[ cross-posted to the FFI list ]
I remember that one of the ForeignObj-like datatypes pretty much requires threads to implement it because the finaliser is an arbitrary piece of Haskell code. This would be hard to support in Hugs and I expect NHC would have problems too.
I seem to recall NHC having some difficulty with this... Malcolm?
Yes and no. An arbitrary Haskell-code finaliser for a ForeignObj is stored in nhc98's runtime system as a StablePtr to the code. When the ForeignObj is garbage-collected, its finaliser is placed onto a 'pending' queue. Then when GC finishes, the pending queue is executed before control returns to the main evaluation. (Each item on the queue is thrown away after execution). This is all implemented, but the evaluation of the pending queue currently causes a seg fault. Chasing the bug wasn't a high priority at the time (18 months ago), and I have simply never got round to looking at it again. There is also the problem that executing a finaliser might itself cause another GC, but I can't remember if I had a solution for that. Regards, Malcolm