[racket] call/cc not working as expected in DrScheme

From: Bas Steunebrink (bas at idsia.ch)
Date: Fri Aug 20 04:55:38 EDT 2010

  Thanks Matthew, that was very enlightening!

I do get the behaviour I had expected if I type `((lambda () (stop) 
(stop) 42))' instead.

The funny thing is, I thought `(begin exp ...)' was nothing more than a 
syntactic abbreviation of `((lambda () exp ...))', but apparently there 
is an exception for the top level. Granted, this exception is described 
in R5RS, but I had failed to see the consequences of this on captured 


On 8/19/10 20:13 PM, Matthew Flatt wrote:
> At Thu, 19 Aug 2010 19:04:49 +0200, Bas Steunebrink wrote:
>>    Hi all,
>>   From what I understand about call/cc, when a saved continuation is
>> invoked, the current continuation should be discarded. So I expected the
>> following program to just print `stop?' once; however, it doesn't stop.
>> Welcome to DrScheme, version 4.2.4 [3m].
>> Language: Pretty Big; memory limit: 128 megabytes.
>>   >  (define stop
>>       (let ((k (call-with-current-continuation (lambda (k) k))))
>>         (lambda ()
>>           (display "stop?\n")
>>           (k k))))
>>   >  (begin (stop) (stop) 42)
>> stop?
>> stop?
>> 42
>>   >
>> So is this an error in my understanding of call/cc or in DrScheme?
> You need two more pieces of information:
>   * When `begin' appears in a top-level position, its expressions are
>     spliced into the top level.
>   * Every top-level expression is evaluated inside a "prompt" that
>     delimits continuation capture and invocation.
> So, each call to `stop' discards only the continuation up to the prompt
> that wraps the call to `stop'.

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