From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (from majordomo@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id MAA21759; Tue, 10 Apr 2001 12:44:10 +0200 (MET DST)
X-Authentication-Warning: pauillac.inria.fr: majordomo set sender to owner-caml-list@pauillac.inria.fr using -f
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id MAA21755 for <caml-list@pauillac.inria.fr>; Tue, 10 Apr 2001 12:44:09 +0200 (MET DST)
Received: from iona.dcs.gla.ac.uk (iona.dcs.gla.ac.uk [130.209.240.35])
	by nez-perce.inria.fr (8.11.1/8.10.0) with ESMTP id f3AAi4n05075;
	Tue, 10 Apr 2001 12:44:04 +0200 (MET DST)
Received: from crab.dcs.gla.ac.uk ([130.209.241.150])
	by iona.dcs.gla.ac.uk with esmtp (Exim 3.13 #1)
	id 14mvcx-0003HK-00; Tue, 10 Apr 2001 11:44:03 +0100
Received: by crab.dcs.gla.ac.uk (8.8.8+Sun/Dumb)
	id LAA10626; Tue, 10 Apr 2001 11:44:02 +0100 (BST)
From: reig@dcs.gla.ac.uk
Message-Id: <200104101044.LAA10626@crab.dcs.gla.ac.uk>
Subject: Re: [Caml-list] variant with tuple arg in pattern match?
To: caml-list@inria.fr
Date: Tue, 10 Apr 2001 11:44:02 +0100 (BST)
Cc: Michel.Mauny@inria.fr
In-Reply-To: <20010410102323.A13306@quincy.inria.fr> from "Michel Mauny" at Apr 10, 2001 10:23:23 AM
X-Mailer: ELM [version 2.5 PL3]
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Sender: owner-caml-list@pauillac.inria.fr
Precedence: bulk

Michel Mauny wrote:

> Well, as far as I understand, there is an interaction with
> pattern-matching. Consider the following example:
> 
>   type 'a t =
>     Int : int -> int t
>   | Bool : bool -> bool t
>   | Node : 'a t -> 'a t -> 'a t
> 
> This definition implies that data constructors Int and Bool cannot
> appear in the same tree (no way to be at the same time an int t, and a
> bool t, unless being an 'a t, which cannot occur at all).
> [...]

This is how you do it in haskell:

data T a = I Int
	 | B Bool
	 | N (T a) (T a) 
	 | Uncurried (Int,Int)

hugs (a haskell interpreter) reports this:

Main> :i T
-- type constructor
data T a
 
-- constructors:
I :: Int -> T a
B :: Bool -> T a
N :: T a -> T a -> T a
Uncurried :: (Int,Int) -> T a

Of course, you can have trees with bools and ints:

Main> N (I 1) (B True)
N (I 1) (B True) :: T a

Notice that I has type Int -> T a, not Int -> T Int

The equivalent in ocaml, using Pierre's new syntax, would be:

   type 'a t =
     Int : int -> 'a t
   | Bool : bool -> 'a t
   | Node : 'a t -> 'a t -> 'a t
   | Uncurried : int * int -> 'a t

Moreover, the last part (-> 'a t) is the same for all constructors and
can be omitted like you do in haskell (so that Xavier is happier :)

   type 'a t =
     Int : int
   | Bool : bool
   | Node : 'a t -> 'a t
   | Uncurried : int * int

[I assume there would be no type checking problems for ocaml, but I
don't know for sure]

One other thing changes too: data constructors can be partially
applied in haskell. Ex:

Main> :t N (I 1)
N (I 1) :: T a -> T a

Main> map I [1,2,3]
[I 1,I 2,I 3] :: [T a]

[Don't need to eta expand (\x -> I x)]

Personally, I quite like this feature of haskell compared to ML.

Fermin Reig
-------------------
To unsubscribe, mail caml-list-request@inria.fr.  Archives: http://caml.inria.fr