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To: caml-list@inria.fr
From: eijiro_sumii@anet.ne.jp
Cc: Pierre.Weis@inria.fr, sumii@yl.is.s.u-tokyo.ac.jp
Subject: Re: [Caml-list] exceptions and the polymorphic equality
In-Reply-To: <200107161511.RAA23787@pauillac.inria.fr>
References: <20010715220500D.sumii@yl.is.s.u-tokyo.ac.jp>
	<200107161511.RAA23787@pauillac.inria.fr>
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Date: Tue, 17 Jul 2001 09:59:12 +0900
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> On the other hand, the structural equality (=) when applied to
> exceptions and constructors is not completely specified, hence
> unreliable. 

I see, this makes sense - indeed, exn is not an equality type in SML.

> You're right, having e <> e' should be desirable. However, in this
> case you should not test structural equality, since it is very likely
> the case that e and e' are represented by the same kind of value. You
> should test identity (==) instead:
> 
> # e == e';;
> - : bool = false
> 
> That's what the compiler generates when pattern matching exception
> values (more precisely it uses == for the exception constructor and
> regular pattern matching for the rest of the pattern).

Several people have suggested using == instead of =, but doing so
seems even more problematic because:

        Objective Caml version 3.01

# exception Foo;;
exception Foo
# Foo == Foo;;
- : bool = false
# 

Regards,

	Eijiro
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