[racket] Call/cc captures lexical continuation?...

From: Rüdiger Asche (rac at ruediger-asche.de)
Date: Wed Jan 18 08:00:24 EST 2012

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)

    (emulated-times (lambda (accumulated-result rest multiplicator)
                         (if (eq? rest 0)
                                (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)  
                                (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


(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?


- Ruediger (IU alumnus and spare time Scheme afficionado)

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