From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <kosmo.zb@gmail.com>
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 CDAC0820A1
	for <caml-list@sympa.inria.fr>; Sun, 11 Aug 2013 03:55:17 +0200 (CEST)
Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender
  authenticity information available from domain of
  kosmo.zb@gmail.com) identity=pra; client-ip=209.85.217.181;
  receiver=mail3-smtp-sop.national.inria.fr;
  envelope-from="kosmo.zb@gmail.com";
  x-sender="kosmo.zb@gmail.com"; x-conformance=sidf_compatible
Received-SPF: Pass (mail3-smtp-sop.national.inria.fr: domain of
  kosmo.zb@gmail.com designates 209.85.217.181 as permitted
  sender) identity=mailfrom; client-ip=209.85.217.181;
  receiver=mail3-smtp-sop.national.inria.fr;
  envelope-from="kosmo.zb@gmail.com";
  x-sender="kosmo.zb@gmail.com"; x-conformance=sidf_compatible;
  x-record-type="v=spf1"
Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender
  authenticity information available from domain of
  postmaster@mail-lb0-f181.google.com) identity=helo;
  client-ip=209.85.217.181;
  receiver=mail3-smtp-sop.national.inria.fr;
  envelope-from="kosmo.zb@gmail.com";
  x-sender="postmaster@mail-lb0-f181.google.com";
  x-conformance=sidf_compatible
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AucFAJvtBlLRVdm1lGdsb2JhbABZhAusQJMwCBYOAQEBAQcLFBIqglIZATkDDQUQXRIBBQEiiBEDD5cXgwCPU4NxJw1XiAEBBQyURwOXZI9sFimBXYJlOw
X-IPAS-Result: AucFAJvtBlLRVdm1lGdsb2JhbABZhAusQJMwCBYOAQEBAQcLFBIqglIZATkDDQUQXRIBBQEiiBEDD5cXgwCPU4NxJw1XiAEBBQyURwOXZI9sFimBXYJlOw
X-IronPort-AV: E=Sophos;i="4.89,854,1367964000"; 
   d="scan'208";a="23687875"
Received: from mail-lb0-f181.google.com ([209.85.217.181])
  by mail3-smtp-sop.national.inria.fr with ESMTP/TLS/RC4-SHA; 11 Aug 2013 03:55:17 +0200
Received: by mail-lb0-f181.google.com with SMTP id o10so4057670lbi.12
        for <caml-list@inria.fr>; Sat, 10 Aug 2013 18:55:16 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=20120113;
        h=mime-version:sender:date:message-id:subject:from:to:content-type;
        bh=CC4FZV2BKK+xqs8YU9BxnLD+kFX53YnOHl3vtDI4cuQ=;
        b=BOSQt64Yr7bWO17zdk9IL70msuQ6niX55WQR0T7XNOIJFCJ7ACCbNr8F7zraD3DSjw
         kvZmV+zupr1WRgpvpPd20grJbiBBLvoaFjm43l/0kQObFyuK+8RCPH/2GWCKYKMbwOm9
         9O9Mcuo1EpwgijJfHY0JwZxxTewXXbRhXUbHs05wGoRKWOVwmdqmNopuqp+PIY4P0mag
         kwwtM/Zx86GNnxJwP3H85RIy7PwYnqm4o8+MwK37+Hor3MEvlJQf22DVSFDvbzLLmdiB
         Ctk1OC0EnBRg4hSSTsONMzjOal0Kk6niwkrfjfY1W+KPKSlgO2Cbg2TeN8SikdQV4YU2
         shUQ==
MIME-Version: 1.0
X-Received: by 10.112.51.101 with SMTP id j5mr2802129lbo.17.1376186116211;
 Sat, 10 Aug 2013 18:55:16 -0700 (PDT)
Sender: kosmo.zb@gmail.com
Received: by 10.112.61.130 with HTTP; Sat, 10 Aug 2013 18:55:16 -0700 (PDT)
Date: Sun, 11 Aug 2013 02:55:16 +0100
X-Google-Sender-Auth: NM0qODV9nVvVdFh-LanVAPPmh0w
Message-ID: <CAAWM5TyT4kryhLbDzsNHyLw76QpWFzBXHR4+Vf9bSwKGy16-=Q@mail.gmail.com>
From: David Sheets <sheets@alum.mit.edu>
To: Caml List <caml-list@inria.fr>
Content-Type: text/plain; charset=ISO-8859-1
Subject: [Caml-list] First-class Functor Forgetting for Free

What issue prevents functions over first-class modules from forgetting
the first-class modules' type constraints?

That is, given:

###############

module type D = sig
  type t

  val x : t
  val f : t -> int
end

module type C = sig
  val q : int
end

module F(X : D) : C = struct
  let q = X.(f x)
end

let f x =
  let module X = (val x : D) in
  let module Q = struct
    let q = X.(f x)
  end in
  (module Q : C)

let x =
  let module X = struct
    type t = int

    let x = 2
    let f x = x * 3
  end in
  (module X : D with type t = int)

;;
let module X = (val x : D with type t = int) in
let module M = F(X) in
(* let x = (module (val x : D with type t = int) : D) in*)
let module M' = (val f x : C) in

Printf.printf "M.q  is %d\n%!" M.q;
Printf.printf "M'.q is %d\n%!" M'.q;
()

###############

Why must I uncomment (* let x = (module (val x : D with type t = int)
: D) in*) to compile?

I understand why structural subtyping requires a module cast but I
don't see why type relaxation would.
I looked at the generated assembly and this line seems to disappear.

Why is it needed?

Thanks,

David