On Sun, Sep 06, 2009 at 04:06:46AM -0400, John Cowan wrote:
> Here's the chibi-scheme implementation:
>
> (define *values-tag* (list 'values))
>
> (define (values . ls)
> (if (and (pair? ls) (null? (cdr ls)))
> (car ls)
> (cons *values-tag* ls)))
>
> (define (call-with-values producer consumer)
> (let ((res (producer)))
> (if (and (pair? res) (eq? *values-tag* (car res)))
> (apply consumer (cdr res))
> (consumer res))))
Which, though less wrong than MIT Scheme, is incomplete:
(define call-with-current-continuation
(let ((ur-call/cc call-with-current-continuation))
(lambda (f)
(ur-call/cc
(lambda (k)
(f (lambda a (k (apply values a)))))))))
As I may have mentioned the last time this came up, values is just
semantic sugar for call/cc, and call-with-values is the inverse of
(delimited) continuation capture. More to the point, as long as
call/cc is in the language, the equivalence between multiple value
return and multiple argument passing is manifest; relegating only one
to second-class-blob status may not have the desired effect on the
language's elegance.
--
(let ((C call-with-current-continuation)) (apply (lambda (x y) (x y)) (map
((lambda (r) ((C C) (lambda (s) (r (lambda l (apply (s s) l)))))) (lambda
(f) (lambda (l) (if (null? l) C (lambda (k) (display (car l)) ((f (cdr l))
(C k))))))) '((#\J #\d #\D #\v #\s) (#\e #\space #\a #\i #\newline)))))
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