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Subject: RE: [Caml-list] Pattern matcher no more supposed to warn on non  exhaustive patterns ?
Date: Thu, 4 Oct 2001 00:29:47 -0400
Message-ID: <706871B20764CD449DB0E8E3D81C4D4301EE6C00@opus.cs.cornell.edu>
Thread-Topic: [Caml-list] Pattern matcher no more supposed to warn on non  exhaustive patterns ?
Thread-Index: AcFMTcrWmyUi6r9ZS0+5yqXqzDAnTwAPdn/A
From: "Gregory Morrisett" <jgm@cs.cornell.edu>
To: "Pierre Weis" <pierre.weis@inria.fr>,
        "Daniel Grossman" <danieljg@sundial.cs.cornell.edu>
Cc: <caml-list@inria.fr>
Sender: owner-caml-list@pauillac.inria.fr
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The issue with threads is a bit more troublesome.  Consider:

  let x : (int->int) option ref =3D ref (Some (fun x -> x));;

  let foo z =3D
      match z with
        {contents=3DNone} -> 0
      | {contents=3DSome(f)} -> f(0);

Now suppose I fork two threads:

Thread 1:  foo x
Thread 2:  x :=3D None

And suppose that Thread 1 runs long enough that it does the first
match, so it assumes the contents of x is not of the form None. =20
Now Thread 1 gets descheduled, Thread 2 runs, and sets the
contents of x to None.  Then Thread 2 continues with the second
match...

The question is, does Caml core dump because the pattern matcher
assumes that the contents *has* to be a Some(-) in the second
case?  Or does it do the derefence and check atomically?  Or
does it add a default case that raises a Match exception? =20

The same thing shows up in SML/NJ with CML.  The problem is that
in the presence of threads, you really shouldn't be able to=20
dereference a mutable value in your patterns.

-Greg
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