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b860f382 | 1 | ;;; GNU Guix --- Functional package management for GNU |
dcb95c1f | 2 | ;;; Copyright © 2013, 2014, 2015, 2017 Ludovic Courtès <ludo@gnu.org> |
b860f382 LC |
3 | ;;; |
4 | ;;; This file is part of GNU Guix. | |
5 | ;;; | |
6 | ;;; GNU Guix is free software; you can redistribute it and/or modify it | |
7 | ;;; under the terms of the GNU General Public License as published by | |
8 | ;;; the Free Software Foundation; either version 3 of the License, or (at | |
9 | ;;; your option) any later version. | |
10 | ;;; | |
11 | ;;; GNU Guix is distributed in the hope that it will be useful, but | |
12 | ;;; WITHOUT ANY WARRANTY; without even the implied warranty of | |
13 | ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
14 | ;;; GNU General Public License for more details. | |
15 | ;;; | |
16 | ;;; You should have received a copy of the GNU General Public License | |
17 | ;;; along with GNU Guix. If not, see <http://www.gnu.org/licenses/>. | |
18 | ||
19 | (define-module (guix monads) | |
aeb7ec5c LC |
20 | #:use-module ((system syntax) |
21 | #:select (syntax-local-binding)) | |
b860f382 | 22 | #:use-module (ice-9 match) |
45adbd62 | 23 | #:use-module (srfi srfi-1) |
aeb7ec5c | 24 | #:use-module (srfi srfi-9) |
b860f382 LC |
25 | #:use-module (srfi srfi-26) |
26 | #:export (;; Monads. | |
aeb7ec5c | 27 | define-monad |
b860f382 LC |
28 | monad? |
29 | monad-bind | |
30 | monad-return | |
31 | ||
dcb95c1f LC |
32 | template-directory |
33 | ||
b860f382 LC |
34 | ;; Syntax. |
35 | >>= | |
36 | return | |
37 | with-monad | |
38 | mlet | |
39 | mlet* | |
405a9d4e | 40 | mbegin |
21caa6de LC |
41 | mwhen |
42 | munless | |
b307c064 | 43 | lift0 lift1 lift2 lift3 lift4 lift5 lift6 lift7 lift |
b860f382 LC |
44 | listm |
45 | foldm | |
46 | mapm | |
47 | sequence | |
48 | anym | |
49 | ||
50 | ;; Concrete monads. | |
81a97734 LC |
51 | %identity-monad |
52 | ||
53 | %state-monad | |
54 | state-return | |
55 | state-bind | |
56 | current-state | |
57 | set-current-state | |
58 | state-push | |
59 | state-pop | |
60 | run-with-state)) | |
b860f382 LC |
61 | |
62 | ;;; Commentary: | |
63 | ;;; | |
64 | ;;; This module implements the general mechanism of monads, and provides in | |
561fb6c3 | 65 | ;;; particular an instance of the "state" monad. The API was inspired by that |
b860f382 LC |
66 | ;;; of Racket's "better-monads" module (see |
67 | ;;; <http://planet.racket-lang.org/package-source/toups/functional.plt/1/1/planet-docs/better-monads-guide/index.html>). | |
68 | ;;; The implementation and use case were influenced by Oleg Kysielov's | |
69 | ;;; "Monadic Programming in Scheme" (see | |
70 | ;;; <http://okmij.org/ftp/Scheme/monad-in-Scheme.html>). | |
71 | ;;; | |
b860f382 LC |
72 | ;;; Code: |
73 | ||
aeb7ec5c LC |
74 | ;; Record type for monads manipulated at run time. |
75 | (define-record-type <monad> | |
76 | (make-monad bind return) | |
b860f382 LC |
77 | monad? |
78 | (bind monad-bind) | |
79 | (return monad-return)) ; TODO: Add 'plus' and 'zero' | |
80 | ||
aeb7ec5c LC |
81 | (define-syntax define-monad |
82 | (lambda (s) | |
83 | "Define the monad under NAME, with the given bind and return methods." | |
84 | (define prefix (string->symbol "% ")) | |
85 | (define (make-rtd-name name) | |
86 | (datum->syntax name | |
87 | (symbol-append prefix (syntax->datum name) '-rtd))) | |
88 | ||
89 | (syntax-case s (bind return) | |
90 | ((_ name (bind b) (return r)) | |
91 | (with-syntax ((rtd (make-rtd-name #'name))) | |
92 | #`(begin | |
93 | (define rtd | |
94 | ;; The record type, for use at run time. | |
95 | (make-monad b r)) | |
96 | ||
dcb95c1f LC |
97 | ;; Instantiate all the templates, specialized for this monad. |
98 | (template-directory instantiations name) | |
99 | ||
aeb7ec5c LC |
100 | (define-syntax name |
101 | ;; An "inlined record", for use at expansion time. The goal is | |
102 | ;; to allow 'bind' and 'return' to be resolved at expansion | |
103 | ;; time, in the common case where the monad is accessed | |
104 | ;; directly as NAME. | |
105 | (lambda (s) | |
106 | (syntax-case s (%bind %return) | |
107 | ((_ %bind) #'b) | |
108 | ((_ %return) #'r) | |
109 | (_ #'rtd)))))))))) | |
110 | ||
dcb95c1f LC |
111 | ;; Expansion- and run-time state of the template directory. This needs to be |
112 | ;; available at run time (and not just at expansion time) so we can | |
113 | ;; instantiate templates defined in other modules, or use instances defined | |
114 | ;; elsewhere. | |
115 | (eval-when (load expand eval) | |
116 | ;; Mapping of syntax objects denoting the template to a pair containing (1) | |
117 | ;; the syntax object of the parameter over which it is templated, and (2) | |
118 | ;; the syntax of its body. | |
119 | (define-once %templates (make-hash-table)) | |
120 | ||
121 | (define (register-template! name param body) | |
122 | (hash-set! %templates name (cons param body))) | |
123 | ||
124 | ;; List of template instances, where each entry is a triplet containing the | |
125 | ;; syntax of the name, the actual parameter for which the template is | |
126 | ;; specialized, and the syntax object referring to this specialization (the | |
127 | ;; procedure's identifier.) | |
128 | (define-once %template-instances '()) | |
129 | ||
130 | (define (register-template-instance! name actual instance) | |
131 | (set! %template-instances | |
132 | (cons (list name actual instance) %template-instances)))) | |
133 | ||
134 | (define-syntax template-directory | |
135 | (lambda (s) | |
136 | "This is a \"stateful macro\" to register and lookup templates and | |
137 | template instances." | |
138 | (define location | |
139 | (syntax-source s)) | |
140 | ||
141 | (define current-info-port | |
142 | ;; Port for debugging info. | |
143 | (const (%make-void-port "w"))) | |
144 | ||
145 | (define location-string | |
146 | (format #f "~a:~a:~a" | |
147 | (assq-ref location 'filename) | |
148 | (and=> (assq-ref location 'line) 1+) | |
149 | (assq-ref location 'column))) | |
150 | ||
151 | (define (matching-instance? name actual) | |
152 | (match-lambda | |
153 | ((name* instance-param proc) | |
154 | (and (free-identifier=? name name*) | |
155 | (or (equal? actual instance-param) | |
156 | (and (identifier? actual) | |
157 | (identifier? instance-param) | |
158 | (free-identifier=? instance-param | |
159 | actual))) | |
160 | proc)))) | |
161 | ||
162 | (define (instance-identifier name actual) | |
163 | (define stem | |
164 | (string-append | |
165 | " " | |
166 | (symbol->string (syntax->datum name)) | |
167 | (if (identifier? actual) | |
168 | (string-append " " (symbol->string (syntax->datum actual))) | |
169 | "") | |
170 | " instance")) | |
171 | (datum->syntax actual (string->symbol stem))) | |
172 | ||
173 | (define (instance-definition name template actual) | |
174 | (match template | |
175 | ((formal . body) | |
176 | (let ((instance (instance-identifier name actual))) | |
177 | (format (current-info-port) | |
178 | "~a: info: specializing '~a' for '~a' as '~a'~%" | |
179 | location-string | |
180 | (syntax->datum name) (syntax->datum actual) | |
181 | (syntax->datum instance)) | |
182 | ||
183 | (register-template-instance! name actual instance) | |
184 | ||
185 | #`(begin | |
186 | (define #,instance | |
187 | (let-syntax ((#,formal (identifier-syntax #,actual))) | |
188 | #,body)) | |
189 | ||
190 | ;; Generate code to register the thing at run time. | |
191 | (register-template-instance! #'#,name #'#,actual | |
192 | #'#,instance)))))) | |
193 | ||
194 | (syntax-case s (register! lookup exists? instantiations) | |
195 | ((_ register! name param body) | |
196 | ;; Register NAME as a template on PARAM with the given BODY. | |
197 | (begin | |
198 | (register-template! #'name #'param #'body) | |
199 | ||
200 | ;; Generate code to register the template at run time. XXX: Because | |
201 | ;; of this, BODY must not contain ellipses. | |
202 | #'(register-template! #'name #'param #'body))) | |
203 | ((_ lookup name actual) | |
204 | ;; Search for an instance of template NAME for this ACTUAL parameter. | |
205 | ;; On success, expand to the identifier of the instance; otherwise | |
206 | ;; expand to #f. | |
207 | (any (matching-instance? #'name #'actual) %template-instances)) | |
208 | ((_ exists? name actual) | |
209 | ;; Likewise, but return a Boolean. | |
210 | (let ((result (->bool | |
211 | (any (matching-instance? #'name #'actual) | |
212 | %template-instances)))) | |
213 | (unless result | |
214 | (format (current-warning-port) | |
215 | "~a: warning: no specialization of template '~a' for '~a'~%" | |
216 | location-string | |
217 | (syntax->datum #'name) (syntax->datum #'actual))) | |
218 | result)) | |
219 | ((_ instantiations actual) | |
220 | ;; Expand to the definitions of all the existing templates | |
221 | ;; specialized for ACTUAL. | |
222 | #`(begin | |
223 | #,@(hash-map->list (cut instance-definition <> <> #'actual) | |
224 | %templates)))))) | |
225 | ||
226 | (define-syntax define-template | |
227 | (lambda (s) | |
228 | "Define a template, which is a procedure that can be specialized over its | |
229 | first argument. In our case, the first argument is typically the identifier | |
230 | of a monad. | |
231 | ||
232 | Defining templates for procedures like 'mapm' allows us to make have a | |
233 | specialized version of those procedures for each monad that we define, such | |
234 | that calls to: | |
235 | ||
236 | (mapm %state-monad proc lst) | |
237 | ||
238 | automatically expand to: | |
239 | ||
240 | (#{ mapm %state-monad instance}# proc lst) | |
241 | ||
242 | Here, #{ mapm %state-monad instance}# is specialized for %state-monad, and | |
243 | thus it contains inline calls to %state-bind and %state-return. This avoids | |
244 | repeated calls to 'struct-ref' to get the 'bind' and 'return' procedure of the | |
245 | monad, and allows 'bind' and 'return' to be inlined, which in turn allows for | |
246 | more optimizations." | |
247 | (syntax-case s () | |
248 | ((_ (name arg0 args ...) body ...) | |
249 | (with-syntax ((generic-name (datum->syntax | |
250 | #'name | |
251 | (symbol-append '#{ %}# | |
252 | (syntax->datum #'name) | |
253 | '-generic))) | |
254 | (original-name #'name)) | |
255 | #`(begin | |
256 | (template-directory register! name arg0 | |
257 | (lambda (args ...) | |
258 | body ...)) | |
259 | (define (generic-name arg0 args ...) | |
260 | ;; The generic instance of NAME, for when no specialization was | |
261 | ;; found. | |
262 | body ...) | |
263 | ||
264 | (define-syntax name | |
265 | (lambda (s) | |
266 | (syntax-case s () | |
267 | ((_ arg0* args ...) | |
268 | ;; Expand to either the specialized instance or the | |
269 | ;; generic instance of template ORIGINAL-NAME. | |
270 | #'(if (template-directory exists? original-name arg0*) | |
271 | ((template-directory lookup original-name arg0*) | |
272 | args ...) | |
273 | (generic-name arg0* args ...))) | |
274 | (_ | |
275 | #'generic-name)))))))))) | |
276 | ||
4f621a2b | 277 | (define-syntax-parameter >>= |
b860f382 LC |
278 | ;; The name 'bind' is already taken, so we choose this (obscure) symbol. |
279 | (lambda (s) | |
280 | (syntax-violation '>>= ">>= (bind) used outside of 'with-monad'" s))) | |
281 | ||
4f621a2b | 282 | (define-syntax-parameter return |
b860f382 LC |
283 | (lambda (s) |
284 | (syntax-violation 'return "return used outside of 'with-monad'" s))) | |
285 | ||
751630c9 LC |
286 | (define-syntax-rule (bind-syntax bind) |
287 | "Return a macro transformer that handles the expansion of '>>=' expressions | |
288 | using BIND as the binary bind operator. | |
289 | ||
290 | This macro exists to allow the expansion of n-ary '>>=' expressions, even | |
291 | though BIND is simply binary, as in: | |
292 | ||
293 | (with-monad %state-monad | |
294 | (>>= (return 1) | |
295 | (lift 1+ %state-monad) | |
296 | (lift 1+ %state-monad))) | |
297 | " | |
298 | (lambda (stx) | |
299 | (define (expand body) | |
300 | (syntax-case body () | |
301 | ((_ mval mproc) | |
302 | #'(bind mval mproc)) | |
303 | ((x mval mproc0 mprocs (... ...)) | |
304 | (expand #'(>>= (>>= mval mproc0) | |
305 | mprocs (... ...)))))) | |
306 | ||
307 | (expand stx))) | |
308 | ||
b860f382 LC |
309 | (define-syntax with-monad |
310 | (lambda (s) | |
311 | "Evaluate BODY in the context of MONAD, and return its result." | |
312 | (syntax-case s () | |
313 | ((_ monad body ...) | |
aeb7ec5c LC |
314 | (eq? 'macro (syntax-local-binding #'monad)) |
315 | ;; MONAD is a syntax transformer, so we can obtain the bind and return | |
316 | ;; methods by directly querying it. | |
751630c9 | 317 | #'(syntax-parameterize ((>>= (bind-syntax (monad %bind))) |
aeb7ec5c LC |
318 | (return (identifier-syntax (monad %return)))) |
319 | body ...)) | |
320 | ((_ monad body ...) | |
321 | ;; MONAD refers to the <monad> record that represents the monad at run | |
322 | ;; time, so use the slow method. | |
751630c9 | 323 | #'(syntax-parameterize ((>>= (bind-syntax |
b860f382 LC |
324 | (monad-bind monad))) |
325 | (return (identifier-syntax | |
326 | (monad-return monad)))) | |
327 | body ...))))) | |
328 | ||
329 | (define-syntax mlet* | |
330 | (syntax-rules (->) | |
331 | "Bind the given monadic values MVAL to the given variables VAR. When the | |
332 | form is (VAR -> VAL), bind VAR to the non-monadic value VAL in the same way as | |
333 | 'let'." | |
334 | ;; Note: the '->' symbol corresponds to 'is:' in 'better-monads.rkt'. | |
335 | ((_ monad () body ...) | |
336 | (with-monad monad body ...)) | |
337 | ((_ monad ((var mval) rest ...) body ...) | |
338 | (with-monad monad | |
339 | (>>= mval | |
340 | (lambda (var) | |
341 | (mlet* monad (rest ...) | |
342 | body ...))))) | |
343 | ((_ monad ((var -> val) rest ...) body ...) | |
344 | (let ((var val)) | |
345 | (mlet* monad (rest ...) | |
346 | body ...))))) | |
347 | ||
348 | (define-syntax mlet | |
349 | (lambda (s) | |
350 | (syntax-case s () | |
351 | ((_ monad ((var mval ...) ...) body ...) | |
352 | (with-syntax (((temp ...) (generate-temporaries #'(var ...)))) | |
353 | #'(mlet* monad ((temp mval ...) ...) | |
354 | (let ((var temp) ...) | |
355 | body ...))))))) | |
356 | ||
405a9d4e | 357 | (define-syntax mbegin |
21caa6de | 358 | (syntax-rules (%current-monad) |
8bc2183f CM |
359 | "Bind MEXP and the following monadic expressions in sequence, returning |
360 | the result of the last expression. Every expression in the sequence must be a | |
361 | monadic expression." | |
21caa6de LC |
362 | ((_ %current-monad mexp) |
363 | mexp) | |
364 | ((_ %current-monad mexp rest ...) | |
365 | (>>= mexp | |
366 | (lambda (unused-value) | |
367 | (mbegin %current-monad rest ...)))) | |
405a9d4e LC |
368 | ((_ monad mexp) |
369 | (with-monad monad | |
370 | mexp)) | |
371 | ((_ monad mexp rest ...) | |
372 | (with-monad monad | |
373 | (>>= mexp | |
374 | (lambda (unused-value) | |
375 | (mbegin monad rest ...))))))) | |
376 | ||
21caa6de LC |
377 | (define-syntax mwhen |
378 | (syntax-rules () | |
60a9fcb1 CM |
379 | "When CONDITION is true, evaluate the sequence of monadic expressions |
380 | MEXP0..MEXP* as in an 'mbegin'. When CONDITION is false, return *unspecified* | |
381 | in the current monad. Every expression in the sequence must be a monadic | |
382 | expression." | |
d922c8e4 | 383 | ((_ condition mexp0 mexp* ...) |
21caa6de LC |
384 | (if condition |
385 | (mbegin %current-monad | |
d922c8e4 | 386 | mexp0 mexp* ...) |
21caa6de LC |
387 | (return *unspecified*))))) |
388 | ||
389 | (define-syntax munless | |
390 | (syntax-rules () | |
60a9fcb1 CM |
391 | "When CONDITION is false, evaluate the sequence of monadic expressions |
392 | MEXP0..MEXP* as in an 'mbegin'. When CONDITION is true, return *unspecified* | |
393 | in the current monad. Every expression in the sequence must be a monadic | |
394 | expression." | |
d922c8e4 | 395 | ((_ condition mexp0 mexp* ...) |
21caa6de LC |
396 | (if condition |
397 | (return *unspecified*) | |
398 | (mbegin %current-monad | |
d922c8e4 | 399 | mexp0 mexp* ...))))) |
21caa6de | 400 | |
b860f382 LC |
401 | (define-syntax define-lift |
402 | (syntax-rules () | |
403 | ((_ liftn (args ...)) | |
b6c6105c LC |
404 | (define-syntax liftn |
405 | (lambda (s) | |
406 | "Lift PROC to MONAD---i.e., return a monadic function in MONAD." | |
407 | (syntax-case s () | |
408 | ((liftn proc monad) | |
409 | ;; Inline the result of lifting PROC, such that 'return' can in | |
410 | ;; turn be open-coded. | |
411 | #'(lambda (args ...) | |
412 | (with-monad monad | |
413 | (return (proc args ...))))) | |
414 | (id | |
415 | (identifier? #'id) | |
416 | ;; Slow path: Return a closure-returning procedure (we don't | |
417 | ;; guarantee (eq? LIFTN LIFTN), but that's fine.) | |
dbbc248a LC |
418 | #'(lambda (proc monad) |
419 | (lambda (args ...) | |
420 | (with-monad monad | |
421 | (return (proc args ...)))))))))))) | |
b860f382 | 422 | |
b307c064 | 423 | (define-lift lift0 ()) |
b860f382 LC |
424 | (define-lift lift1 (a)) |
425 | (define-lift lift2 (a b)) | |
426 | (define-lift lift3 (a b c)) | |
427 | (define-lift lift4 (a b c d)) | |
428 | (define-lift lift5 (a b c d e)) | |
429 | (define-lift lift6 (a b c d e f)) | |
430 | (define-lift lift7 (a b c d e f g)) | |
431 | ||
e4bed284 LC |
432 | (define (lift proc monad) |
433 | "Lift PROC, a procedure that accepts an arbitrary number of arguments, to | |
434 | MONAD---i.e., return a monadic function in MONAD." | |
b860f382 LC |
435 | (lambda args |
436 | (with-monad monad | |
437 | (return (apply proc args))))) | |
438 | ||
dcb95c1f | 439 | (define-template (foldm monad mproc init lst) |
b734996f LC |
440 | "Fold MPROC over LST and return a monadic value seeded by INIT. |
441 | ||
442 | (foldm %state-monad (lift2 cons %state-monad) '() '(a b c)) | |
443 | => '(c b a) ;monadic | |
444 | " | |
b860f382 LC |
445 | (with-monad monad |
446 | (let loop ((lst lst) | |
447 | (result init)) | |
448 | (match lst | |
449 | (() | |
450 | (return result)) | |
dcb95c1f | 451 | ((head . tail) |
b734996f LC |
452 | (>>= (mproc head result) |
453 | (lambda (result) | |
454 | (loop tail result)))))))) | |
b860f382 | 455 | |
dcb95c1f | 456 | (define-template (mapm monad mproc lst) |
b734996f LC |
457 | "Map MPROC over LST and return a monadic list. |
458 | ||
459 | (mapm %state-monad (lift1 1+ %state-monad) '(0 1 2)) | |
460 | => (1 2 3) ;monadic | |
461 | " | |
dcb95c1f LC |
462 | ;; XXX: We don't use 'foldm' because template specialization wouldn't work |
463 | ;; in this context. | |
464 | (with-monad monad | |
465 | (let mapm ((lst lst) | |
466 | (result '())) | |
467 | (match lst | |
468 | (() | |
469 | (return (reverse result))) | |
470 | ((head . tail) | |
471 | (>>= (mproc head) | |
472 | (lambda (head) | |
473 | (mapm tail (cons head result))))))))) | |
474 | ||
475 | (define-template (sequence monad lst) | |
b860f382 LC |
476 | "Turn the list of monadic values LST into a monadic list of values, by |
477 | evaluating each item of LST in sequence." | |
b860f382 | 478 | (with-monad monad |
8d7dc5d9 LC |
479 | (let seq ((lstx lst) |
480 | (result '())) | |
481 | (match lstx | |
482 | (() | |
483 | (return (reverse result))) | |
484 | ((head . tail) | |
485 | (>>= head | |
486 | (lambda (item) | |
487 | (seq tail (cons item result))))))))) | |
b860f382 | 488 | |
dcb95c1f | 489 | (define-template (anym monad mproc lst) |
b734996f LC |
490 | "Apply MPROC to the list of values LST; return as a monadic value the first |
491 | value for which MPROC returns a true monadic value or #f. For example: | |
492 | ||
493 | (anym %state-monad (lift1 odd? %state-monad) '(0 1 2)) | |
494 | => #t ;monadic | |
495 | " | |
b860f382 LC |
496 | (with-monad monad |
497 | (let loop ((lst lst)) | |
498 | (match lst | |
499 | (() | |
500 | (return #f)) | |
dcb95c1f | 501 | ((head . tail) |
b734996f LC |
502 | (>>= (mproc head) |
503 | (lambda (result) | |
504 | (if result | |
505 | (return result) | |
506 | (loop tail))))))))) | |
b860f382 LC |
507 | |
508 | (define-syntax listm | |
509 | (lambda (s) | |
510 | "Return a monadic list in MONAD from the monadic values MVAL." | |
511 | (syntax-case s () | |
512 | ((_ monad mval ...) | |
513 | (with-syntax (((val ...) (generate-temporaries #'(mval ...)))) | |
514 | #'(mlet monad ((val mval) ...) | |
515 | (return (list val ...)))))))) | |
516 | ||
517 | ||
518 | \f | |
519 | ;;; | |
520 | ;;; Identity monad. | |
521 | ;;; | |
522 | ||
aeb7ec5c | 523 | (define-inlinable (identity-return value) |
b860f382 LC |
524 | value) |
525 | ||
aeb7ec5c | 526 | (define-inlinable (identity-bind mvalue mproc) |
b860f382 LC |
527 | (mproc mvalue)) |
528 | ||
aeb7ec5c LC |
529 | (define-monad %identity-monad |
530 | (bind identity-bind) | |
531 | (return identity-return)) | |
b860f382 | 532 | |
81a97734 LC |
533 | \f |
534 | ;;; | |
535 | ;;; State monad. | |
536 | ;;; | |
537 | ||
538 | (define-inlinable (state-return value) | |
539 | (lambda (state) | |
540 | (values value state))) | |
541 | ||
542 | (define-inlinable (state-bind mvalue mproc) | |
543 | "Bind MVALUE, a value in the state monad, and pass it to MPROC." | |
544 | (lambda (state) | |
545 | (call-with-values | |
546 | (lambda () | |
547 | (mvalue state)) | |
548 | (lambda (value state) | |
549 | ;; Note: as of Guile 2.0.11, declaring a variable to hold the result | |
550 | ;; of (mproc value) prevents a bit of unfolding/inlining. | |
551 | ((mproc value) state))))) | |
552 | ||
553 | (define-monad %state-monad | |
554 | (bind state-bind) | |
555 | (return state-return)) | |
556 | ||
557 | (define* (run-with-state mval #:optional (state '())) | |
558 | "Run monadic value MVAL starting with STATE as the initial state. Return | |
559 | two values: the resulting value, and the resulting state." | |
560 | (mval state)) | |
561 | ||
562 | (define-inlinable (current-state) | |
563 | "Return the current state as a monadic value." | |
564 | (lambda (state) | |
565 | (values state state))) | |
566 | ||
567 | (define-inlinable (set-current-state value) | |
568 | "Set the current state to VALUE and return the previous state as a monadic | |
569 | value." | |
570 | (lambda (state) | |
571 | (values state value))) | |
572 | ||
573 | (define (state-pop) | |
574 | "Pop a value from the current state and return it as a monadic value. The | |
575 | state is assumed to be a list." | |
576 | (lambda (state) | |
577 | (match state | |
578 | ((head . tail) | |
579 | (values head tail))))) | |
580 | ||
581 | (define (state-push value) | |
582 | "Push VALUE to the current state, which is assumed to be a list, and return | |
583 | the previous state as a monadic value." | |
584 | (lambda (state) | |
585 | (values state (cons value state)))) | |
586 | ||
b860f382 | 587 | ;;; monads.scm end here |