guile/srfi/srfi-16.scm

;;;; srfi-16.scm --- `case-lambda' for Guile

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;;; 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.

;;; Author: Martin Grabmueller

;;; 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)))