Re: Some questions about signatures

From: Xavier Leroy (
Date: Tue Apr 27 1999 - 21:47:01 MET DST

Date: Tue, 27 Apr 1999 21:47:01 +0200
From: Xavier Leroy <>
To: Juan Jose Garcia Ripoll <>,
Subject: Re: Some questions about signatures
In-Reply-To: <>; from Juan Jose Garcia Ripoll on Wed, Apr 14, 1999 at 09:36:33AM +0200

> 1) Are OCaml's signatures opaque? I mean if they do hide the real
> implementation of data types and of non-declared values to the public, much
> like SML's :>

Yes, they are. OCaml's ": SIG" construct behaves like ":> SIG" in

> 2) How can I extend a signature with new values and types? I've been
> using the 'include' directive, but I did not find it in the
> manual. Is this supposed to work or is it just a Caml-Light
> reminiscent.

No, it's an oversight in the manual. "include" in signatures was
originally an experimental feature, but it seems useful enough that it
will stay. I'll fix the manual.

> 3) What is the simplest way to build toplevel modules using
> functors? In SML everything on every file is bound to the toplevel
> environment and one can do just 'structure NewStruct =
> AFunctor(structure A = AParameter)' to get a brand new module. But
> in OCaml toplevel modules are defined in term of two files (mli/ml)
> so what are my options if I dont want to add a spurious path name?

Currently, you must put your functors inside toplevel structures,
which are then mapped to .ml/.mli files. So, continuing your example
above, you have to write

        module NewStruct = ModA.Functor(AParameter)

        open ModA
        module NewStruct = Functor(AParameter)

One could imagine having compilation units that can be functors by
themselves. It's mostly a problem of syntax: currently, the contents
of an implementation file (.ml) are systematically taken to be the
inside of a "module X = struct ... end" declaration; extending this
to functors would require special syntax for the parameters.


- Xavier Leroy

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