One of the annoyance of the stateless Web application language that comes with the PLT Web Server is that you can't call third-party higher-order library procedures with arguments that try to capture serializable continuations. (I know, you try to do that all the time.) For example:
(build-list 3 (lambda (i) (call-with-serializable-current-continuation (lambda (k) (serialize k)))))
The problem is that the stateless language performs a transformation on your program to extract the continuations into a serializable representation. If you really need to do this, we've developed a compromise called "The Two State Solution": one state on the client and the other on the server. Only the third-party parts of the continuation (in this case, the code inside
build-list) are stored on the server; everything else is shipped to the client. You just need to annotate your code slightly to indicate where the transition is:
(serial->native (build-list 3 (lambda (i) (native->serial (call-with-serializable-current-continuation (lambda (k) (serialize k)))))))
serial->native signals the transition to the third-party and
native->serial signals the transition back.
It is still a little annoying to find when you've called these third-party higher-order library procedures with arguments that try to capture serializable continuations, so there's a simple macro that provides a transitioning wrapper for you:
(define-native (build-list/native _ ho) build-list)
(define (build-list/native fst snd) (serial->native (build-list fst (lambda args (native->serial (apply snd args))))))
This new feature is documented in the online manual, of course.