[plt-scheme] Re: Puzzle about amb and memory

From: Mark Engelberg (mark.engelberg at gmail.com)
Date: Tue Apr 1 00:08:23 EDT 2008

On Mon, Mar 31, 2008 at 1:36 PM, Danny Yoo <dyoo at cs.wpi.edu> wrote:
> > I've isolated and resolved a problem in my project.  But I don't quite
>  > understand why the problem existed!
>  >
>  > I've distilled what I ran into.  I've been using Will Farr's excellent
>  > amb.plt PLaneT library to write nondeterministic programs.  Here's a
>  > small program that demonstrates the issue.

Well, your post encouraged me to refresh my memory on amb and look at
the implementation in the planet library.

I think the crux of the problem is that your test-1 creates 7
continuations, and test-2 creates 70 continuations.  Apparently, 70
continuations is enough to cause PLT Scheme to segfault, but 7 isn't.

test-1 creates one choice point continuation for the entire list of
numbers.  Each time a choice fails, it pops back to the same choice
point and tries it again with the next thing in the list.

On the other hand, test-2 uses recursion to create a separate choice
point for each number in the list.


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