Hi,

I've problems in writing an Identity Functor in OCAML that I want to use as a wrapper module.

Consider the following OCAML definition containing the type IHelloWorld, the module World, an Identity functor
defined in the module Wrapper, and the binding of World to Wrapper in the module TestWrapper:

module type IHelloWorld =
sig
     type helloWorldType = Hello | World
val hello : unit -> helloWorldType
end

      module Wrapper (X: IHelloWorld) : IHelloWorld =
struct
    type helloWorldType = X.helloWorldType
    let hello = X.hello
end

      module World : IHelloWorld =
struct
   type helloWorldType = Hello | World
     let hello () = Hello
end

      module TestWrapper = Wrapper(World)

If I compile this module I get the following output from the OCAML compiler

mdejonge> ocamlc wrapper-demo.ml
File "wrapper-demo.ml", line 8, characters 3-85:
Signature mismatch:
Modules do not match:
      sig
      type helloWorldType = X.helloWorldType
      val hello : unit -> X.helloWorldType
      end
is not included in
      IHelloWorld
   Type declarations do not match:
      type helloWorldType = X.helloWorldType
is not included in
      type helloWorldType = Hello | World

If I change helloWorldType into "int" and let the definition of hello yield (say) 10:

type helloWorldType = int
let hello () = 10

Then the compiler issues no error at all!

Using a "with type" construct in the module definition of Wrapper does not work:

      module Wrapper (X: IHelloWorld) : IHelloWorld with type helloWorldType = X.helloWorldType =

I don't understand what is going on. Can anyone help me understanding this problem and help me
writing a wrapper module as above?

Many thanks in advance

Best regards,
Merijn de Jonge