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 HAA12974; Wed, 21 Aug 2002 07:27:33 +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 HAA12773 for <caml-list@pauillac.inria.fr>; Wed, 21 Aug 2002 07:27:32 +0200 (MET DST)
Received: from kurims.kurims.kyoto-u.ac.jp (kurims.kurims.kyoto-u.ac.jp [130.54.16.1])
	by nez-perce.inria.fr (8.11.1/8.11.1) with ESMTP id g7L5RUP12184
	for <caml-list@inria.fr>; Wed, 21 Aug 2002 07:27:31 +0200 (MET DST)
Received: from localhost (suiren.kurims.kyoto-u.ac.jp [130.54.16.25])
	by kurims.kurims.kyoto-u.ac.jp (8.9.3/3.7W) with ESMTP id OAA09006;
	Wed, 21 Aug 2002 14:27:24 +0900 (JST)
To: nalexander@amavi.com
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Naming polymorphic variant types
In-Reply-To: <1413.207.194.7.18.1029903741.squirrel@mail.gx.ca>
References: <1413.207.194.7.18.1029903741.squirrel@mail.gx.ca>
X-Mailer: Mew version 1.94.2 on Emacs 21.2 / Mule 5.0 (SAKAKI)
Mime-Version: 1.0
Content-Type: Text/Plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-Id: <20020821142724W.garrigue@kurims.kyoto-u.ac.jp>
Date: Wed, 21 Aug 2002 14:27:24 +0900
From: Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>
X-Dispatcher: imput version 20000228(IM140)
Sender: owner-caml-list@pauillac.inria.fr
Precedence: bulk

From: "Nick Alexander" <nalexander@amavi.com>

> module type TEST =
>   sig
>     type outer = [ `A | `B]
>     and inner = [ `B]
>     val x : outer -> outer
>     val y : inner
>   end
> module Test : TEST
> # let p = Test.y;;
> val p : Test.inner = `B
> # let q = Test.x p;;
>                  ^
> This expression has type Test.inner = [ `B] but is here used with type
>   Test.outer = [ `A | `B]
> The first variant type does not allow tag(s) `A
>
> I'm confused.  Can I get an explanation for why the parameter to a
> function with a sum variant type needs all summands to be possible
> arguments? 

Inner is a subtype of outer, but they cannot be unified (unification
requires equality, not subtyping).
 
You can get subtyping by writing explicit coercions:

# let q = Test.x (p :> Test.outer);;
val q : Test.outer = `A

You can also give polymorphic types to your values in the interface:
  val x : [< outer] -> outer
and
  val y : [> inner]
are both valid, and one of them is enough to allow (Test.x p) without
coercion. I.e., [< outer] can be unified with inner, and [> inner] can
be unified with outer.

        Jacques Garrigue
-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners