Re: [Chicken-users] continuation example: different behavior from other Scheme implementations

[Ricardo Gabriel Herdt]

Am 19.10.2019 14:18 schrieb address@hidden:
> The exact behaviour of re-
> entering a continuation captured during execution of "map" is, I think,
> dependent on implementation details (there may be a note about this in 
> the
> SRFI-1 document or R7RS, I can't remember right now).

Many thanks for the fast and detailed answer.

I found this in R5RS/R7RS/SRFI-1: "The dynamic order in which proc is 
applied to the elements of the lists is unspecified". Indeed redefining 
map in all implementations as

(define (map f l)
  (cond ((null? l) '())
(else (cons (f (car l))
    (map f (cdr l))))))

make all behave the same way (I know this is not a complete map 
implementation, is just an example).

R7RS adds this: "If multiple returns occur from map, the values returned 
by earlier returns are not mutated". Does this mean returns through 
continuation calls?

I was curious how CHICKEN implements map and found the following in 
library.scm:

(define (##sys#map p lst0)
  (let loop ((lst lst0))
    (cond ((eq? lst '()) lst)
  ((pair? lst)
   (cons (p (##sys#slot lst 0)) (loop (##sys#slot lst 1))) )
  (else (##sys#error-not-a-proper-list lst0 'map)) ) ))

...
(set! scheme#map
    (lambda (fn lst1 . lsts)
      (if (null? lsts)
  (##sys#map fn lst1)
  (let loop ((all (cons lst1 lsts)))
    (let ((first (##sys#slot all 0)))
      (cond ((pair? first)
     (cons (apply fn (mapsafe (lambda (x) (car x)) all #t 'map))
           (loop (mapsafe (lambda (x) (cdr x)) all #t 'map)) ) )
    (else (check (##core#inline "C_i_cdr" all) #t 'map)
          '() ) ) ) ) ) ) )
...

So still I don't get why calling any stored continuation appends the 
result to the previously computed "numbers", but if the standard allows 
this behavior it's not a big deal.

Regards,

Ricardo