Message-Id: <14288.9611141020@subnode.epcc.ed.ac.uk>
Subject: Re: functors and type constraints
To: caml-list@margaux.inria.fr
Date: Thu, 14 Nov 1996 10:20:02 +0000 (GMT)
From: Jocelyn Serot <jserot@epcc.ed.ac.uk>
Hello,
In his last post, P. Boulet <Pierre.Boulet@prism.uvsq.fr> says:
> hello,
>
> I have a problem with some complicated type constraints with Objective
> Caml version 1.02. Let have an example:
>
> [stuff deleted]
>
> Everything goes find till now. Let's instanciate the modules now.
>
> module T1 : TAG struct ... end;;
> module T2 : TAG struct ... end;;
> module A1 : ARBRE Make_arbre (T1);;
> module A2 : ARBRE Make_arbre (T2);;
> module Trad_Tag : TRAD =
>
> struct =
>
> type from T1.t
> type into T2.t
> val f ... =
>
> end;;
>
> The problem appears during the compilation of the following statement:
> module Trad_Arbre : TRAD Make_trad (A1) (A2) (Trad_Tag);;
> Signature mismatch:
> Modules do not match:
> sig =
> type from Trad_Tag.from =
> type into Trad_Tag.into =
> val f : from -> into =
> end
> is not included in
> sig type from A1.tag type into A2.tag val f : from -> into end
> Type declarations do not match:
> type from Trad_Tag.from
> is not included in
> type from A1.tag
>
> These types are equal though. I imagine a problem of abstract
> definition of some types, but how can I signify to the compiler that
> these types are equal?
I understand your problem as follows: given a function f to convert between
TAGs, you want to build a function to convert between ARBREs.
I recently came across a very similar problem dealing, respectively,
with PIXELs and IMAGEs.
Here's the solution i finally wrote, adapted a bit to fit to your definitions:
----------------------------- trad.ml ------------------------------------------
module type TAG = sig type t end;;
module type ARBRE = sig type tag type quast end;;
module type MAKE_ARBRE =
functor (T : TAG) -> (ARBRE with type tag = T.t);;
module Make_arbre : MAKE_ARBRE =
functor (T : TAG) ->
struct
type tag = T.t
type quast = {tag : tag}
end;;
module type TRAD = sig type t1 type t2 val f : t1 -> t2 end;;
module type Make_trad = functor
(A1 : ARBRE) -> functor
(A2 : ARBRE) -> functor
(F : sig val f : A1.tag -> A2.tag end)
-> TRAD with type t1 = A1.tag and type t2 = A2.tag;;
module T1 : TAG = struct type t = float end;;
module T2 : TAG = struct type t = int end;;
module A1 : ARBRE = Make_arbre(T1);;
module A2 : ARBRE = Make_arbre(T2);;
module Trad = Make_trad (A1) (A2) (struct let f = truncate end);;
----------------------------- trad.ml ------------------------------------------
Hope that helps.
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E-mail: jserot@epcc.ed.ac.uk (->Nov 29 1996) ................................
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