Re: [Caml-list] type of ==

[Remi Vanicat <remi.vanicat@gmail.com>]

On 4/18/05, Jon Harrop <jon@ffconsultancy.com> wrote:
> On Monday 18 April 2005 10:18, Christophe DEHLINGER wrote:
> > I was recently in a situation where I would have liked to be able to
> > test physical equality on terms of possibly different types, but
> > couldn't because of =='s 'a -> 'a -> bool type.
> > Why isn't its type 'a -> 'b -> bool ? Is there more to the
> > implementation of == than a simple physical, asm-level equality test ?
> > Is it because of the semantics of == ? If so, is a 'a -> 'b -> bool
> > equivalent possible / already existing ?
> 
> If there were an "'a -> 'b -> bool" physical equality test, how could it ever
> return "true"?

Well, when I wrote my Weak memo module (see
http://aspellfr.free.fr/hweak/doc/Weak_memo.html), I add the need too
compare two object having a different type (too know if they are
actually the same or not). So I needed a egalyty having the type <..>
-> <..> -> bool

One could wrote one :

# let eq x y = (x : < .. > :> < > ) == (y : < .. > :> < >);;
val eq : < .. > -> < .. > -> bool = <fun>

But I don't know how to write it for polymorphic variant (without the
Obj module) for example, and someone may want to do a similar weak
memo on them (Notice that if one could write this, then one can do the
same on structural equality, and I'm not sure that one want to use
this on structural equality as I don't know what will appen if you try
to compare `Foo 10 with `Foo (10.) or some nasty things like that)