1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
|
;;; srfi-11.scm --- let-values and let*-values
;; Copyright (C) 2000, 2001, 2002, 2004, 2006 Free Software Foundation, Inc.
;;
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public
;; License as published by the Free Software Foundation; either
;; version 2.1 of the License, or (at your option) any later version.
;;
;; This library is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
;;; Commentary:
;; This module exports two syntax forms: let-values and let*-values.
;;
;; Sample usage:
;;
;; (let-values (((x y . z) (foo a b))
;; ((p q) (bar c)))
;; (baz x y z p q))
;;
;; This binds `x' and `y' to the first to values returned by `foo',
;; `z' to the rest of the values from `foo', and `p' and `q' to the
;; values returned by `bar'. All of these are available to `baz'.
;;
;; let*-values : let-values :: let* : let
;;
;; This module is fully documented in the Guile Reference Manual.
;;; Code:
(define-module (srfi srfi-11)
:use-module (ice-9 syncase)
:export-syntax (let-values let*-values))
(cond-expand-provide (current-module) '(srfi-11))
;;;;;;;;;;;;;;
;; let-values
;;
;; Current approach is to translate
;;
;; (let-values (((x y . z) (foo a b))
;; ((p q) (bar c)))
;; (baz x y z p q))
;;
;; into
;;
;; (call-with-values (lambda () (foo a b))
;; (lambda (<tmp-x> <tmp-y> . <tmp-z>)
;; (call-with-values (lambda () (bar c))
;; (lambda (<tmp-p> <tmp-q>)
;; (let ((x <tmp-x>)
;; (y <tmp-y>)
;; (z <tmp-z>)
;; (p <tmp-p>)
;; (q <tmp-q>))
;; (baz x y z p q))))))
;; I originally wrote this as a define-macro, but then I found out
;; that guile's gensym/gentemp was broken, so I tried rewriting it as
;; a syntax-rules statement.
;; [make-symbol now fixes gensym/gentemp problems.]
;;
;; Since syntax-rules didn't seem powerful enough to implement
;; let-values in one definition without exposing illegal syntax (or
;; perhaps my brain's just not powerful enough :>). I tried writing
;; it using a private helper, but that didn't work because the
;; let-values expands outside the scope of this module. I wonder why
;; syntax-rules wasn't designed to allow "private" patterns or
;; similar...
;;
;; So in the end, I dumped the syntax-rules implementation, reproduced
;; here for posterity, and went with the define-macro one below --
;; gensym/gentemp's got to be fixed anyhow...
;
; (define-syntax let-values-helper
; (syntax-rules ()
; ;; Take the vars from one let binding (i.e. the (x y z) from ((x y
; ;; z) (values 1 2 3)) and turn it in to the corresponding (lambda
; ;; (<tmp-x> <tmp-y> <tmp-z>) ...) from above, keeping track of the
; ;; temps you create so you can use them later...
; ;;
; ;; I really don't fully understand why the (var-1 var-1) trick
; ;; works below, but basically, when all those (x x) bindings show
; ;; up in the final "let", syntax-rules forces a renaming.
; ((_ "consumer" () lambda-tmps final-let-bindings lv-bindings
; body ...)
; (lambda lambda-tmps
; (let-values-helper "cwv" lv-bindings final-let-bindings body ...)))
; ((_ "consumer" (var-1 var-2 ...) (lambda-tmp ...) final-let-bindings lv-bindings
; body ...)
; (let-values-helper "consumer"
; (var-2 ...)
; (lambda-tmp ... var-1)
; ((var-1 var-1) . final-let-bindings)
; lv-bindings
; body ...))
; ((_ "cwv" () final-let-bindings body ...)
; (let final-let-bindings
; body ...))
; ((_ "cwv" ((vars-1 binding-1) other-bindings ...) final-let-bindings
; body ...)
; (call-with-values (lambda () binding-1)
; (let-values-helper "consumer"
; vars-1
; ()
; final-let-bindings
; (other-bindings ...)
; body ...)))))
;
; (define-syntax let-values
; (syntax-rules ()
; ((let-values () body ...)
; (begin body ...))
; ((let-values (binding ...) body ...)
; (let-values-helper "cwv" (binding ...) () body ...))))
;
;
; (define-syntax let-values
; (letrec-syntax ((build-consumer
; ;; Take the vars from one let binding (i.e. the (x
; ;; y z) from ((x y z) (values 1 2 3)) and turn it
; ;; in to the corresponding (lambda (<tmp-x> <tmp-y>
; ;; <tmp-z>) ...) from above.
; (syntax-rules ()
; ((_ () new-tmps tmp-vars () body ...)
; (lambda new-tmps
; body ...))
; ((_ () new-tmps tmp-vars vars body ...)
; (lambda new-tmps
; (lv-builder vars tmp-vars body ...)))
; ((_ (var-1 var-2 ...) new-tmps tmp-vars vars body ...)
; (build-consumer (var-2 ...)
; (tmp-1 . new-tmps)
; ((var-1 tmp-1) . tmp-vars)
; bindings
; body ...))))
; (lv-builder
; (syntax-rules ()
; ((_ () tmp-vars body ...)
; (let tmp-vars
; body ...))
; ((_ ((vars-1 binding-1) (vars-2 binding-2) ...)
; tmp-vars
; body ...)
; (call-with-values (lambda () binding-1)
; (build-consumer vars-1
; ()
; tmp-vars
; ((vars-2 binding-2) ...)
; body ...))))))
;
; (syntax-rules ()
; ((_ () body ...)
; (begin body ...))
; ((_ ((vars binding) ...) body ...)
; (lv-builder ((vars binding) ...) () body ...)))))
(define-macro (let-values vars . body)
(define (map-1-dot proc elts)
;; map over one optionally dotted (a b c . d) list, producing an
;; optionally dotted result.
(cond
((null? elts) '())
((pair? elts) (cons (proc (car elts)) (map-1-dot proc (cdr elts))))
(else (proc elts))))
(define (undot-list lst)
;; produce a non-dotted list from a possibly dotted list.
(cond
((null? lst) '())
((pair? lst) (cons (car lst) (undot-list (cdr lst))))
(else (list lst))))
(define (let-values-helper vars body prev-let-vars)
(let* ((var-binding (car vars))
(new-tmps (map-1-dot (lambda (sym) (make-symbol "let-values-var"))
(car var-binding)))
(let-vars (map (lambda (sym tmp) (list sym tmp))
(undot-list (car var-binding))
(undot-list new-tmps))))
(if (null? (cdr vars))
`(call-with-values (lambda () ,(cadr var-binding))
(lambda ,new-tmps
(let ,(apply append let-vars prev-let-vars)
,@body)))
`(call-with-values (lambda () ,(cadr var-binding))
(lambda ,new-tmps
,(let-values-helper (cdr vars) body
(cons let-vars prev-let-vars)))))))
(if (null? vars)
`(begin ,@body)
(let-values-helper vars body '())))
;;;;;;;;;;;;;;
;; let*-values
;;
;; Current approach is to translate
;;
;; (let*-values (((x y z) (foo a b))
;; ((p q) (bar c)))
;; (baz x y z p q))
;;
;; into
;;
;; (call-with-values (lambda () (foo a b))
;; (lambda (x y z)
;; (call-with-values (lambda (bar c))
;; (lambda (p q)
;; (baz x y z p q)))))
(define-syntax let*-values
(syntax-rules ()
((let*-values () body ...)
(begin body ...))
((let*-values ((vars-1 binding-1) (vars-2 binding-2) ...) body ...)
(call-with-values (lambda () binding-1)
(lambda vars-1
(let*-values ((vars-2 binding-2) ...)
body ...))))))
; Alternate define-macro implementation...
;
; (define-macro (let*-values vars . body)
; (define (let-values-helper vars body)
; (let ((var-binding (car vars)))
; (if (null? (cdr vars))
; `(call-with-values (lambda () ,(cadr var-binding))
; (lambda ,(car var-binding)
; ,@body))
; `(call-with-values (lambda () ,(cadr var-binding))
; (lambda ,(car var-binding)
; ,(let-values-helper (cdr vars) body))))))
; (if (null? vars)
; `(begin ,@body)
; (let-values-helper vars body)))
;;; srfi-11.scm ends here
|