OCaml functors :: counter-intuitive behaviour

Question

I am experimenting with the module language of OCaml (3.12.1), defining functors and signatures for modules and so on, mostly following the examples from Chapter 2 of the OC

Answer 1

If you look at the inferred signature when you don't write something explicitly:

# module IntType = struct type t = int end ;;
module IntType : sig type t = int end

The signature exposes that t is an int. Your signature, on the contrary:

# module IntType : SOMETYPE = struct type t = int end ;;
module IntType : SOMETYPE

is really:

# module IntType : sig type t end = struct type t = int end ;;
module IntType : sig type t end

This seems to solve your problem:

# module IntType : (SOMETYPE with type t = int) = struct type t = int end ;;
module IntType : sig type t = int end
# module IdentityInt = Identity(IntType) ;;
module IdentityInt : sig val f : IntType.t -> IntType.t end
# IdentityInt.f 0 ;;
- : IntType.t = 0

(you don't need the parens, but they help mentally parse). Basically, you expose the fact that t is an int with your signature. So that OCaml knows the equality IntType.t = int.

For more details of the internals, I'll leave it to more knowledgeable people.