From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mail1-relais-roc.national.inria.fr (mail1-relais-roc.national.inria.fr [192.134.164.82])
	by walapai.inria.fr (8.13.6/8.13.6) with ESMTP id p388idfa008640
	for <caml-list@sympa-roc.inria.fr>; Fri, 8 Apr 2011 10:44:39 +0200
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AhYCAMDJnk3B/BfXkWdsb2JhbACmJQEBAQEJCwsHFAMiwEuFbQSNToNqiR8
X-IronPort-AV: E=Sophos;i="4.63,322,1299452400"; 
   d="scan'208";a="105243311"
Received: from msa06.smtpout.orange.fr (HELO msa.smtpout.orange.fr) ([193.252.23.215])
  by mail1-smtp-roc.national.inria.fr with ESMTP; 08 Apr 2011 10:44:34 +0200
Received: from [192.168.1.63] ([83.204.229.18])
	by mwinf5d22 with ME
	id UwkZ1g00x0QTpRf03wkZQG; Fri, 08 Apr 2011 10:44:34 +0200
Message-ID: <4D9ECAF2.7070300@lexifi.com>
Date: Fri, 08 Apr 2011 10:44:34 +0200
From: Alain Frisch <alain.frisch@lexifi.com>
Organization: LexiFi
User-Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9.2.15) Gecko/20110303 Thunderbird/3.1.9
MIME-Version: 1.0
To: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
CC: caml-list <caml-list@yquem.inria.fr>
References: <4D9E28D2.1050808@wp.pl> <1302212990.8429.1150.camel@thinkpad> <0F248A34-05CF-4640-B122-75C4CE7C2CD2@mpi-sws.org> <4D9EC172.1060205@lexifi.com> <F8F08069-5BEC-4E15-B359-FE70CF1105A6@math.nagoya-u.ac.jp>
In-Reply-To: <F8F08069-5BEC-4E15-B359-FE70CF1105A6@math.nagoya-u.ac.jp>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [Caml-list] What is an applicative functor?

On 04/08/2011 10:20 AM, Jacques Garrigue wrote:
> Applicative functors have other advantages, like the fact you can refer to a
> type produced by a functor without having really applied it.
>
> For instance think of the following functor definition
>
>     module F(O : Set.OrderedType)(S : sig type t = Set.Make(O).t val union : t ->  t ->  t end) = ...
>
> If you want to do the same thing with generative functors, I believe you have to
> pass the result of Set.Make(O) around physically.
> I do think this is a significant weakness.

I can imagine uses for:

module F(O : Set.OrderedType)(S : Set.S with type elt = O.t) =

but I don't see a real-life case where one would want functor F to know 
that the type S.t was really produced by applying Set.Make.  Do you have 
a specific example in mind?


Alain