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1 | ;;; GNU Guix --- Functional package management for GNU |
2 | ;;; Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org> | |
3 | ;;; Copyright © 2014 Eric Bavier <bavier@member.fsf.org> | |
4 | ;;; | |
5 | ;;; This file is part of GNU Guix. | |
6 | ;;; | |
7 | ;;; GNU Guix is free software; you can redistribute it and/or modify it | |
8 | ;;; under the terms of the GNU General Public License as published by | |
9 | ;;; the Free Software Foundation; either version 3 of the License, or (at | |
10 | ;;; your option) any later version. | |
11 | ;;; | |
12 | ;;; GNU Guix is distributed in the hope that it will be useful, but | |
13 | ;;; WITHOUT ANY WARRANTY; without even the implied warranty of | |
14 | ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
15 | ;;; GNU General Public License for more details. | |
16 | ;;; | |
17 | ;;; You should have received a copy of the GNU General Public License | |
18 | ;;; along with GNU Guix. If not, see <http://www.gnu.org/licenses/>. | |
19 | ||
20 | (define-module (guix combinators) | |
21 | #:use-module (ice-9 match) | |
22 | #:use-module (ice-9 vlist) | |
23 | #:export (memoize | |
24 | fold2 | |
25 | fold-tree | |
26 | fold-tree-leaves | |
27 | compile-time-value)) | |
28 | ||
29 | ;;; Commentary: | |
30 | ;;; | |
31 | ;;; This module provides useful combinators that complement SRFI-1 and | |
32 | ;;; friends. | |
33 | ;;; | |
34 | ;;; Code: | |
35 | ||
36 | (define (memoize proc) | |
37 | "Return a memoizing version of PROC." | |
38 | (let ((cache (make-hash-table))) | |
39 | (lambda args | |
40 | (let ((results (hash-ref cache args))) | |
41 | (if results | |
42 | (apply values results) | |
43 | (let ((results (call-with-values (lambda () | |
44 | (apply proc args)) | |
45 | list))) | |
46 | (hash-set! cache args results) | |
47 | (apply values results))))))) | |
48 | ||
49 | (define fold2 | |
50 | (case-lambda | |
51 | ((proc seed1 seed2 lst) | |
52 | "Like `fold', but with a single list and two seeds." | |
53 | (let loop ((result1 seed1) | |
54 | (result2 seed2) | |
55 | (lst lst)) | |
56 | (if (null? lst) | |
57 | (values result1 result2) | |
58 | (call-with-values | |
59 | (lambda () (proc (car lst) result1 result2)) | |
60 | (lambda (result1 result2) | |
61 | (loop result1 result2 (cdr lst))))))) | |
62 | ((proc seed1 seed2 lst1 lst2) | |
63 | "Like `fold', but with a two lists and two seeds." | |
64 | (let loop ((result1 seed1) | |
65 | (result2 seed2) | |
66 | (lst1 lst1) | |
67 | (lst2 lst2)) | |
68 | (if (or (null? lst1) (null? lst2)) | |
69 | (values result1 result2) | |
70 | (call-with-values | |
71 | (lambda () (proc (car lst1) (car lst2) result1 result2)) | |
72 | (lambda (result1 result2) | |
73 | (fold2 proc result1 result2 (cdr lst1) (cdr lst2))))))))) | |
74 | ||
75 | (define (fold-tree proc init children roots) | |
76 | "Call (PROC NODE RESULT) for each node in the tree that is reachable from | |
77 | ROOTS, using INIT as the initial value of RESULT. The order in which nodes | |
78 | are traversed is not specified, however, each node is visited only once, based | |
79 | on an eq? check. Children of a node to be visited are generated by | |
80 | calling (CHILDREN NODE), the result of which should be a list of nodes that | |
81 | are connected to NODE in the tree, or '() or #f if NODE is a leaf node." | |
82 | (let loop ((result init) | |
83 | (seen vlist-null) | |
84 | (lst roots)) | |
85 | (match lst | |
86 | (() result) | |
87 | ((head . tail) | |
88 | (if (not (vhash-assq head seen)) | |
89 | (loop (proc head result) | |
90 | (vhash-consq head #t seen) | |
91 | (match (children head) | |
92 | ((or () #f) tail) | |
93 | (children (append tail children)))) | |
94 | (loop result seen tail)))))) | |
95 | ||
96 | (define (fold-tree-leaves proc init children roots) | |
97 | "Like fold-tree, but call (PROC NODE RESULT) only for leaf nodes." | |
98 | (fold-tree | |
99 | (lambda (node result) | |
100 | (match (children node) | |
101 | ((or () #f) (proc node result)) | |
102 | (else result))) | |
103 | init children roots)) | |
104 | ||
105 | (define-syntax compile-time-value ;not quite at home | |
106 | (syntax-rules () | |
107 | "Evaluate the given expression at compile time. The expression must | |
108 | evaluate to a simple datum." | |
109 | ((_ exp) | |
110 | (let-syntax ((v (lambda (s) | |
111 | (let ((val exp)) | |
112 | (syntax-case s () | |
113 | (_ #`'#,(datum->syntax s val))))))) | |
114 | v)))) | |
115 | ||
116 | ;;; combinators.scm ends here |