[plt-scheme] PLT 371 Web-Server Generating Error "call-with-composable-continuation: cannot capture past continuation barrier"

[Eric Domeshek]

Matthias,

Thanks.  That did it. 

Still I'm curious about what changed to make the old call/cc approach 
not work any more.  Likewise, what changed with define-macro...

--Eric

Matthias Felleisen wrote:
> Use let/ec for local function exists, e.g.,
>
> (module foo mzscheme
>   (require (lib "list.ss"))
>
>   (define (f lx)
>     (let/ec exit-from-f
>       (foldr (lambda (x a) (if (= x 0) (exit-from-f 0) (* x a))) 1 lx)))
>
>   (printf "~s=0\n" (f '(1 2 3 0 4)))
>   (printf "~s=24\n" (f '(1 2 3 4))))
>
> -- Matthias
>
>
>
> On Oct 22, 2007, at 8:27 PM, Eric Domeshek wrote:
>
>> Noel,
>>
>> Thanks for the advice.  By dint of about 12 hours of hacking, I got 
>> my stuff mostly ported to 371.  I got explicit about managing session 
>> state in the move from unit-based to module-based servlets.  I also 
>> ran into a problem with all my old "define-macro" macros no longer 
>> working so I finally had to learn about "define-syntax" macros and 
>> then rewrite all my macros.
>>
>> The good news is that the symptom that motivated this upgrade seems 
>> to have been fixed by moving to 371.  The server no longer hangs if I 
>> click a second time before getting the result of the first click.
>>
>> The bad news is I've just spent another hour diagnosing a new problem 
>> that's turned up: it seems that now I can't use call/cc in my code.  
>> I have an implementation of the Porter stemming algorithm that uses 
>> call/cc to emulate CommonLisp's ability to do non-local returns from 
>> a block or function.  When I invoke it from inside my servlet I get 
>> the error message:  "call-with-composable-continuation: cannot 
>> capture past continuation barrier"
>>
>> Any clue what this is about?  Or do you happen to have an 
>> implementation of the Porter stemming algorithm hanging about that 
>> doesn't use call/cc, or have a better way to do non-local exits?
>>
>> Thanks again,
>>
>> --Eric