From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <rossberg@ps.uni-sb.de>
X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on yquem.inria.fr
X-Spam-Level: 
X-Spam-Status: No, score=0.4 required=5.0 tests=AWL,NO_REAL_NAME 
	autolearn=disabled version=3.1.3
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39])
	by yquem.inria.fr (Postfix) with ESMTP id D32D7BC6B
	for <caml-list@yquem.inria.fr>; Sat,  4 Aug 2007 20:51:42 +0200 (CEST)
Received: from triton.rz.uni-saarland.de (triton.rz.uni-saarland.de [134.96.7.25])
	by concorde.inria.fr (8.13.6/8.13.6) with ESMTP id l74Ipg0T027425
	for <caml-list@inria.fr>; Sat, 4 Aug 2007 20:51:42 +0200
Received: from mail.cs.uni-sb.de (mail.cs.uni-sb.de [134.96.254.200])
	by triton.rz.uni-saarland.de (8.12.11.20060614/8.12.10) with ESMTP id l74IpeUp24378071
	for <caml-list@inria.fr>; Sat, 4 Aug 2007 20:51:40 +0200 (CEST)
Received: from mail.ps.uni-sb.de (james.ps.uni-sb.de [134.96.186.68])
	by mail.cs.uni-sb.de (8.14.1/2007062800) with ESMTP id l74IpdKS000120
	for <caml-list@inria.fr>; Sat, 4 Aug 2007 20:51:39 +0200 (CEST)
Received: from localhost
	([127.0.0.1] helo=www.ps.uni-sb.de ident=www-data)
	by mail.ps.uni-sb.de with esmtp (Exim 4.63)
	(envelope-from <rossberg@ps.uni-sb.de>)
	id 1IHOix-0005Le-G9
	for caml-list@inria.fr; Sat, 04 Aug 2007 20:51:39 +0200
Received: from 84.165.140.254
        (SquirrelMail authenticated user rossberg)
        by www.ps.uni-sb.de with HTTP;
        Sat, 4 Aug 2007 20:51:39 +0200 (CEST)
Message-ID: <64963.84.165.140.254.1186253499.squirrel@www.ps.uni-sb.de>
In-Reply-To: <46B4A343.5030900@lix.polytechnique.fr>
References:  <46B4A343.5030900@lix.polytechnique.fr>
Date: Sat, 4 Aug 2007 20:51:39 +0200 (CEST)
Subject: Re: [Caml-list] Instanciating functor types with extra parameters
From: rossberg@ps.uni-sb.de
To: "Caml List" <caml-list@inria.fr>
User-Agent: SquirrelMail/1.4.9a
MIME-Version: 1.0
Content-Type: text/plain;charset=iso-8859-1
Content-Transfer-Encoding: 8bit
X-Priority: 3 (Normal)
Importance: Normal
X-SA-Exim-Connect-IP: 127.0.0.1
X-SA-Exim-Mail-From: rossberg@ps.uni-sb.de
X-SA-Exim-Scanned: No (on mail.ps.uni-sb.de); SAEximRunCond expanded to false
X-DCC-CTc-dcc2-Metrics: itelist
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.5.1 (triton.rz.uni-saarland.de [134.96.7.25]); Sat, 04 Aug 2007 20:51:40 +0200 (CEST)
X-AntiVirus: checked by AntiVir MailGate (version: 2.1.2-14; AVE: 7.4.0.57; VDF: 6.39.0.213; host: AntiVir0)
X-Miltered: at concorde with ID 46B4CABE.000 by Joe's j-chkmail (http://j-chkmail . ensmp . fr)!
X-Spam: no; 0.00; functor:01 rossberg:01 lix:01 functor:01 orderedtype:01 sig:01 val:01 orderedtype:01 struct:01 ocaml:01 sig:01 val:01 struct:01 innocent:98 arnaud:01 

From: "Arnaud Spiwack" <aspiwack@lix.polytechnique.fr>
>
> When I have a functor type, like for example (not too innocent
> I'm afraid) :
>
> module type OrderedType =
>   sig
>     type t
>     val compare : t -> t -> int
>   end
>
> Something that naive intuition would allow you to do is something like :
>
> module GenOrder : OrderedType =
>   struct
>     type t = 'a
>     let compare = compare
>   end
>
> Though this is more or less nonsense. And is not currently possible
> under OCaml type system.
>
> My point is that I know absolutely no way of doing such a thing.

If you have control over the functor type then you can do this:

  module type OrderedPolyType =
  sig
     type 'a t
     val compare : 'a t -> 'a t -> int
  end

  module GenOrder : OrdererPolyType =
  struct
     type 'a t = 'a
     let compare = compare
  end

  module InvIntOrder : OrdererPolyType =
  struct
     type 'a t = int
     let compare x y = compare y x
  end

Not a general solution, but one that's often good enough.

- Andreas