Commit | Line | Data |
---|---|---|
b860f382 | 1 | ;;; GNU Guix --- Functional package management for GNU |
462a3fa3 | 2 | ;;; Copyright © 2013, 2014, 2015 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 | ||
32 | ;; Syntax. | |
33 | >>= | |
34 | return | |
35 | with-monad | |
36 | mlet | |
37 | mlet* | |
405a9d4e | 38 | mbegin |
21caa6de LC |
39 | mwhen |
40 | munless | |
b307c064 | 41 | lift0 lift1 lift2 lift3 lift4 lift5 lift6 lift7 lift |
b860f382 LC |
42 | listm |
43 | foldm | |
44 | mapm | |
45 | sequence | |
46 | anym | |
47 | ||
48 | ;; Concrete monads. | |
81a97734 LC |
49 | %identity-monad |
50 | ||
51 | %state-monad | |
52 | state-return | |
53 | state-bind | |
54 | current-state | |
55 | set-current-state | |
56 | state-push | |
57 | state-pop | |
58 | run-with-state)) | |
b860f382 LC |
59 | |
60 | ;;; Commentary: | |
61 | ;;; | |
62 | ;;; This module implements the general mechanism of monads, and provides in | |
561fb6c3 | 63 | ;;; particular an instance of the "state" monad. The API was inspired by that |
b860f382 LC |
64 | ;;; of Racket's "better-monads" module (see |
65 | ;;; <http://planet.racket-lang.org/package-source/toups/functional.plt/1/1/planet-docs/better-monads-guide/index.html>). | |
66 | ;;; The implementation and use case were influenced by Oleg Kysielov's | |
67 | ;;; "Monadic Programming in Scheme" (see | |
68 | ;;; <http://okmij.org/ftp/Scheme/monad-in-Scheme.html>). | |
69 | ;;; | |
b860f382 LC |
70 | ;;; Code: |
71 | ||
aeb7ec5c LC |
72 | ;; Record type for monads manipulated at run time. |
73 | (define-record-type <monad> | |
74 | (make-monad bind return) | |
b860f382 LC |
75 | monad? |
76 | (bind monad-bind) | |
77 | (return monad-return)) ; TODO: Add 'plus' and 'zero' | |
78 | ||
aeb7ec5c LC |
79 | (define-syntax define-monad |
80 | (lambda (s) | |
81 | "Define the monad under NAME, with the given bind and return methods." | |
82 | (define prefix (string->symbol "% ")) | |
83 | (define (make-rtd-name name) | |
84 | (datum->syntax name | |
85 | (symbol-append prefix (syntax->datum name) '-rtd))) | |
86 | ||
87 | (syntax-case s (bind return) | |
88 | ((_ name (bind b) (return r)) | |
89 | (with-syntax ((rtd (make-rtd-name #'name))) | |
90 | #`(begin | |
91 | (define rtd | |
92 | ;; The record type, for use at run time. | |
93 | (make-monad b r)) | |
94 | ||
95 | (define-syntax name | |
96 | ;; An "inlined record", for use at expansion time. The goal is | |
97 | ;; to allow 'bind' and 'return' to be resolved at expansion | |
98 | ;; time, in the common case where the monad is accessed | |
99 | ;; directly as NAME. | |
100 | (lambda (s) | |
101 | (syntax-case s (%bind %return) | |
102 | ((_ %bind) #'b) | |
103 | ((_ %return) #'r) | |
104 | (_ #'rtd)))))))))) | |
105 | ||
b860f382 LC |
106 | (define-syntax-parameter >>= |
107 | ;; The name 'bind' is already taken, so we choose this (obscure) symbol. | |
108 | (lambda (s) | |
109 | (syntax-violation '>>= ">>= (bind) used outside of 'with-monad'" s))) | |
110 | ||
111 | (define-syntax-parameter return | |
112 | (lambda (s) | |
113 | (syntax-violation 'return "return used outside of 'with-monad'" s))) | |
114 | ||
115 | (define-syntax with-monad | |
116 | (lambda (s) | |
117 | "Evaluate BODY in the context of MONAD, and return its result." | |
118 | (syntax-case s () | |
119 | ((_ monad body ...) | |
aeb7ec5c LC |
120 | (eq? 'macro (syntax-local-binding #'monad)) |
121 | ;; MONAD is a syntax transformer, so we can obtain the bind and return | |
122 | ;; methods by directly querying it. | |
123 | #'(syntax-parameterize ((>>= (identifier-syntax (monad %bind))) | |
124 | (return (identifier-syntax (monad %return)))) | |
125 | body ...)) | |
126 | ((_ monad body ...) | |
127 | ;; MONAD refers to the <monad> record that represents the monad at run | |
128 | ;; time, so use the slow method. | |
b860f382 LC |
129 | #'(syntax-parameterize ((>>= (identifier-syntax |
130 | (monad-bind monad))) | |
131 | (return (identifier-syntax | |
132 | (monad-return monad)))) | |
133 | body ...))))) | |
134 | ||
135 | (define-syntax mlet* | |
136 | (syntax-rules (->) | |
137 | "Bind the given monadic values MVAL to the given variables VAR. When the | |
138 | form is (VAR -> VAL), bind VAR to the non-monadic value VAL in the same way as | |
139 | 'let'." | |
140 | ;; Note: the '->' symbol corresponds to 'is:' in 'better-monads.rkt'. | |
141 | ((_ monad () body ...) | |
142 | (with-monad monad body ...)) | |
143 | ((_ monad ((var mval) rest ...) body ...) | |
144 | (with-monad monad | |
145 | (>>= mval | |
146 | (lambda (var) | |
147 | (mlet* monad (rest ...) | |
148 | body ...))))) | |
149 | ((_ monad ((var -> val) rest ...) body ...) | |
150 | (let ((var val)) | |
151 | (mlet* monad (rest ...) | |
152 | body ...))))) | |
153 | ||
154 | (define-syntax mlet | |
155 | (lambda (s) | |
156 | (syntax-case s () | |
157 | ((_ monad ((var mval ...) ...) body ...) | |
158 | (with-syntax (((temp ...) (generate-temporaries #'(var ...)))) | |
159 | #'(mlet* monad ((temp mval ...) ...) | |
160 | (let ((var temp) ...) | |
161 | body ...))))))) | |
162 | ||
405a9d4e | 163 | (define-syntax mbegin |
21caa6de | 164 | (syntax-rules (%current-monad) |
405a9d4e LC |
165 | "Bind the given monadic expressions in sequence, returning the result of |
166 | the last one." | |
21caa6de LC |
167 | ((_ %current-monad mexp) |
168 | mexp) | |
169 | ((_ %current-monad mexp rest ...) | |
170 | (>>= mexp | |
171 | (lambda (unused-value) | |
172 | (mbegin %current-monad rest ...)))) | |
405a9d4e LC |
173 | ((_ monad mexp) |
174 | (with-monad monad | |
175 | mexp)) | |
176 | ((_ monad mexp rest ...) | |
177 | (with-monad monad | |
178 | (>>= mexp | |
179 | (lambda (unused-value) | |
180 | (mbegin monad rest ...))))))) | |
181 | ||
21caa6de LC |
182 | (define-syntax mwhen |
183 | (syntax-rules () | |
184 | "When CONDITION is true, evaluate EXP0..EXP* as in an 'mbegin'. When | |
185 | CONDITION is false, return *unspecified* in the current monad." | |
186 | ((_ condition exp0 exp* ...) | |
187 | (if condition | |
188 | (mbegin %current-monad | |
189 | exp0 exp* ...) | |
190 | (return *unspecified*))))) | |
191 | ||
192 | (define-syntax munless | |
193 | (syntax-rules () | |
194 | "When CONDITION is false, evaluate EXP0..EXP* as in an 'mbegin'. When | |
195 | CONDITION is true, return *unspecified* in the current monad." | |
196 | ((_ condition exp0 exp* ...) | |
197 | (if condition | |
198 | (return *unspecified*) | |
199 | (mbegin %current-monad | |
200 | exp0 exp* ...))))) | |
201 | ||
b860f382 LC |
202 | (define-syntax define-lift |
203 | (syntax-rules () | |
204 | ((_ liftn (args ...)) | |
205 | (define (liftn proc monad) | |
206 | "Lift PROC to MONAD---i.e., return a monadic function in MONAD." | |
207 | (lambda (args ...) | |
208 | (with-monad monad | |
209 | (return (proc args ...)))))))) | |
210 | ||
b307c064 | 211 | (define-lift lift0 ()) |
b860f382 LC |
212 | (define-lift lift1 (a)) |
213 | (define-lift lift2 (a b)) | |
214 | (define-lift lift3 (a b c)) | |
215 | (define-lift lift4 (a b c d)) | |
216 | (define-lift lift5 (a b c d e)) | |
217 | (define-lift lift6 (a b c d e f)) | |
218 | (define-lift lift7 (a b c d e f g)) | |
219 | ||
e4bed284 LC |
220 | (define (lift proc monad) |
221 | "Lift PROC, a procedure that accepts an arbitrary number of arguments, to | |
222 | MONAD---i.e., return a monadic function in MONAD." | |
b860f382 LC |
223 | (lambda args |
224 | (with-monad monad | |
225 | (return (apply proc args))))) | |
226 | ||
227 | (define (foldm monad mproc init lst) | |
228 | "Fold MPROC over LST, a list of monadic values in MONAD, and return a | |
229 | monadic value seeded by INIT." | |
230 | (with-monad monad | |
231 | (let loop ((lst lst) | |
232 | (result init)) | |
233 | (match lst | |
234 | (() | |
235 | (return result)) | |
236 | ((head tail ...) | |
237 | (mlet* monad ((item head) | |
238 | (result (mproc item result))) | |
239 | (loop tail result))))))) | |
240 | ||
241 | (define (mapm monad mproc lst) | |
242 | "Map MPROC over LST, a list of monadic values in MONAD, and return a monadic | |
f62435e2 LC |
243 | list. LST items are bound from left to right, so effects in MONAD are known |
244 | to happen in that order." | |
245 | (mlet monad ((result (foldm monad | |
246 | (lambda (item result) | |
247 | (mlet monad ((item (mproc item))) | |
248 | (return (cons item result)))) | |
249 | '() | |
250 | lst))) | |
251 | (return (reverse result)))) | |
b860f382 | 252 | |
8d7dc5d9 | 253 | (define-syntax-rule (sequence monad lst) |
b860f382 LC |
254 | "Turn the list of monadic values LST into a monadic list of values, by |
255 | evaluating each item of LST in sequence." | |
8d7dc5d9 LC |
256 | ;; XXX: Making it a macro is a bit brutal as it leads to a lot of code |
257 | ;; duplication. However, it allows >>= and return to be open-coded, which | |
258 | ;; avoids struct-ref's to MONAD and a few closure allocations when using | |
259 | ;; %STATE-MONAD. | |
b860f382 | 260 | (with-monad monad |
8d7dc5d9 LC |
261 | (let seq ((lstx lst) |
262 | (result '())) | |
263 | (match lstx | |
264 | (() | |
265 | (return (reverse result))) | |
266 | ((head . tail) | |
267 | (>>= head | |
268 | (lambda (item) | |
269 | (seq tail (cons item result))))))))) | |
b860f382 LC |
270 | |
271 | (define (anym monad proc lst) | |
272 | "Apply PROC to the list of monadic values LST; return the first value, | |
273 | lifted in MONAD, for which PROC returns true." | |
274 | (with-monad monad | |
275 | (let loop ((lst lst)) | |
276 | (match lst | |
277 | (() | |
278 | (return #f)) | |
279 | ((head tail ...) | |
593c3fe6 LC |
280 | (mlet* monad ((value head) |
281 | (result -> (proc value))) | |
282 | (if result | |
283 | (return result) | |
b860f382 LC |
284 | (loop tail)))))))) |
285 | ||
286 | (define-syntax listm | |
287 | (lambda (s) | |
288 | "Return a monadic list in MONAD from the monadic values MVAL." | |
289 | (syntax-case s () | |
290 | ((_ monad mval ...) | |
291 | (with-syntax (((val ...) (generate-temporaries #'(mval ...)))) | |
292 | #'(mlet monad ((val mval) ...) | |
293 | (return (list val ...)))))))) | |
294 | ||
295 | ||
296 | \f | |
297 | ;;; | |
298 | ;;; Identity monad. | |
299 | ;;; | |
300 | ||
aeb7ec5c | 301 | (define-inlinable (identity-return value) |
b860f382 LC |
302 | value) |
303 | ||
aeb7ec5c | 304 | (define-inlinable (identity-bind mvalue mproc) |
b860f382 LC |
305 | (mproc mvalue)) |
306 | ||
aeb7ec5c LC |
307 | (define-monad %identity-monad |
308 | (bind identity-bind) | |
309 | (return identity-return)) | |
b860f382 | 310 | |
81a97734 LC |
311 | \f |
312 | ;;; | |
313 | ;;; State monad. | |
314 | ;;; | |
315 | ||
316 | (define-inlinable (state-return value) | |
317 | (lambda (state) | |
318 | (values value state))) | |
319 | ||
320 | (define-inlinable (state-bind mvalue mproc) | |
321 | "Bind MVALUE, a value in the state monad, and pass it to MPROC." | |
322 | (lambda (state) | |
323 | (call-with-values | |
324 | (lambda () | |
325 | (mvalue state)) | |
326 | (lambda (value state) | |
327 | ;; Note: as of Guile 2.0.11, declaring a variable to hold the result | |
328 | ;; of (mproc value) prevents a bit of unfolding/inlining. | |
329 | ((mproc value) state))))) | |
330 | ||
331 | (define-monad %state-monad | |
332 | (bind state-bind) | |
333 | (return state-return)) | |
334 | ||
335 | (define* (run-with-state mval #:optional (state '())) | |
336 | "Run monadic value MVAL starting with STATE as the initial state. Return | |
337 | two values: the resulting value, and the resulting state." | |
338 | (mval state)) | |
339 | ||
340 | (define-inlinable (current-state) | |
341 | "Return the current state as a monadic value." | |
342 | (lambda (state) | |
343 | (values state state))) | |
344 | ||
345 | (define-inlinable (set-current-state value) | |
346 | "Set the current state to VALUE and return the previous state as a monadic | |
347 | value." | |
348 | (lambda (state) | |
349 | (values state value))) | |
350 | ||
351 | (define (state-pop) | |
352 | "Pop a value from the current state and return it as a monadic value. The | |
353 | state is assumed to be a list." | |
354 | (lambda (state) | |
355 | (match state | |
356 | ((head . tail) | |
357 | (values head tail))))) | |
358 | ||
359 | (define (state-push value) | |
360 | "Push VALUE to the current state, which is assumed to be a list, and return | |
361 | the previous state as a monadic value." | |
362 | (lambda (state) | |
363 | (values state (cons value state)))) | |
364 | ||
b860f382 | 365 | ;;; monads.scm end here |