From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <alain@frisch.fr>
X-Original-To: caml-list@sympa.inria.fr
Delivered-To: caml-list@sympa.inria.fr
Received: from mail3-relais-sop.national.inria.fr (mail3-relais-sop.national.inria.fr [192.134.164.104])
	by sympa.inria.fr (Postfix) with ESMTPS id D3C4F7FA31
	for <caml-list@sympa.inria.fr>; Wed, 16 Jul 2014 13:17:17 +0200 (CEST)
Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender
  authenticity information available from domain of
  alain@frisch.fr) identity=pra; client-ip=193.252.23.215;
  receiver=mail3-smtp-sop.national.inria.fr;
  envelope-from="alain@frisch.fr"; x-sender="alain@frisch.fr";
  x-conformance=sidf_compatible
Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender
  authenticity information available from domain of
  alain@frisch.fr) identity=mailfrom; client-ip=193.252.23.215;
  receiver=mail3-smtp-sop.national.inria.fr;
  envelope-from="alain@frisch.fr"; x-sender="alain@frisch.fr";
  x-conformance=sidf_compatible
Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender
  authenticity information available from domain of
  postmaster@msa.smtpout.orange.fr) identity=helo;
  client-ip=193.252.23.215;
  receiver=mail3-smtp-sop.national.inria.fr;
  envelope-from="alain@frisch.fr";
  x-sender="postmaster@msa.smtpout.orange.fr";
  x-conformance=sidf_compatible
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AgABAIRexlPB/BfXnGdsb2JhbABZDoNSV8Iqh0IBgScPAQEBAQEIFAk8hAQBBScRQAEQCw4KCRYPCQMCAQIBRQYNAQcBAYhCCcpOEwSPGDMHhEMBBJschxSQFURqAQ
X-IPAS-Result: AgABAIRexlPB/BfXnGdsb2JhbABZDoNSV8Iqh0IBgScPAQEBAQEIFAk8hAQBBScRQAEQCw4KCRYPCQMCAQIBRQYNAQcBAYhCCcpOEwSPGDMHhEMBBJschxSQFURqAQ
X-IronPort-AV: E=Sophos;i="5.01,671,1400018400"; 
   d="scan'208";a="71572200"
Received: from msa06.smtpout.orange.fr (HELO msa.smtpout.orange.fr) ([193.252.23.215])
  by mail3-smtp-sop.national.inria.fr with ESMTP; 16 Jul 2014 13:17:17 +0200
Received: from [192.168.1.133] ([92.151.54.109])
	by mwinf5d47 with ME
	id SzHG1o00G2MNqdG03zHGxa; Wed, 16 Jul 2014 13:17:16 +0200
X-ME-Helo: [192.168.1.133]
X-ME-Auth: bGV4aWZpQHdhbmFkb28uZnI=
X-ME-Date: Wed, 16 Jul 2014 13:17:16 +0200
X-ME-IP: 92.151.54.109
Message-ID: <53C65F3C.6000704@frisch.fr>
Date: Wed, 16 Jul 2014 13:17:16 +0200
From: Alain Frisch <alain@frisch.fr>
User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0
MIME-Version: 1.0
To: Leo White <lpw25@cam.ac.uk>
CC: caml-list <caml-list@inria.fr>
References: <53C5481B.3070409@frisch.fr> <86k37e1bzv.fsf@cam.ac.uk>
In-Reply-To: <86k37e1bzv.fsf@cam.ac.uk>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [Caml-list] Existential row types

Hi Leo,

On 7/15/2014 6:11 PM, Leo White wrote:
 > There is a work-around, but it is quite convoluted:
 >
 >    https://sympa.inria.fr/sympa/arc/caml-list/2012-10/msg00131.html

Very clever trick. Thanks!

One can actually use the GADT only to carry the constraint and pass the 
value argument(s) separately.  This means the GADT definition remains 
very simple even if we need to add many fields:


=============================================================
class type c = object end

module type SIG =
sig
   type t = private #c
   val f: t -> unit
   val g: unit -> t
end

type 'a aux = Aux: (#c as 'a) aux

let create f g =
   let helper (type u) (Aux : u aux) f g =
     (module struct
        type t = u
        let f = f
        let g = g
      end : SIG)
   in
   helper Aux f g

(* or even: *)

let create f g =
   let module Aux = struct type 'a t = E: (#c as 'a) t end in
   begin fun (type u) (Aux.E : u Aux.t) f g ->
     (module struct
        type t = u
        let f = f
        let g = g
      end : SIG)
   end Aux.E f g
=============================================================



-- Alain