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6be07c52 | 1 | ;;; srfi-16.scm --- case-lambda |
6851c8a4 | 2 | |
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3 | ;; Copyright (C) 2001, 2002 Free Software Foundation, Inc. |
4 | ;; | |
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5 | ;; This library is free software; you can redistribute it and/or |
6 | ;; modify it under the terms of the GNU Lesser General Public | |
7 | ;; License as published by the Free Software Foundation; either | |
8 | ;; version 2.1 of the License, or (at your option) any later version. | |
9 | ;; | |
10 | ;; This library is distributed in the hope that it will be useful, | |
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11 | ;; but WITHOUT ANY WARRANTY; without even the implied warranty of |
12 | ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
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13 | ;; Lesser General Public License for more details. |
14 | ;; | |
15 | ;; You should have received a copy of the GNU Lesser General Public | |
16 | ;; License along with this library; if not, write to the Free Software | |
17 | ;; Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA | |
6851c8a4 | 18 | |
6be07c52 | 19 | ;;; Author: Martin Grabmueller |
6851c8a4 | 20 | |
6be07c52 | 21 | ;;; Commentary: |
6851c8a4 | 22 | |
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23 | ;; Implementation of SRFI-16. `case-lambda' is a syntactic form |
24 | ;; which permits writing functions acting different according to the | |
25 | ;; number of arguments passed. | |
26 | ;; | |
27 | ;; The syntax of the `case-lambda' form is defined in the following | |
28 | ;; EBNF grammar. | |
29 | ;; | |
30 | ;; <case-lambda> | |
31 | ;; --> (case-lambda <case-lambda-clause>) | |
32 | ;; <case-lambda-clause> | |
33 | ;; --> (<signature> <definition-or-command>*) | |
34 | ;; <signature> | |
35 | ;; --> (<identifier>*) | |
36 | ;; | (<identifier>* . <identifier>) | |
37 | ;; | <identifier> | |
38 | ;; | |
39 | ;; The value returned by a `case-lambda' form is a procedure which | |
40 | ;; matches the number of actual arguments against the signatures in | |
41 | ;; the various clauses, in order. The first matching clause is | |
42 | ;; selected, the corresponding values from the actual parameter list | |
43 | ;; are bound to the variable names in the clauses and the body of the | |
44 | ;; clause is evaluated. | |
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45 | |
46 | ;;; Code: | |
6be07c52 | 47 | |
1a179b03 MD |
48 | (define-module (srfi srfi-16) |
49 | :export-syntax (case-lambda)) | |
6851c8a4 | 50 | |
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51 | (cond-expand-provide (current-module) '(srfi-16)) |
52 | ||
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53 | (define-macro (case-lambda . clauses) |
54 | ||
55 | ;; Return the length of the list @var{l}, but allow dotted list. | |
56 | ;; | |
57 | (define (alength l) | |
58 | (cond ((null? l) 0) | |
59 | ((pair? l) (+ 1 (alength (cdr l)))) | |
60 | (else 0))) | |
61 | ||
62 | ;; Return @code{#t} if @var{l} is a dotted list, @code{#f} if it is | |
63 | ;; a normal list. | |
64 | ;; | |
65 | (define (dotted? l) | |
66 | (cond ((null? l) #f) | |
67 | ((pair? l) (dotted? (cdr l))) | |
68 | (else #t))) | |
69 | ||
70 | ;; Return the expression for accessing the @var{index}th element of | |
71 | ;; the list called @var{args-name}. If @var{tail?} is true, code | |
72 | ;; for accessing the list-tail is generated, otherwise for accessing | |
73 | ;; the list element itself. | |
74 | ;; | |
75 | (define (accessor args-name index tail?) | |
76 | (if tail? | |
77 | (case index | |
78 | ((0) `,args-name) | |
79 | ((1) `(cdr ,args-name)) | |
80 | ((2) `(cddr ,args-name)) | |
81 | ((3) `(cdddr ,args-name)) | |
82 | ((4) `(cddddr ,args-name)) | |
83 | (else `(list-tail ,args-name ,index))) | |
84 | (case index | |
85 | ((0) `(car ,args-name)) | |
86 | ((1) `(cadr ,args-name)) | |
87 | ((2) `(caddr ,args-name)) | |
88 | ((3) `(cadddr ,args-name)) | |
89 | (else `(list-ref ,args-name ,index))))) | |
90 | ||
91 | ;; Generate the binding lists of the variables of one case-lambda | |
92 | ;; clause. @var{vars} is the (possibly dotted) list of variables | |
93 | ;; and @var{args-name} is the generated name used for the argument | |
94 | ;; list. | |
95 | ;; | |
96 | (define (gen-temps vars args-name) | |
97 | (let lp ((v vars) (i 0)) | |
98 | (cond ((null? v) '()) | |
6be07c52 | 99 | ((pair? v) |
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100 | (cons `(,(car v) ,(accessor args-name i #f)) |
101 | (lp (cdr v) (+ i 1)))) | |
102 | (else `((,v ,(accessor args-name i #t))))))) | |
103 | ||
104 | ;; Generate the cond clauses for each of the clauses of case-lambda, | |
105 | ;; including the parameter count check, binding of the parameters | |
106 | ;; and the code of the corresponding body. | |
107 | ;; | |
108 | (define (gen-clauses l length-name args-name) | |
109 | (cond ((null? l) (list '(else (error "too few arguments")))) | |
110 | (else | |
111 | (cons | |
112 | `((,(if (dotted? (caar l)) '>= '=) | |
113 | ,length-name ,(alength (caar l))) | |
114 | (let ,(gen-temps (caar l) args-name) | |
115 | ,@(cdar l))) | |
116 | (gen-clauses (cdr l) length-name args-name))))) | |
117 | ||
118 | (let ((args-name (gensym)) | |
119 | (length-name (gensym))) | |
120 | (let ((proc | |
121 | `(lambda ,args-name | |
122 | (let ((,length-name (length ,args-name))) | |
123 | (cond ,@(gen-clauses clauses length-name args-name)))))) | |
124 | proc))) | |
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125 | |
126 | ;;; srfi-16.scm ends here |