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Message-ID: <4A6FC8CB.4010009@gmail.com>
Date: Tue, 28 Jul 2009 22:58:03 -0500
From: Edgar Friendly <thelema314@gmail.com>
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To: Elnatan Reisner <elnatan@cs.umd.edu>, caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Physical counterpart to Pervasives.compare?
References: <1501.208.59.169.96.1248830737.squirrel@webmail.cs.umd.edu> <4A6FBCBF.4070900@gmail.com>
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X-Spam: no; 0.00; pervasives:01 annotations:01 compile-time:01 pointers:01 pointers:01 edgar:98 edgar:98 wrote:01 wrote:01 ints:01 ints:01 compiles:01 caml-list:01 compares:01 compares:01 

Edgar Friendly wrote:
> Elnatan Reisner wrote:
>> I'm afraid of getting into trouble with Obj.magic, but what would this do:
>> let f (x:'a) (y:'a) = compare (Obj.magic x) (Obj.magic y)
>> ? Or would annotations make any difference:
>> let f (x:'a) (y:'a) = compare (Obj.magic x : int) (Obj.magic y : int)
>>
>> -Elnatan
> 
> Nope, Obj.magic and casting only have compile-time effects, the code
> given compiles exactly the same as [let f x y = compare x y].
>
It looks like I'm wrong on this - there's versions of compare
specialized for floats, strings and ints.  Maybe you *could* trigger the
use of the caml_int_compare by this kind of type system manipulation.
It looks quite safe to use on non-ints.

Testing on your platform is possible, by trying to find store pointers
to ints, so that caml_compare compares the ints, but the tricked
caml_int_compare compares the pointers, and gets a different result.

E