Unfortunately it isn't possible to add a finalizer to a normal - Ptr a. We already have a generic finalization mechanism: - see the Weak module in package lang. But the only reliable way to - use finalizers is to attach one to an atomic heap object - that - way the compiler's optimiser can't interfere with the lifetime of - the object. - -
The Ptr type is really just a boxed address - it's - defined like - -
-data Ptr a = Ptr Addr# -- -
where Addr# is an unboxed native address (just a 32- - or 64- bit word). Putting a finalizer on a Ptr is - dangerous, because the compiler's optimiser might remove the box - altogether. - -
ForeignPtr is defined like this - -
-data ForeignPtr a = ForeignPtr ForeignObj# -- -
where ForeignObj# is a *boxed* address, it corresponds - to a real heap object. The heap object is primitive from the - point of view of the compiler - it can't be optimised away. So it - works to attach a finalizer to the ForeignObj# (but not - to the ForeignPtr!). - -
There are several primitive objects to which we can attach - finalizers: MVar#, MutVar#, ByteArray#, - etc. We have special functions for some of these: eg. - MVar.addMVarFinalizer. - -
So a nicer interface might be something like - -
-class Finalizable a where - addFinalizer :: a -> IO () -> IO () - -instance Finalizable (ForeignPtr a) where ... -instance Finalizable (MVar a) where ... -- -
So you might ask why we don't just get rid of Ptr and - rename ForeignPtr to Ptr. The reason for that - is just efficiency, I think. - -
- -Last modified: Wed Sep 26 09:49:37 BST 2001 - - - -