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

When you write:

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

You are constraining the signature of InType to be exactly SOMETYPE. This means, for instance, that the type t is now becoming an abstract type (whose implementation is unknown to the typer).

So IdentityInt.f type is still IntType.t -> IntType.t, but, by using a signature constraint, you've explicitly removed the equation IntType.t = int from the typer knowledge. The error message you get tells you exactly that.