GC bug

The attached program (needs the Hugs Graphics Library) reveals what I
presume is a GC bug in the Dec 2001 hugs.

John

----- Original Message -----
From: "John Hughes" <[email protected]>
To: "Alastair Reid" <[email protected]>
Sent: Friday, November 08, 2002 2:58 PM
Subject: Re: HGL


>
> >
> > > Alastair, Just for a bit of fun, I solved the first programming lab
> > > on our Java course using HGL instead. You might like to run the
> > > attached program to see the result!
> >
> > That's pretty cool.  Thanks.
> >
> > > Give it a BIG heap, or you'll hit the same GC bug in Hugs that I did.
> >
> > I do indeed need a big heap.
> >
> > How do you know it's a GC bug and not just a space leak?
> > If we can, I'd like to fix it if it's a bug (or even if it's not a 'bug'
> > but can be fixed.)
> >
>
> Well, I get different behaviour depending on the heap size. For example,
> with a heap size of 250000,
>
> Main> main
>
> Thread raised exception: {{Cell ...}}
> Uncaught Error: {{Cell ...}}
>
> or, in a fresh winhugs,
>
> Main> foldr1 (.+) (take 10000 (points ()))
>
> Program error: {primIntegerToInt (-2147483648)}
>
> Repeating that, by the way, gives
>
> Main> foldr1 (.+) (take 10000 (points ()))
>
> ERROR - Garbage collection fails to reclaim sufficient space
>
> With a heap size of 249999, I get¨
>
> Main> main
>
> ERROR - Garbage collection fails to reclaim sufficient space
>
> Main> foldr1 (.+) (take 10000 (points ()))
>
> Program error: {primIntegerToInt 352126429244364}
>
> Behaviour that depends on the heap size looks like a GC bug to me.
>
> I would have liked to pare the program down to a small one exhibiting the
> same problem, but that proved difficult. What I did discover is that the
> type signature on transform is crucial: removing it changes the behaviour,
> for reasons which are not at all clear to me.
>
> Good luck with bug-hunting!
>
> John
>
>