addForeignPtrFinalizer

[Alastair Reid]

> You do have to worry about the impact of running Haskell finalizers
> at any point during execution of Haskell code.  Since you can't do
> any concurrency synchronisation between the finalizer and the main
> Haskell thread, shared access to mutable data is impossible.  This
> is just something you'd have to document carefully (note, it's not
> something you can do with the current finalization mechanism either,
> but there it's obvious).

This is a pretty large restriction since the only reason for running
Haskell finalizers is to manipulate shared Haskell state.  

Here's a typical example of where you would need Haskell finalizers
instead of C finalizers: consider a simple graphics library which
maintains a list of windows and deletes a window from the list when
its finalizer runs (it presumably uses weak pointers to avoid the
space leak).  If the finalizer blindly deletes the object from the
list, we get a race condition.  If the object is protected by a lock,
the finalizer can't access it.  If we want to allow finalizers to
access objects protected by a lock, we have to be prepared to block
execution of the finalizer until the main thread releases the lock.

I claim that Haskell finalizers are no use unless we provide
synchronization primitives, those primitives are able to block
execution of a currently running finalizer.  I also claim that
implementing blocking is a significant undertaking.

Simon's patch doesn't make Hugs' concurrency implementation work with
finalizers and it will take a fairly significant rewrite of the
implementation to make it work.  (At present, it doesn't work with
unsafePerformIO.)  I don't know if I'd be able to support blocking 
in any useful way without reimplementing the whole thing on top of
a native thread library (pthreads, etc.)

AFAIK, NHC does not implement any concurrency primitives so NHC would
have to add them before it can provide any useful form of Haskell
finalizers.

--
Alastair Reid                 alastair at reid-consulting-uk.ltd.uk  
Reid Consulting (UK) Limited  http://www.reid-consulting-uk.ltd.uk/alastair/