[Alastair Reid <[email protected]>] Re: cvs commit: haskell-report/ffi finalizers.txt

Alastair Reid alastair at reid-consulting-uk.ltd.uk
Tue Oct 15 04:59:13 EDT 2002


> If you have a concurrent Haskell program, with
> mutable state and MVars and all, it's entirely likely that you'll
> have mutable state that reflects the state of the external (e.g. C)
> world.  For example, the GHC I/O library has Haskell I/O handles
> that reflect the state of the underlying C handle. 

It sounds like you have Haskell objects that need finalized and that
they just happen to contain a C object.  For this it seems appropriate
to use Weak pointers which provide Haskell finalizers and, indeed,
that is what GHC does.  I didn't notice any use of ForeignPtrs in the 
IO Handle implementation.

> Incidentally, as I understand it, it is common ground that, by
> scheduling finalizers on an ordinary Hugs thread, Hugs could provide
> Haskell finalizers exactly as GHC does.  (Although that is not what
> Simon's patch does, I know.)  

It would mean ripping the scheduler apart but yes that could be done.

> The issue then becomes only one of promptness, as Simon described in
> the writeup.  Am I right about that?

I think 'starvation' is the better term since the concern is that a
single-threaded program will never hit a preemption point so the
finalizer will never get to run.  That is, 'promptness' implies that
the finalizer gets delayed for a small but unknown amount of time
while 'starvation' implies that the finalizer is delayed for a large,
possibly infinite amount of time.

With that change in terminology, I think I can agree that the problem
with the easily implemented thread option is starvation.

--
Alastair



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