Well, idiot me. First question and I already goofed up - didn't take
tail recursion optimization into consideration. this here does the
trick. sorry for the noise.
(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)
(+ accumulated-result (call/cc
(lambda (x) (set! glbSnapshot x)
(emulated-times 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)
)
Quoting Rüdiger Asche <r...@ruediger-asche.de>:
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)
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