From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <furr@cs.umd.edu>
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,DNS_FROM_RFC_ABUSE 
	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 10A99BC69
	for <caml-list@yquem.inria.fr>; Mon,  6 Aug 2007 15:47:43 +0200 (CEST)
Received: from dispatch.cs.umd.edu (dispatch.cs.umd.edu [128.8.128.60])
	by concorde.inria.fr (8.13.6/8.13.6) with ESMTP id l76DlfoV025149
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=FAIL)
	for <caml-list@inria.fr>; Mon, 6 Aug 2007 15:47:42 +0200
Received: from [192.168.0.2] (c-69-243-63-39.hsd1.md.comcast.net [69.243.63.39])
	(authenticated bits=0)
	by dispatch.cs.umd.edu (8.13.1/8.12.5) with ESMTP id l76DlTDA012219
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO)
	for <caml-list@inria.fr>; Mon, 6 Aug 2007 09:47:34 -0400
Message-ID: <46B72671.3040301@cs.umd.edu>
Date: Mon, 06 Aug 2007 09:47:29 -0400
From: Mike Furr <furr@cs.umd.edu>
User-Agent: Mozilla-Thunderbird 2.0.0.4 (X11/20070622)
MIME-Version: 1.0
To: Caml List <caml-list@inria.fr>
Subject: Re: [Caml-list] Instanciating functor types with extra parameters
References: <46B4A343.5030900@lix.polytechnique.fr>
In-Reply-To: <46B4A343.5030900@lix.polytechnique.fr>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-CSD-MailScanner-Information: Please email staff@cs.umd.edu for more information
X-CSD-MailScanner: Found to be clean
X-CSD-MailScanner-SpamCheck: not spam, SpamAssassin (not cached,
	score=-4.389, required 5, autolearn=not spam, ALL_TRUSTED -1.80,
	AWL 0.01, BAYES_00 -2.60)
X-CSD-MailScanner-From: furr@cs.umd.edu
X-Miltered: at concorde with ID 46B7267D.000 by Joe's j-chkmail (http://j-chkmail . ensmp . fr)!
X-Spam: no; 0.00; functor:01 orderedtype:01 struct:01 struct:01 elt:01 node:01 elt:01 val:01 val:01 pervasives:01 orderedtype:01 cheers:01 -mike:01 polymorphic:01 wrote:01 

Arnaud Spiwack wrote:

> Something that naive intuition would allow you to do is something like :
> 
> module GenOrder : OrderedType =
>  struct
>    type t = 'a
>    let compare = compare
>  end

I had a similar problem while developing my OCaml-Reins data structure 
library (first release will be "real-soon-now").  Stephen Weeks offered 
the following solution:  the main idea is that you build a generic core 
module that is parameterized by the maximum number of type variables 
desired.  So for a set, you might define

module BaseSet = struct
   type 'a elt_
   type 'a set_ = Empty | Node of 'a set_ * 'a elt_ * 'a set_
   val empty : 'a set_
   val add : 'a elt_ -> 'a set_ -> 'a set_
   ...
   val compare_ : ('a elt_ -> 'a elt_ -> int) ->
      'a set_ -> 'a set_ -> int
   ...
end

Then, you can create a polymorphic version
module PolySet = struct
   include BaseSet
   type 'a t = 'a set_
   type 'a elt = 'a
   let compare = compare_ Pervasives.compare
   ...
end

or a monomorphic version:
module MonoSet(C : OrderedType) = struct
   type elt = C.t
   type 'a elt_ = elt
   type t = C.t set_
   let compare = compare_ C.compare
end

Thus allowing you to share all of the underlying code with only a little 
boilerplate.

Cheers,
-mike