Finalizers etcetera

Simon Peyton-Jones simonpj at
Fri Oct 11 03:41:31 EDT 2002

| I claim that the major thing that finalizers do is manipulate shared
| state. 

That is certainly true.  If they were pure, you'd never know they'd run!

| To get any benefit from writing finalizers in Haskell, I have to have
| MVars which protect against finalizers.  

Nearly right, but not quite.  You might write a Haskell finalizer that
did lots of useful things (e.g. consulted a large pure data structure)
before doing its state-mutation by calling C.

| > And if they existed, everything would be fine, right?  We could
| > just use Haskell finalizers as we all want.  Or am I missing
| > something.
| Finalizers would have to be scheduled by the same scheduler as normal
| threads. 


| The scheduler would need to prioritize finalizers over normal threads.

Desirable but not necessary.   The programmer cannot expect finalizers
to run promptly (there is always lots of literature on the GC mailing
list about this point).  

| Most of the cooperative concurrency implementation is written in
| Haskell.  That would have to be rewritten in C to make the operations
| atomic wrt garbage collection.

No, I disagree with this.  When GC runs, the finalizers can be put in
the ready queue.  After GC completes, the thread that was interrupted by
GC continues.  So a thread switch takes place ONLY when the thread
yields, as now.  (Again, promptness is not a reqt. If the thread never
yields, the finaliser will never run.  That is 100% ok. You absolutely
should not RELY on finalizers.

So the question before the house is to choose between:

A) Haskell finalizers: flexible; continue to be what we want when we
have concurrency; but if your impl does not support MVars you have to
call C to do state mutation.

B) C finalizers: less flexible; GHC will have (A) anyway; but arguably
one less trap for the unwary.

I still prefer (A), albeit not unto death, because
(i) I believe that supporting MVars in Hugs is not as hard as you think.
(I'm agnostic about NHC.)

(ii) Less incompatibility... (i.e. programs that use GHC extensions that
won't run on Hugs)

(iii) Moves in the direction we will ultimately want to go.  It seems
bizarre to write finalizers in a different language!


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