From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <rossberg@mpi-sws.org>
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: from mail2-relais-roc.national.inria.fr (mail2-relais-roc.national.inria.fr [192.134.164.83])
	by yquem.inria.fr (Postfix) with ESMTP id D4605BC57
	for <caml-list@yquem.inria.fr>; Mon, 26 Apr 2010 18:54:50 +0200 (CEST)
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AtsAAIli1UuLEwExkWdsb2JhbACcUQEBAQEJCwoHEQUdtBEBjmAFhQs
X-IronPort-AV: E=Sophos;i="4.52,274,1270418400"; 
   d="scan'208";a="49673683"
Received: from infao0809.mpi-sb.mpg.de (HELO hera.mpi-sb.mpg.de) ([139.19.1.49])
  by mail2-smtp-roc.national.inria.fr with ESMTP/TLS/AES256-SHA; 26 Apr 2010 18:54:50 +0200
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
	d=mpi-sb.mpg.de; s=mail200803; h=Message-ID:In-Reply-To:
	References:Date:Subject:From:To:Cc:MIME-Version:Content-Type:
	Content-Transfer-Encoding; bh=cJ0i7HIpj83Q09paL432dx/cMSNVuGKjX7
	QzlFgBRHg=; b=M2urYtZgR+fsLRofTdxWESHKsAbRLdRUMUhUUN6hK28Ghhukdl
	DNK69ZzP08eJCwgowjgWEt5/0PS4CyAvfE8rpRkLMagz4ImauuhgBydAVsZVZKdJ
	VpnCPEPXOfUBOwo0Ev+5syw5HLgfx9mtpVu9BwAMjKZN81P32H0VZrkuI=
Received: from maniac.mpi-sb.mpg.de ([139.19.1.26]:32946)
	by hera.mpi-sb.mpg.de (envelope-from <rossberg@mpi-sws.org>) 
	with esmtp (Exim 4.69) id 1O6Ra3-0004Sy-UJ; Mon, 26 Apr 2010 18:54:50 +0200
Received: from www-data by maniac.mpi-sb.mpg.de with local (Exim 4.69)
	(envelope-from <rossberg@mpi-sws.org>)
	id 1O6Ra3-000469-JS; Mon, 26 Apr 2010 18:54:47 +0200
Received: from 80.146.185.83
        (SquirrelMail authenticated user rossberg)
        by mail.mpi-sws.org with HTTP;
        Mon, 26 Apr 2010 18:54:47 +0200 (CEST)
Message-ID: <5ac779c6dcc89fe009150414e374043c.squirrel@mail.mpi-sws.org>
In-Reply-To: <4BD5C057.7020800@wp.pl>
References: <4BD229E4.4050003@wp.pl> <4BD5C057.7020800@wp.pl>
Date: Mon, 26 Apr 2010 18:54:47 +0200 (CEST)
Subject: Re: [Caml-list] Re: Module type of a structure returned by functor
From: rossberg@mpi-sws.org
To: "Dawid Toton" <d0@wp.pl>
Cc: "caml-list" <caml-list@yquem.inria.fr>
User-Agent: SquirrelMail/1.4.15
MIME-Version: 1.0
Content-Type: text/plain;charset=iso-8859-15
Content-Transfer-Encoding: 8bit
X-Priority: 3 (Normal)
Importance: Normal
X-Spam: no; 0.00; functor:01 rossberg:01 foo:01 functor:01 sig:01 val:01 foo:01 functors:01 ocaml:01 struct:01 sig:01 val:01 wrote:01 signatures:01 signatures:01 

Dawid Toton <d0@wp.pl> wrote:
> I've found that I have more fundamental problem. What is the exact
> meaning of the following line?
>
> module type Foo = functor (X:X) -> sig val foo : X.t end
>
> (1) Foo is not a functor, but it is a type of some functors that map
> modules to modules
> (2) Foo is a mapping from modules to module types
>
> Currently I think that it (1) is true and (2) is false. Let me know if
> I'm wrong.

That's right. A construct for (2), sometimes referred to as "parameterized
signatures", does not exist in OCaml directly. However, you can encode it
using a functor with a nested signature in its result:

  module FooOf (X : X) =
  struct
    module type S = sig val foo : X.t end
  end

Now, for example:

  module F (X : X) (Y : FooOf(X).S) = ...

> It means that there is no easy way to get module type of what results
> from functor application. I think that the solution is to separately
> define signature of results of the functor and use "with type" clauses
> to recreate all result module types that are needed.
>
> This is not very bad, but I'm still wondering if "module type of..." of
> 3.12 will provide elegant solution for this.

Interesting, it looks like it may. I wasn't aware that singleton
signatures will be coming in 3.12 - nice.

/Andreas