OCaml Signature Functors

Question

I am trying to extend a functor in OCaml. For example, assume the following functor X:

module type X = functor (A : ModuleA) -> I with type t := A.t

I am trying to create a similar functor Y that also accepts A : Module A but returns an extended version of I. I am trying something like:

module type Y = functor (A : ModuleA) ->
  sig
    include X(A)
    val blah : A.t -> int
  end

But I get a syntax error on this. I am trying to extend the resulting signature from X with more functions. Is this possible in OCaml? What am I doing wrong?

Thanks!

EDIT:

I guess my question is: why don't functors behave the same way for modules and module types?

The functor X above returns a module type (or at least that's how I read that expression). If this expression is allowed, then why does OCaml forbid extending the resulting module type?

Answer 1

Unfortunately, to my knowledge this is not possible. You will have to do

module type Y = functor (A : ModuleA) ->
  sig
    include I with type t := A.t
    val blah : A.t -> int
  end

Hopefully someone else can elaborate why the feature you were trying to use is not implemented. Possibly there is a good reason.

EDIT:

If you already have a module XX of type X (an instance), you can do

module type Y = functor (A : ModuleA) ->
  sig
    include module type of XX(A)
    val blah : A.t -> int
  end