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 JAA06052; Wed, 17 Oct 2001 09:42:11 +0200 (MET DST)
X-Authentication-Warning: pauillac.inria.fr: majordomo set sender to owner-caml-list@pauillac.inria.fr using -f
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id JAA06362 for <caml-list@pauillac.inria.fr>; Wed, 17 Oct 2001 09:42:10 +0200 (MET DST)
Received: from paille.inria.fr (paille.inria.fr [128.93.11.15])
	by concorde.inria.fr (8.11.1/8.10.0) with ESMTP id f9H7g9118024;
	Wed, 17 Oct 2001 09:42:09 +0200 (MET DST)
Received: by paille.inria.fr (Postfix, from userid 11683)
	id D42A67876; Wed, 17 Oct 2001 09:05:15 +0200 (CEST)
From: Tom Hirschowitz <Tom.Hirschowitz@inria.fr>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <15309.11691.752373.956131@paille.inria.fr>
Date: Wed, 17 Oct 2001 09:05:15 +0200
To: Lauri Alanko <la@iki.fi>
Cc: caml-list@pauillac.inria.fr
Subject: [Caml-list] Module equivalence across definitions
In-Reply-To: <20011013210733.G412@la.iki.fi>
References: <20011013210733.G412@la.iki.fi>
X-Mailer: VM 6.92 under Emacs 20.6.1
Sender: owner-caml-list@pauillac.inria.fr
Precedence: bulk



Hi, 

I think it is more a question of datatypes than of 
applicative functors, in the sense that if you replace 
the module B with something like

module B(X:T) = struct
  type t = int
  let v = 2
end

then it works.

It's just because sum types are some kind of abstract 
types, the constructors interfacing them with the outer 
world. For instance :

# module A = struct type t = A end;;

# module B = struct type t = A end;;

# A.A = B.A;;
This expression has type A.t but is here used with type B.t

Do you agree?

Lauri Alanko writes:
 > [Pardon for possible duplicates]
 > 
 > Hello. Given:
 > 
 > 
 > module type T = sig
 >   type t
 > end
 > 
 > module A : T = struct
 >   type t = A
 > end
 > 
 > module B(X:T) = struct
 >   type t = B
 >   let v = B
 > end
 > 
 > module type CT = sig
 >   val a : B(A).t
 > end
 > 
 > module C : CT = struct
 >   module Aalias = A
 >   module BA = B(Aalias)
 >   let a = BA.v
 > end
 > 
 > 
 > I get an error:
 > 
 > 
 > Signature mismatch:
 > Modules do not match:
 >   sig
 >     module Aalias : sig type t = A.t end
 >     module BA : sig type t = B(Aalias).t = B val v : t end
 >     val a : BA.t
 >   end
 > is not included in
 >   CT
 > Values do not match: val a : BA.t is not included in val a : B(A).t
 > 
 > 
 > But changing "module BA = B(Aalias)" to "module BA = B(A)" makes everything
 > work.
 > 
 > What gives? I'm used to seeing structural equivalence being used everywhere
 > in O'Caml's type system, but here simply renaming a module seems to give it
 > a distinct new identity. Is this a bug or a feature?
 > 
 > 
 > Lauri Alanko
 > la@iki.fi
 > -------------------
 > Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
 > 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/
To unsubscribe, mail caml-list-request@inria.fr  Archives: http://caml.inria.fr