From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <darioteixeira@yahoo.com>
X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on yquem.inria.fr
X-Spam-Level: 
X-Spam-Status: No, score=0.0 required=5.0 tests=none autolearn=disabled 
	version=3.1.3
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: from mail3-relais-sop.national.inria.fr (mail3-relais-sop.national.inria.fr [192.134.164.104])
	by yquem.inria.fr (Postfix) with ESMTP id 59CA7BBAF
	for <caml-list@yquem.inria.fr>; Wed, 25 Jun 2008 16:26:32 +0200 (CEST)
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AiwBAODxYUjOvjGxfGdsb2JhbACSVQEBCwUCBgcRA59G
X-IronPort-AV: E=Sophos;i="4.27,702,1204498800"; 
   d="scan'208";a="14514280"
Received: from web54607.mail.re2.yahoo.com ([206.190.49.177])
  by mail3-smtp-sop.national.inria.fr with SMTP; 25 Jun 2008 16:26:31 +0200
Received: (qmail 4069 invoked by uid 60001); 25 Jun 2008 14:26:30 -0000
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=yahoo.com;
  h=Received:X-Mailer:Date:From:Subject:To:MIME-Version:Content-Type:Content-Transfer-Encoding:Message-ID;
  b=Ilcj61eNWB8ex9BDoHjmfDRUTwXcMM6xJryMAUCM1mQ8gtrdsVTBJba61jROK2emlRn0ZsCaCoC/1gQrQEkecoFCk5je1b60yRgC1759xuC1UxrkUJC/D6vrnEGO43uXEVDOpQfTGcPjOMI4hgiBkTK5HM0359GzRs4/qSdFllM=;
Received: from [212.13.62.47] by web54607.mail.re2.yahoo.com via HTTP; Wed, 25 Jun 2008 07:26:30 PDT
X-Mailer: YahooMailWebService/0.7.199
Date: Wed, 25 Jun 2008 07:26:30 -0700 (PDT)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Phantom types: transparency vs rogue unification
To: caml-list@yquem.inria.fr
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Message-ID: <767974.3892.qm@web54607.mail.re2.yahoo.com>
X-Spam: no; 0.00; unification:01 module's:01 unification:01 sig:01 val:01 val:01 struct:01 foo:01 sig:01 struct:01 foo:01 cheers:01 1.0:98 2.0:98 1.0:98 

Hi,

I've been experimenting with phantom types since I've read Richard Jones'
excellent intro to the subject [1].  I've now come across a tricky problem,
however.  Essentially, I need to reconcile two seemingly incompatible goals=
:
making a module's phantom type opaque to avoid the "rogue unification" issu=
e,
and making it transparent so that external modules can produce values of
that type.

So, suppose I have a "Stringies" module, which uses a phantom string to
parametrise floats.  Because the phantom type is opaque, the rogue unificat=
ion
is prevented in the last line:

_______________________________________________________________
module Stringies:
sig
        type 'a t
        val make: float -> string t
        val add: string t -> string t -> string t
end =3D
struct
        type 'a t =3D float
        let make x =3D x
        let add a b =3D a +. b
end

open Stringies
let foo =3D add (make 1.0) (make 2.0);;
let bar =3D add (make 1.0) 2.0;;  (* Error! *)
_______________________________________________________________


But suppose you wish to create an external module with a function to parse
strings into Stringies.t.  The phantom must become visible, but that allows
for rogue unification to take place:

_______________________________________________________________
module Stringies:
sig
        type 'a t =3D float
        val make: float -> string t
        val add: string t -> string t -> string t
end =3D
struct
        type 'a t =3D float
        let make x =3D x
        let add a b =3D a +. b
end


module Stringies_IO:
sig
        val parse: string -> string Stringies.t
end =3D
struct
        let parse =3D float_of_string
end


open Stringies
let foo =3D add (make 1.0) (Stringies_IO.parse "2.0");;
let bar =3D add (make 1.0) 2.0;;  (* Rogue unification! *)
_______________________________________________________________


How would you go about solving this problem?

Thanks in advance!
cheers,
Dario Teixeira

[1] http://camltastic.blogspot.com/2008/05/phantom-types.html
=0A=0A=0A      __________________________________________________________=
=0ASent from Yahoo! Mail.=0AA Smarter Email http://uk.docs.yahoo.com/nowyou=
can.html