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To: "Mattias Waldau" <mattias.waldau@abc.se>
Cc: caml-list@inria.fr
Subject: Re: Why can't I use val mover : < move : int -> unit; .. > list -> 
	unit ?
In-Reply-To: Your message of "Wed, 10 Jan 2001 13:39:47 +0100."
             <HDEEKOMJILGEIHIMAPCDCEFFDNAA.mattias.waldau@abc.se> 
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Date: Thu, 11 Jan 2001 10:29:55 +0100
From: "Sylvain BOULM'E" <Sylvain.Boulme@lip6.fr>
Sender: weis@pauillac.inria.fr

Hi,

Lists are polymorphic but they are uniform : elements of a list must have all 
the same type (which can be any type).

Hence, "mover" is a function that expects a list of objects, all of the same 
type, and which type contains a method "move".

Thus, with 
 q: p1d_1
 p: p1d_2

Expression "[q;q]" is of type "p1d_1 list" and "[p;p]" is of type "p1d_2 list",
thus "mover [q;q]" and "mover [p;p]" are well-typed.

But "[p;q]" is not well-typed.

Here, p1d_2 is subtype of p1d_1 : you can coerce p to have the type p1d_1.
Thus "[(p:>p1d_1);q]" is of type p1d_1 list.
And "mover [(p:>p1d_1);q]" is well-typed.

Sylvain.