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

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Mon Oct 22 20:50:21 EDT 2007

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  

   (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
> Noel Welsh wrote:
>> Hi Eric,
>> I'm afraid the best advice I can give is to upgrade to 371, as it
>> incorporates many important improvements and fixes (and I've even
>> encourage you to try one of the nightly builds).  It shouldn't be  
>> that
>> onerous.  The conversion from unit to module based servlets should be
>> straightforward, and you get better error checking with modules,  
>> so it
>> is a bit of a win.  To load the web server configuration you can take
>> example code from instaweb.  Search for
>> configuration-table->web-config@
>> in
>> http://planet.plt-scheme.org/package-source/schematics/ 
>> instaweb.plt/1/8/instaweb.ss
>> HTH,
>> Noel
>> On 10/19/07, Eric Domeshek <domeshek at stottlerhenke.com> wrote:
>>> Gang,
>>> I have used PLT 360 to build an elaborate web application.  I have
>>> noticed that if, from a particular page, I click a link that  
>>> invokes an
>>> extended computation/display, and before the new page comes up, I  
>>> click
>>> another link on the original page, then the server seems to hang and
>>> never respond to either click.  All these links are generated using
>>> embed/url inside a send/suspend/dispatch.  I've been using  
>>> Firefox 2 as
>>> my browser.
>>> Before I get all hung about about possible threading issues and the
>>> possibility of unsafe data modifications, I thought I'd ask if  
>>> there is
>>> a simpler explanation (and fix) or if anyone else has seen and can
>>> account for this behavior.
>>> Also, as an aside, I've downloaded PLT 371 and considered  
>>> upgrading, but
>>> it doesn't appear to be a smooth path, since many things have  
>>> changed
>>> (but I'm not sure the docs have kept up).  The way I set my
>>> configuration no longer works (I think the 'load-configuration'  
>>> function
>>> is gone and I can't figure out what has replaced it).  I also  
>>> think I
>>> read in a post to this list that unit-based servlets are no longer
>>> supported (which is how I've built my two major servlets).
>>> Anyway, thanks for any help or pointers you folks can offer...
>>> --Eric
>>> ====================================================================
>>> Eric Domeshek                      Phone: 617-616-1291
>>> AI Project Manager                   Fax: 617-616-1290
>>> Stottler Henke Associates, Inc.    EMail: domeshek at stottlerhenke.com
>>> 48 Grove St., Somerville, MA 02144   Web: www.stottlerhenke.com
>>> ====================================================================
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