[MLton-devel] Re: finalization in MLton

[Ken Friis Larsen]

Stephen Weeks <sweeks@sweeks.com> writes:
> Yes.  The implementations are somewhat similar, although I used
> closures to hide the type of the value while you used exceptions.  One
> problem I see with your withValue implementation is that it doesn't
> ensure the finalizer runs after the function completes.

Hmm, I don't think I understand exactly what your point is.  So let me
state what my intentions for withValue are:

  (1) I don't want to ensure that the finalizer runs after the
      function completes.  I fact I think that would be wrong becase
      the finalized value might not be dead at that point.

  (2) I want to ensure that that the finalizer don't run while in the
      "body" of a withValue.

> That is essential -- imagine if the finalizable value were a pointer
> to a C structure that were freed by the finalizer before the
> function could complete.

This corresponds with my intention (2) above.  Hence I agree.  But I
don't understand *why* my withValue function don't work.  Example,
let's say tha v is a finalized value:

  withValue (fn x => ...) v 


can v be collected in the ... part?  I guess that is what you are
saying.  Is that because MLton with inline withValue and then realise
that v can be collected?  If so, how should I "obfuscate" my code so
that v is protected?

Trying to adobt your code I get:

        fun wind(thunk, cleanup) =
            let val a = thunk()
            in cleanup(); a
            end handle exn => (cleanup(); raise exn)

        fun touch v =
            case v of
                Domain => raise Fail "Finalized.touch bug\n"
              | _      => ()
	    
        fun withValue f (v as {value, getVal}) =
            wind (fn () => f(getVal value),
		  fn () => touch value)

Would this be enough to protect v?  Or is MLton smart enough to
eliminate the whole touch call, because can v never be the Domain
exception.


> > The example works with mlton-20030419 but if I try to use yesterdays
> > CVS mlton, then MLton spews tons of SSA SML at me and ends with:
> >       unhandled exception: TypeError
> 
> I've checked in a fix for that bug.

Thanks.  I checked that it works for my code.


> Hmmm.  I compiled and ran this with 20030419 without problems.

That is strange.

I just verified that I get the error I reported:

$ /home/kfl/programming/ml/mlton-20030419/install/usr/bin/mlton 
MLton 20030419 (built Sun Apr 27 23:00:53 2003 on vips)
$ /home/kfl/programming/ml/mlton-20030419/install/usr/bin/mlton omega-mlton.cm
shrinker raised Prim.apply raised assertion failure: SmallIntInf.fromWord

> So, you must either be using an SML/NJ-compiled version or modified
> sources.

Nope, it was compiled with the MLton from Debian sid.  And the only
file I've modified is mlton-20030419/install/usr/bin/mlton where I've
changed what $lib points to.

> I would like to check my guess though, so could you please send more
> details on the configuration of MLton with which you generated the
> error.

OK.  What do you want me to send?  I usually just compile mlton with
the command:

        make clean bootstrap install

and then edit install/usr/bin/mlton by hand.  Is this the intended
procedure?

> > If you want it I can send you my implementation together with test
> > program.
> 
> Sure, we can take a look.  The MLton.FFI structure is under active
> development as we speak.

I'll package it up and send it during the week.

> Excellent.  You can alpha test our new stuff. :-)

I shall (and I'm looking forward to do so, expect lots of feedback
next week ;-)


Cheers,

--Ken


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