diff options
Diffstat (limited to 'module/srfi/srfi-16.scm')
-rw-r--r-- | module/srfi/srfi-16.scm | 126 |
1 files changed, 126 insertions, 0 deletions
diff --git a/module/srfi/srfi-16.scm b/module/srfi/srfi-16.scm new file mode 100644 index 000000000..0b213fde7 --- /dev/null +++ b/module/srfi/srfi-16.scm @@ -0,0 +1,126 @@ +;;; srfi-16.scm --- case-lambda + +;; Copyright (C) 2001, 2002, 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 + +;;; Author: Martin Grabmueller + +;;; Commentary: + +;; Implementation of SRFI-16. `case-lambda' is a syntactic form +;; which permits writing functions acting different according to the +;; number of arguments passed. +;; +;; The syntax of the `case-lambda' form is defined in the following +;; EBNF grammar. +;; +;; <case-lambda> +;; --> (case-lambda <case-lambda-clause>) +;; <case-lambda-clause> +;; --> (<signature> <definition-or-command>*) +;; <signature> +;; --> (<identifier>*) +;; | (<identifier>* . <identifier>) +;; | <identifier> +;; +;; The value returned by a `case-lambda' form is a procedure which +;; matches the number of actual arguments against the signatures in +;; the various clauses, in order. The first matching clause is +;; selected, the corresponding values from the actual parameter list +;; are bound to the variable names in the clauses and the body of the +;; clause is evaluated. + +;;; Code: + +(define-module (srfi srfi-16) + :export-syntax (case-lambda)) + +(cond-expand-provide (current-module) '(srfi-16)) + +(define-macro (case-lambda . clauses) + + ;; Return the length of the list @var{l}, but allow dotted list. + ;; + (define (alength l) + (cond ((null? l) 0) + ((pair? l) (+ 1 (alength (cdr l)))) + (else 0))) + + ;; Return @code{#t} if @var{l} is a dotted list, @code{#f} if it is + ;; a normal list. + ;; + (define (dotted? l) + (cond ((null? l) #f) + ((pair? l) (dotted? (cdr l))) + (else #t))) + + ;; Return the expression for accessing the @var{index}th element of + ;; the list called @var{args-name}. If @var{tail?} is true, code + ;; for accessing the list-tail is generated, otherwise for accessing + ;; the list element itself. + ;; + (define (accessor args-name index tail?) + (if tail? + (case index + ((0) `,args-name) + ((1) `(cdr ,args-name)) + ((2) `(cddr ,args-name)) + ((3) `(cdddr ,args-name)) + ((4) `(cddddr ,args-name)) + (else `(list-tail ,args-name ,index))) + (case index + ((0) `(car ,args-name)) + ((1) `(cadr ,args-name)) + ((2) `(caddr ,args-name)) + ((3) `(cadddr ,args-name)) + (else `(list-ref ,args-name ,index))))) + + ;; Generate the binding lists of the variables of one case-lambda + ;; clause. @var{vars} is the (possibly dotted) list of variables + ;; and @var{args-name} is the generated name used for the argument + ;; list. + ;; + (define (gen-temps vars args-name) + (let lp ((v vars) (i 0)) + (cond ((null? v) '()) + ((pair? v) + (cons `(,(car v) ,(accessor args-name i #f)) + (lp (cdr v) (+ i 1)))) + (else `((,v ,(accessor args-name i #t))))))) + + ;; Generate the cond clauses for each of the clauses of case-lambda, + ;; including the parameter count check, binding of the parameters + ;; and the code of the corresponding body. + ;; + (define (gen-clauses l length-name args-name) + (cond ((null? l) (list '(else (error "too few arguments")))) + (else + (cons + `((,(if (dotted? (caar l)) '>= '=) + ,length-name ,(alength (caar l))) + (let ,(gen-temps (caar l) args-name) + ,@(cdar l))) + (gen-clauses (cdr l) length-name args-name))))) + + (let ((args-name (gensym)) + (length-name (gensym))) + (let ((proc + `(lambda ,args-name + (let ((,length-name (length ,args-name))) + (cond ,@(gen-clauses clauses length-name args-name)))))) + proc))) + +;;; srfi-16.scm ends here |