Re: [Caml-list] Expressing module sig and impl in mli file

[Gabriel Scherer <gabriel.scherer@gmail.com>]

From the outside, there is no point in trying to make a difference
between an "interface" and "concrete types": what users see of the
module is exactly its interface, just as what you see of a value
(through an interface boundary) is its type only. There are exotic
cases where you could want to publish a value definition (or module
implementation) through a component boundary, but that's probably not
your concern here.

So my intuitive answer (but maybe some other posters will differ) is
that, in an interface, only "module Foo : Bar" makes sense. I do not
understand why you would want the implementation to also be available.
You may be facing a problem that is different than the one you think.
Could you provide a concrete example of what you're trying to do?

Making a guess: a mistake people often make when using the OCaml
module system is to seal a module with abstract types when they
actually don't want to hide the types, but only to check that the
signature are compatible. For example, with the signature "module type
Ty = sig type t end", people will try to write "module M : Ty = struct
type t = int end" and then be surprised than "(1 : M.t)" does not
type-check. This is because "module M : Ty = <Foo>" is equivalent to
"module M = (<Foo> : Ty)" which coerces <Foo> into the signature Ty
(where "t" is abstract), and no richer. A workaround is to define
"module M = <Foo>" and then separately "module M_unused = (M : Ty)" if
you want to make sure that the interface is compatible, or to refine
it with "module M : (Ty with type t = int) = <Foo>". Not sure that is
actually your problem, though.

Finally, if you have a module implementation and are not sure what its
precise, most permissive signature is, "ocamlc -i" can help you by
printing the inferred interface.

On Tue, Sep 18, 2012 at 11:02 PM, Malcolm Matalka <mmatalka@gmail.com> wrote:
> I'm sure this question has been asked before but I didn't see it in
> the Ocaml FAQ and I'm not sure how to express it for a search.
>
> In short, how do I express the following line in a .mli file:
>
> module Foo : Bar = Baz
>
> What I want to accomplish is make present the module Foo to the user,
> where they know both the interface and the concerete types on Foo.
>
> Thanks!
>
> /Malcolm
>
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