Re: How often are continuations created?

[Keisuke Nishida <kxn30 at po dot cwru dot edu>]

Miroslav Silovic <silovic@zesoi.fer.hr> writes:

> >   (do ((i 0 (1+ i)))
> >       ((= i N))
> >    (vector-set! v i (magic-computation i)))  =>
> 
> > Since magic-computation is a procedure, %call creates a new frame for
> > it.  `i' is still a local variable because magic-computation cannot
> > refer to it.  Is there any problem with this?
> 
> Well, suppose magic computation, when first invoked, calls call/cc and
> passes it out through some global, then just returns and does a NOP
> until the loop exits
> 
> (sample:
> 
> 	(define magic-computation
> 	  (let ((capture #t)
> 	     (if (capture)
> 		(begin
> 		   (set! capture #f)
> 		   (set! some-cont (call/cc (lambda (cont) cont))))))))
> 
> )
> 
> Then, when you invoke some-cont, it will return from magic-computation
> and redo the loop - except that the value of i will be 0, when it
> should be N.

Oh, should it?  But Guile does restart it from 0:

  guile> (define some-cont #f)
  guile> (define magic-computation
           (let ((capture #t))
             (lambda (x)
               (if capture
                   (begin
                     (set! capture #f)
                     (set! some-cont (call-with-current-continuation id)))))))
  guile> (do ((v (make-vector 5 #f))
              (i 0 (1+ i)))
             ((= i 5))
           (vector-set! v i (magic-computation i))
           (display i))
  01234guile> (some-cont #f)
  01234guile> 

Which one is the correct behavior?