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Date: Wed, 21 Jul 2010 15:13:28 +0100
Message-ID: <AANLkTikvjA641yGQJDFrC-wWOyLRGuudTh=gZjpKiHYj@mail.gmail.com>
Subject: Re: [Caml-list] Bug in the module system of version 3.12.0+beta1
From: Jeremy Yallop <yallop@gmail.com>
To: dumitru.potop@inria.fr
Cc: caml-list@inria.fr
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X-Spam: no; 0.00; bug:01 functor:01 sig:01 val:01 functor:01 sig:01 val:01 ocaml:01 wrote:01 signatures:01 signatures:01 opaque:01 caml-list:01 int:01 int:01 

On 21 July 2010 14:38, Dumitru Potop-Butucaru
<dumitru.potop_butucaru@inria.fr> wrote:
> module type Abc =3D
> =A0 functor (M:Simple) ->
> =A0 sig
> =A0 =A0 =A0val x : M.t
> =A0 end

You're trying to treat Abc as a functor from signatures to signatures
(i.e. as a parameterised signature).  In fact, it's something quite
different: it's the *type* of a functor from structures to structures.

You can emulate a parameterised signature using a signature with some
opaque components, which are later specified using substitution.
Here's the "parametrised signature":

  module type ABC =3D
  sig
    module M : Simple
    val x : M.t
  end

Here's how to supply a value for M:

  module MyModule :
  sig
     include ABC with module M =3D IntList
     val y : int
  end
=3D ...

In OCaml 3.12 you can use destructive substitution instead, ensuring
that M doesn't appear in the output signature.  Note the ':=3D' in the
line that includes ABC:

  module MyModule :
    sig
       include ABC with module M :=3D IntList
       val y : int
    end
  =3D ...