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From: Markus Mottl <mottl@miss.wu-wien.ac.at>
Message-Id: <199912231044.LAA07631@miss.wu-wien.ac.at>
Subject: Re: parameterized signatures
To: cet@webcriteria.com (Chris Tilt)
Date: Thu, 23 Dec 1999 11:44:53 +0100 (MET)
Cc: caml-list@inria.fr (OCAML)
In-Reply-To: <38612749.528896CB@webcriteria.com> from "Chris Tilt" at Dec 22, 99 11:32:25 am
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Hello,

> Is there a way to refer to the type of a functor defined by a .mli file
> in another .mli file?

> =====> the problem area. How do I express the type of this module?
> module type Wgraph = Pgraph.S ???

Acessing .mli-files as signatures is currently not possible. If I remember
right, I have already asked once whether this could be allowed, but I was
told that this would cause confusions about module names and module-type
names.

Therefore, if I use functors I normally do not use an .mli-file to restrict
the interface, but split up the code in something like:

  foo_intf.ml
  foo_impl.ml

and restrict the modules/functors explicitely.

"foo_intf.ml" contains, for example:

  module type SPEC = sig ... end
  module type T = sig ... end

and "foo_impl.ml":

  open Foo_intf

  module Make (Spec : SPEC) : (T with ...) = struct ... end

Sometimes it is necessary to "reexport" types from the "specification"
module.

Then I do this like:

  module Make (Spec_ : SPEC) : (T with ...) = struct
    module Spec = Spec_
    open Spec
    ...
  end

Of course, the signature T will then have to include something like:

  module type SPEC = sig ... end

  module type T = sig
    module Spec : SPEC
    open Spec
  end

and will thus make available all types in "Spec".

I hope this helps!

Best regards,
Markus Mottl

-- 
Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl