[gambit-list] Continuations bug in v4.6.2?

Patrick Li patrickli.2001 at gmail.com
Mon Dec 9 22:10:30 EST 2013


Hello!

I've been writing some tricky code involving continuations, and ran into the
following surprising behavior from Gambit v.4.6.2 running on OSX.

I have two versions of "make-jp", that I believe should be
equivalent.

   Version 1:
      (define (make-jp block)
        (let [(return-point '())]
          (call/cc (lambda (ret)
            (define (jp x)
              (call/cc (lambda (ret)
                 (let [(r return-point)]
                   (set! return-point ret)
                   (r x)))))
            (set! return-point ret)
            (let [(result (block jp))]
              (return-point result))))))

   Version 2:
      (define (make-jp block)
        (let [(return-point '())]
          (call/cc (lambda (ret)
            (define (jp x)
              (call/cc (lambda (ret)
                 (let [(r return-point)]
                   (set! return-point ret)
                   (r x)))))
            (set! return-point ret)
            (return-point (block jp))))))

The only difference is the final line:

   Version 1:
      (let [(result (block jp))]
        (return-point result))

   Version 2:
      (return-point (block jp))


However, I get different behavior when I test with the following code:

   Testing Code:
      (let* [(j '())]
        (make-jp (lambda (y)
           (set! j y)
           (y 0)
           10))
        (println "GOT: " (j "X")))

   Version 1 prints:
      GOT: 10

   Version 2 prints:
      GOT: X

Out of the two behaviours, I expect the answer given by Version 1.

Can anyone verify whether this is a bug, and whether the latest
version also shows this behavior?  This fell out of a piece of code
I'm using to simulate coroutines for an interpreter that I am
programming. Here I've tried to isolate the problem down to as few
lines as possible.

Once I get my environment fixed, I will try it on the latest version
of Gambit as well and see if the problem is still there.

  -Patrick
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