one v p g s (match-one v (and q ...) g s sk fk) fk i))
134 ((match-two v (or) g s sk fk i) fk)
135 ((match-two v (or p) g s sk fk i)
136 (match-one v p g s sk fk i))
137 ((match-two v (or p ...) g s sk fk i)
138 (match-extract-vars (or p ...)
139 (match-gen-or v (p ...) g s sk fk i)
140 i
141 ()))
142 ((match-two v (not p) g s (sk ...) fk i)
143 (match-one v p g s (match-drop-ids fk) (sk ... i) i))
144 ((match-two v (get! getter) g s (sk ...) fk i)
145 (let ((getter (lambda () g))) (sk ... i)))
146 ((match-two v (set! setter) g (s ...) (sk ...) fk i)
147 (let ((setter (lambda (x) (s ... x)))) (sk ... i)))
148 ((match-two v (? pred p ...) g s sk fk i)
149 (if (pred v) (match-one v (and p ...) g s sk fk i) fk))
150 ((match-two v (= proc p) g s sk fk i)
151 (let ((w (proc v)))
152 (match-one w p g s sk fk i)))
153 ((match-two v (p ___ . r) g s sk fk i)
154 (match-extract-vars p (match-gen-ellipses v p r g s sk fk i) i ()))
155 ((match-two v (p) g s sk fk i)
156 (if (and (pair? v) (null? (cdr v)))
157 (let ((w (car v)))
158 (match-one w p (car v) (set-car! v) sk fk i))
159 fk))
160 ((match-two v (p . q) g s sk fk i)
161 (if (pair? v)
162 (let ((w (car v)) (x (cdr v)))
163 (match-one w p (car v) (set-car! v)
164 (match-one x q (cdr v) (set-cdr! v) sk fk)
165 fk
166 i))
167 fk))
168 ((match-two v #(p ...) g s sk fk i)
169 (match-vector v 0 () (p ...) sk fk i))
170 ((match-two v _ g s (sk ...) fk i) (sk ... i))
171 ;; Not a pair or vector or special literal, test to see if it's a
172 ;; new symbol, in which case we just bind it, or if it's an
173 ;; already bound symbol or some other literal, in which case we
174 ;; compare it with EQUAL?.
175 ((match-two v x g s (sk ...) fk (id ...))
176 (let-syntax
177 ((new-sym?
178 (syntax-rules (id ...)
179 ((new-sym? x sk2 fk2) sk2)
180 ((new-sym? y sk2 fk2) fk2))))
181 (new-sym? random-sym-to-match
182 (let ((x v)) (sk ... (id ... x)))
183 (if (equal? v x) (sk ... (id ...)) fk))))
184 ))
185
186 ;; QUASIQUOTE patterns
187
188 (define-syntax match-quasiquote
189 (syntax-rules (unquote unquote-splicing quasiquote)
190 ((_ v (unquote p) g s sk fk i)
191 (match-one v p g s sk fk i))
192 ((_ v ((unquote-splicing p) . rest) g s sk fk i)
193 (if (pair? v)
194 (match-one v
195 (p . tmp)
196 (match-quasiquote tmp rest g s sk fk)
197 fk
198 i)
199 fk))
200 ((_ v (quasiquote p) g s sk fk i . depth)
201 (match-quasiquote v p g s sk fk i #f . depth))
202 ((_ v (unquote p) g s sk fk i x . depth)
203 (match-quasiquote v p g s sk fk i . depth))
204 ((_ v (unquote-splicing p) g s sk fk i x . depth)
205 (match-quasiquote v p g s sk fk i . depth))
206 ((_ v (p . q) g s sk fk i . depth)
207 (if (pair? v)
208 (let ((w (car v)) (x (cdr v)))
209 (match-quasiquote
210 w p g s
211 (match-quasiquote-step x q g s sk fk depth)
212 fk i . depth))
213 fk))
214 ((_ v #(elt ...) g s sk fk i . depth)
215 (if (vector? v)
216 (let ((ls (vector->list v)))
217 (match-quasiquote ls (elt ...) g s sk fk i . depth))
218 fk))
219 ((_ v x g s sk fk i . depth)
220 (match-one v 'x g s sk fk i))))
221
222 (define-syntax match-quasiquote-step
223 (syntax-rules ()
224 ((match-quasiquote-step x q g s sk fk depth i)
225 (match-quasiquote x q g s sk fk i . depth))
226 ))
227
228 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
229 ;; Utilities
230
231 ;; A CPS utility that takes two values and just expands into the
232 ;; first.
233 (define-syntax match-drop-ids
234 (syntax-rules ()
235 ((_ expr ids ...) expr)))
236
237 ;; Generating OR clauses just involves binding the success
238 ;; continuation into a thunk which takes the identifiers common to
239 ;; each OR clause, and trying each clause, calling the thunk as soon
240 ;; as we succeed.
241
242 (define-syntax match-gen-or
243 (syntax-rules ()
244 ((_ v p g s (sk ...) fk (i ...) ((id id-ls) ...))
245 (let ((sk2 (lambda (id ...) (sk ... (i ... id ...)))))
246 (match-gen-or-step
247 v p g s (match-drop-ids (sk2 id ...)) fk (i ...))))))
248
249 (define-syntax match-gen-or-step
250 (syntax-rules ()
251 ((_ v () g s sk fk i)
252 ;; no OR clauses, call the failure continuation
253 fk)
254 ((_ v (p) g s sk fk i)
255 ;; last (or only) OR clause, just expand normally
256 (match-one v p g s sk fk i))
257 ((_ v (p . q) g s sk fk i)
258 ;; match one and try the remaining on failure
259 (match-one v p g s sk (match-gen-or-step v q g s sk fk i) i))
260 ))
261
262 ;; We match a pattern (p ...) by matching the pattern p in a loop on
263 ;; each element of the variable, accumulating the bound ids into lists.
264
265 ;; Look at the body - it's just a named let loop, matching each
266 ;; element in turn to the same pattern. This illustrates the
267 ;; simplicity of this generative-style pattern matching. It would be
268 ;; just as easy to implement a tree searching pattern.
269
270 (define-syntax match-gen-ellipses
271 (syntax-rules ()
272 ((_ v p () g s (sk ...) fk i ((id id-ls) ...))
273 (match-check-identifier p
274 ;; simplest case equivalent to ( . p), just bind the list
275 (let ((p v))
276 (if (list? p)
277 (sk ... i)
278 fk))
279 ;; simple case, match all elements of the list
280 (let loop ((ls v) (id-ls '()) ...)
281 (cond
282 ((null? ls)
283 (let ((id (reverse id-ls)) ...) (sk ... i)))
284 ((pair? ls)
285 (let ((w (car ls)))
286 (match-one w p (car ls) (set-car! ls)
287 (match-drop-ids (loop (cdr ls) (cons id id-ls) ...))
288 fk i)))
289 (else
290 fk)))))
291 ((_ v p (r ...) g s (sk ...) fk i ((id id-ls) ...))
292 ;; general case, trailing patterns to match
293 (match-verify-no-ellipses
294 (r ...)
295 (let* ((tail-len (length '(r ...)))
296 (ls v)
297 (len (length ls)))
298 (if (< len tail-len)
299 fk
300 (let loop ((ls ls) (n len) (id-ls '()) ...)
301 (cond
302 ((= n tail-len)
303 (let ((id (reverse id-ls)) ...)
304 (match-one ls (r ...) #f #f (sk ... i) fk i)))
305 ((pair? ls)
306 (let ((w (car ls)))
307 (match-one w p (car ls) (set-car! ls)
308 (match-drop-ids
309 (loop (cdr ls) (- n 1) (cons id id-ls) ...))
310 fk
311 i)))
312 (else
313 fk)))))))
314 ))
315
316 (define-syntax match-verify-no-ellipses
317 (syntax-rules ()
318 ((_ (x . y) sk)
319 (match-check-ellipse
320 x
321 (match-syntax-error
322 "multiple ellipse patterns not allowed at same level")
323 (match-verify-no-ellipses y sk)))
324 ((_ x sk) sk)
325 ))
326
327 ;; Vector patterns are just more of the same, with the slight
328 ;; exception that we pass around the current vector index being
329 ;; matched.
330
331 (define-syntax match-vector
332 (syntax-rules (___)
333 ((_ v n pats (p q) sk fk i)
334 (match-check-ellipse q
335 (match-vector-ellipses v n pats p sk fk i)
336 (match-vector-two v n pats (p q) sk fk i)))
337 ((_ v n pats (p ___) sk fk i)
338 (match-vector-ellipses v n pats p sk fk i))
339 ((_ . x)
340 (match-vector-two . x))))
341
342 ;; Check the exact vector length, then check each element in turn.
343
344 (define-syntax match-vector-two
345 (syntax-rules ()
346 ((_ v n ((pat index) ...) () sk fk i)
347 (if (vector? v)
348 (let ((len (vector-length v)))
349 (if (= len n)
350 (match-vector-step v ((pat index) ...) sk fk i)
351 fk))
352 fk))
353 ((_ v n (pats ...) (p . q) sk fk i)
354 (match-vector v (+ n 1) (pats ... (p n)) q sk fk i))
355 ))
356
357 (define-syntax match-vector-step
358 (syntax-rules ()
359 ((_ v () (sk ...) fk i) (sk ... i))
360 ((_ v ((pat index) . rest) sk fk i)
361 (let ((w (vector-ref v index)))
362 (match-one w pat (vector-ref v index) (vector-set! v index)
363 (match-vector-step v rest sk fk)
364 fk i)))))
365
366 ;; With a vector ellipse pattern we first check to see if the vector
367 ;; length is at least the required length.
368
369 (define-syntax match-vector-ellipses
370 (syntax-rules ()
371 ((_ v n ((pat index) ...) p sk fk i)
372 (if (vector? v)
373 (let ((len (vector-length v)))
374 (if (>= len n)
375 (match-vector-step v ((pat index) ...)
376 (match-vector-tail v p n len sk fk)
377 fk i)
378 fk))
379 fk))))
380
381 (define-syntax match-vector-tail
382 (syntax-rules ()
383 ((_ v p n len sk fk i)
384 (match-extract-vars p (match-vector-tail-two v p n len sk fk i) i ()))))
385
386 (define-syntax match-vector-tail-two
387 (syntax-rules ()
388 ((_ v p n len (sk ...) fk i ((id id-ls) ...))
389 (let loop ((j n) (id-ls '()) ...)
390 (if (>= j len)
391 (let ((id (reverse id-ls)) ...) (sk ... i))
392 (let ((w (vector-ref v j)))
393 (match-one w p (vector-ref v j) (vetor-set! v j)
394 (match-drop-ids (loop (+ j 1) (cons id id-ls) ...))
395 fk i)))))))
396
397 ;; Extract all identifiers in a pattern. A little more complicated
398 ;; than just looking for symbols, we need to ignore special keywords
399 ;; and not pattern forms (such as the predicate expression in ?
400 ;; patterns).
401 ;;
402 ;; (match-extract-vars pattern continuation (ids ...) (new-vars ...))
403
404 (define-syntax match-extract-vars
405 (syntax-rules (_ ___ ? $ = quote quasiquote and or not get! set!)
406 ((match-extract-vars (? pred . p) k i v)
407 (match-extract-vars p k i v))
408 ((match-extract-vars ($ rec . p) k i v)
409 (match-extract-vars p k i v))
410 ((match-extract-vars (= proc p) k i v)
411 (match-extract-vars p k i v))
412 ((match-extract-vars (quote x) (k ...) i v)
413 (k ... v))
414 ((match-extract-vars (quasiquote x) k i v)
415 (match-extract-quasiquote-vars x k i v (#t)))
416 ((match-extract-vars (and . p) k i v)
417 (match-extract-vars p k i v))
418 ((match-extract-vars (or . p) k i v)
419 (match-extract-vars p k i v))
420 ((match-extract-vars (not . p) k i v)
421 (match-extract-vars p k i v))
422 ;; A non-keyword pair, expand the CAR with a continuation to
423 ;; expand the CDR.
424 ((match-extract-vars (p q . r) k i v)
425 (match-check-ellipse
426 q
427 (match-extract-vars (p . r) k i v)
428 (match-extract-vars p (match-extract-vars-step (q . r) k i v) i ())))
429 ((match-extract-vars (p . q) k i v)
430 (match-extract-vars p (match-extract-vars-step q k i v) i ()))
431 ((match-extract-vars #(p ...) k i v)
432 (match-extract-vars (p ...) k i v))
433 ((match-extract-vars _ (k ...) i v) (k ... v))
434 ((match-extract-vars ___ (k ...) i v) (k ... v))
435 ;; This is the main part, the only place where we might add a new
436 ;; var if it's an unbound symbol.
437 ((match-extract-vars p (k ...) (i ...) v)
438 (let-syntax
439 ((new-sym?
440 (syntax-rules (i ...)
441 ((new-sym? p sk fk) sk)
442 ((new-sym? x sk fk) fk))))
443 (new-sym? random-sym-to-match
444 (k ... ((p p-ls) . v))
445 (k ... v))))
446 ))
447
448 ;; Stepper used in the above so it can expand the CAR and CDR
449 ;; separately.
450
451 (define-syntax match-extract-vars-step
452 (syntax-rules ()
453 ((_ p k i v ((v2 v2-ls) ...))
454 (match-extract-vars p k (v2 ... . i) ((v2 v2-ls) ... . v)))
455 ))
456
457 (define-syntax match-extract-quasiquote-vars
458 (syntax-rules (quasiquote unquote unquote-splicing)
459 ((match-extract-quasiquote-vars (quasiquote x) k i v d)
460 (match-extract-quasiquote-vars x k i v (#t . d)))
461 ((match-extract-quasiquote-vars (unquote-splicing x) k i v d)
462 (match-extract-quasiquote-vars (unquote x) k i v d))
463 ((match-extract-quasiquote-vars (unquote x) k i v (#t))
464 (match-extract-vars x k i v))
465 ((match-extract-quasiquote-vars (unquote x) k i v (#t . d))
466 (match-extract-quasiquote-vars x k i v d))
467 ((match-extract-quasiquote-vars (x . y) k i v (#t . d))
468 (match-extract-quasiquote-vars
469 x
470 (match-extract-quasiquote-vars-step y k i v d) i ()))
471 ((match-extract-quasiquote-vars #(x ...) k i v (#t . d))
472 (match-extract-quasiquote-vars (x ...) k i v d))
473 ((match-extract-quasiquote-vars x (k ...) i v (#t . d))
474 (k ... v))
475 ))
476
477 (define-syntax match-extract-quasiquote-vars-step
478 (syntax-rules ()
479 ((_ x k i v d ((v2 v2-ls) ...))
480 (match-extract-quasiquote-vars x k (v2 ... . i) ((v2 v2-ls) ... . v) d))
481 ))
482
483
484 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
485 ;; Gimme some sugar baby.
486
487 (define-syntax match-lambda
488 (syntax-rules ()
489 ((_ clause ...) (lambda (expr) (match expr clause ...)))))
490
491 (define-syntax match-lambda*
492 (syntax-rules ()
493 ((_ clause ...) (lambda expr (match expr clause ...)))))
494
495 (define-syntax match-let
496 (syntax-rules ()
497 ((_ (vars ...) . body)
498 (match-let/helper let () () (vars ...) . body))
499 ((_ loop . rest)
500 (match-named-let loop () . rest))))
501
502 (define-syntax match-letrec
503 (syntax-rules ()
504 ((_ vars . body) (match-let/helper letrec () () vars . body))))
505
506 (define-syntax match-let/helper
507 (syntax-rules ()
508 ((_ let ((var expr) ...) () () . body)
509 (let ((var expr) ...) . body))
510 ((_ let ((var expr) ...) ((pat tmp) ...) () . body)
511 (let ((var expr) ...)
512 (match-let* ((pat tmp) ...)
513 . body)))
514 ((_ let (v ...) (p ...) (((a . b) expr) . rest) . body)
515 (match-let/helper
516 let (v ... (tmp expr)) (p ... ((a . b) tmp)) rest . body))
517 ((_ let (v ...) (p ...) ((#(a ...) expr) . rest) . body)
518 (match-let/helper
519 let (v ... (tmp expr)) (p ... (#(a ...) tmp)) rest . body))
520 ((_ let (v ...) (p ...) ((a expr) . rest) . body)
521 (match-let/helper let (v ... (a expr)) (p ...) rest . body))
522 ))
523
524 (define-syntax match-named-let
525 (syntax-rules ()
526 ((_ loop ((pat expr var) ...) () . body)
527 (let loop ((var expr) ...)
528 (match-let ((pat var) ...)
529 . body)))
530 ((_ loop (v ...) ((pat expr) . rest) . body)
531 (match-named-let loop (v ... (pat expr tmp)) rest . body))))
532
533 (define-syntax match-let*
534 (syntax-rules ()
535 ((_ () . body)
536 (begin . body))
537 ((_ ((pat expr) . rest) . body)
538 (match expr (pat (match-let* rest . body))))))
539
540
541 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
542 ;; Otherwise COND-EXPANDed bits.
543
544 ;; This *should* work, but doesn't :(
545 ;; (define-syntax match-check-ellipse
546 ;; (syntax-rules (...)
547 ;; ((_ ... sk fk) sk)
548 ;; ((_ x sk fk) fk)))
549
550 ;; This is a little more complicated, and introduces a new let-syntax,
551 ;; but should work portably in any R[56]RS Scheme. Taylor Campbell
552 ;; originally came up with the idea.
553 (define-syntax match-check-ellipse
554 (syntax-rules ()
555 ;; these two aren't necessary but provide fast-case failures
556 ((match-check-ellipse (a . b) success-k failure-k) failure-k)
557 ((match-check-ellipse #(a ...) success-k failure-k) failure-k)
558 ;; matching an atom
559 ((match-check-ellipse id success-k failure-k)
560 (let-syntax ((ellipse? (syntax-rules ()
561 ;; iff `id' is `...' here then this will
562 ;; match a list of any length
563 ((ellipse? (foo id) sk fk) sk)
564 ((ellipse? other sk fk) fk))))
565 ;; this list of three elements will only many the (foo id) list
566 ;; above if `id' is `...'
567 (ellipse? (a b c) success-k failure-k)))))
568
569
570 ;; This is portable but can be more efficient with non-portable
571 ;; extensions. This trick was originally discovered by Oleg Kiselyov.
572
573 (define-syntax match-check-identifier
574 (syntax-rules ()
575 ;; fast-case failures, lists and vectors are not identifiers
576 ((_ (x . y) success-k failure-k) failure-k)
577 ((_ #(x ...) success-k failure-k) failure-k)
578 ;; x is an atom
579 ((_ x success-k failure-k)
580 (let-syntax
581 ((sym?
582 (syntax-rules ()
583 ;; if the symbol `abracadabra' matches x, then x is a
584 ;; symbol
585 ((sym? x sk fk) sk)
586 ;; otherwise x is a non-symbol datum
587 ((sym? y sk fk) fk))))
588 (sym? abracadabra success-k failure-k)))
589 ))
590
591 (match (list 11 99)
592 ((a b ) (display a)))
next reply other threads:[~2019-12-15 23:47 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-15 23:47 Ben [this message]
2019-12-16 0:59 ` Per Bothner
2020-02-19 21:39 ` Duncan Mak
2020-02-20 11:00 ` Kjetil Matheussen
2020-06-29 20:50 ` Duncan Mak
2020-06-29 21:49 ` Per Bothner
2020-06-30 4:38 ` Duncan Mak
2020-06-30 5:48 ` Per Bothner
2020-07-01 4:52 ` Per Bothner
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