From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <jon@jdh30.plus.com>
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78])
	by yquem.inria.fr (Postfix) with ESMTP id 0E426BC40
	for <caml-list@yquem.inria.fr>; Fri, 29 Oct 2004 00:34:45 +0200 (CEST)
Received: from ptb-relay01.plus.net (ptb-relay01.plus.net [212.159.14.212])
	by nez-perce.inria.fr (8.13.0/8.13.0) with ESMTP id i9SMYiAt015342
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO)
	for <caml-list@yquem.inria.fr>; Fri, 29 Oct 2004 00:34:44 +0200
Received: from [80.229.56.224] (helo=chetara)
	 by ptb-relay01.plus.net with esmtp (Exim) id 1CNIqw-000PE1-8c
	for caml-list@yquem.inria.fr; Thu, 28 Oct 2004 22:34:42 +0000
From: Jon Harrop <jon@jdh30.plus.com>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Pattern matching but no construction?
Date: Thu, 28 Oct 2004 23:30:01 +0100
User-Agent: KMail/1.6.2
References: <012676D607FCF54E986746512C22CE7D01FF2E0B@orsmsx407>
In-Reply-To: <012676D607FCF54E986746512C22CE7D01FF2E0B@orsmsx407>
MIME-Version: 1.0
Content-Disposition: inline
Message-Id: <200410282330.01081.jon@jdh30.plus.com>
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
X-Miltered: at nez-perce with ID 41817404.000 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)!
X-Spam: no; 0.00; caml-list:01 constructors:01 sig:01 val:01 val:01 struct:01 bool:01 sig:01 bool:01 stringset:01 cheers:01 ...:98 expression:01 typing:01 constructor:01 
X-Spam-Checker-Version: SpamAssassin 3.0.0 (2004-09-13) on yquem.inria.fr
X-Spam-Status: No, score=0.0 required=5.0 tests=none autolearn=disabled 
	version=3.0.0
X-Spam-Level: 


You can hide the types held by the Integer and Boolean constructors and not 
hide the variant type itself:

# module Thing : sig
  type integer
  type boolean
  type t = Integer of integer | Boolean of boolean
  val mk_thing : int -> t
  val dest_thing : t -> int
end = struct
  type integer = int
  type boolean = bool
  type t = Integer of integer | Boolean of boolean
  let mk_thing i = Integer i
  let dest_thing t = match t with
    Integer i -> i
  | Boolean b -> if b then 1 else 0
end;;
module Thing :
  sig
    type integer
    type boolean
    type t = Integer of integer | Boolean of boolean
    val mk_thing : int -> t
    val dest_thing : t -> int
  end

Then you could "open" the namespace of the Thing module, saving enormously on 
typing:

# open Thing;;

Then you can use pattern matching to determine if a value of type "Thing.t" 
uses the "Integer" or the "Boolean" constructor:

# fun (Boolean b) -> b;;
Warning: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
Integer _
- : Thing.t -> Thing.boolean = <fun>

A better example might be:

# let is_bool = function Integer _ -> false | Boolean _ -> true;;
val is_bool : Thing.t -> bool = <fun>

Of course, if your pattern catches a value of type "Thing.integer" or 
"Thing.boolean" then you can't do anything with it except hand it to a 
function in the "Thing" module.

But you can't actually construct a "Thing.t" because you don't have access to 
the hidden "integer" and "boolean" types in "Thing":

# Integer(3);;
This expression has type int but is here used with type Thing.integer

As an aside which you may well already know, convention is to use a type "t" 
for the main type of a module (e.g. List.t, Array.t, String.t) and a function 
"make" to construct it. Possibly also a function "compare" so you can build 
sets and maps over the type, e.g.:

# module StringSet = Set.Make(String);;
...

Cheers,
Jon.