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Message-ID: <4A6FE88E.7020801@frisch.fr>
Date: Wed, 29 Jul 2009 08:13:34 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Elnatan Reisner <elnatan@cs.umd.edu>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Physical counterpart to Pervasives.compare?
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X-Spam: no; 0.00; frisch:01 frisch:01 pervasives:01 pervasives:01 ocaml:01 hashing:01 val:01 mutable:01 2009:98 polymorphic:01 wrote:01 heap:01 caml-list:01 compares:01 alain:01 

On 7/29/2009 3:25 AM, Elnatan Reisner wrote:
> Is there something that can complete this analogy:
> (=) is to (==) as Pervasives.compare is to ___?
>
> That is, is there a polymorphic total ordering with respect to *physical*
> entities, rather than to their structure?

Not really. The physical location of heap values is not stable (because 
of the GC), so you cannot use it as a total ordering.

It may be useful to know that the generic structural comparison has a 
physical behavior for OCaml objects (it compares their unique ids). So 
wrapping your values inside objects can be a good trick to get physical 
comparison (and hashing), as in:

class ['a] physical_reference init = object
   val mutable current : 'a = init
   method get = current
   method set x = current <- x
end


-- Alain