[Alastair Reid <alastair@reid-consulting-uk.ltd.uk>] Re: cvs commit: haskell-report/ffi finalizers.txt

[Simon Peyton-Jones]

Hi Alastair

| a comment on SimonM's finalizer document.  I decided to comment rather
| than edit since my comments require more than tweaking the odd word
| here and there.

I suggest you edit directly, striving, as Simon did, to be even-handed.

I do have one comment on your rejoinders, though:

|   I don't see how adding concurrency strengthens the argument for
|   Haskell finalizers.
| 
|   Adding mutable Haskell state doesn't seem to strengthen it either.
|   Indeed, I assume the existence of mutable Haskell state in arguing
|   against Haskell finalizers.

Here I disagree!  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.  (For example, they contain some
buffered characters as well as a C pointer to the underlying C object.)
So the finaliser actually watches to the *Haskell* object; when it
becomes unreachable the finaliser should run, and (amongst other things)
call a suitable finaliser on the C object.  But without access to the
Haskell world, the finaliser can't even get hold of the C pointer to
finalise!

So I believe that for a non-stateful, non-concurrent Haskell program the
case for Haskell finalizers is moot, it becomes much more compelling
once you have state and concurrency anyway.   Furthermore, Haskell
finalizers then behave just like threads (which by then exist anyway)
and must be treated with exactly the same care, no more and no less.


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

Simon