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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).

GHC has similar problems for just about all types, due to agressive inlining and other optimisations. We've found that the only reliable types on which you can set a finalizer are the primitive heap-resident ones, like ForeignObj# and MutVar#. That's why using addForeignFinalizer is better than using the generic addFinalizer. We dreamed up various hacks to subvert the optimisations, but haven't found any clean solutions yet. The problem is one of "identity": how to give an object a name such that you can refer to that particular object. Cheers, Simon