Hello, I'm trying to brush up on my scheme/racket, and here's
something I came across when trying to rework the issue of
continuations - I'm sure this qualifies as a beginner's question, so
sorry for probably asking a stupid question - I promise to ask better
ones in the future:
Consider the following code:
(define glbSnapshot 0)
(letrec
[
(emulated-times (lambda (accumulated-result rest multiplicator)
(if (eq? rest 0)
accumulated-result
(begin
(display accumulated-result)
(display " ")
(display rest)
(display "\r\n")
(if (eq? rest 10)
(call/cc (lambda (x) (set! glbSnapshot x)
(emulated-times (+ accumulated-result multiplicator) (- rest 1)
multiplicator)))
(emulated-times (+ accumulated-result
multiplicator) (- rest 1) multiplicator)
)))))
(et-displayed (lambda (multiplicand multiplicator)
(display (emulated-times 0 multiplicand multiplicator))
(display "\r\n")))]
(et-displayed 12 4)
)
I would expect glbSnapshot to be the continuation accumulated upon the
second recursion, i.e. whatever is passed to the continuation would
unwind the stack and complete the partial computation already done
when taking the snapshot, thus:
(glbSnapshot 7) => 8+7 => 15
However,
(glbSnapshot 7) => 7 on Racket 5.1.3. It seems as if the continuation
is only taken at the lexical, not the dynamic time! Somehow I was
under the impression that call/cc would express the "true"
continuation when taken, but either I'm wrong or Racket 5.1.3.
implements it wrong... Almost certainly it's my misunderstanding -
thus, what do I have to do to capture the dynamic continuation?
Thanks!
- Ruediger (IU alumnus and spare time Scheme afficionado)
____________________
Racket Users list:
http://lists.racket-lang.org/users
