Finalizer problems..

[Alastair Reid]

> I would like to run some code when something is
> garbage-collected. Fine, he says, just the job for
> finalizers (module Weak). Unfortunately, they don't
> seem to offer a general solution, as the code below
> demonstrates (presumably, smallish things are copied,
> not shared, so that the finalizers run too early?).

Yes, small Ints (approximately 30 bits worth) and Chars have a special
representation which makes it impossible to tell if they have been GC'd

GHC has a similar problem for Chars and small ints (-15..15 I think).

> Is there a general way to add finalizers to values
> of arbitrary type (and have them run when the value,
> not just one copy of it, disappears)?

No.
Anything that is represented as an Int or Char on the heap could behave this
way.
(The unit value "()", occurences of [], etc may behave the same way.)
The only fix is to wrap the object in a data constructor.

> Or is this behaviour -he hopes- a bug, to be fixed in the next release?

Bug?
Probably

Likely to be fixed anytime soon?
I hesitate to speak for the current maintainers but, since it is caused by a
fundamental part of Hugs' implementation, I wouldn't hold my breath waiting for
a fix.