> >       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
> 
> You have to explicitely define Hello and World in the wrapper. Try this 
:
> 
> module Wrapper(X:IHelloWorld): IHelloWorld =
> struct
>   type helloWorldType = X.helloWorldType = Hello | Word
>   let hello = X.hello
> end

Alternatively, you have defined too much In IHelloWorld
(depends on what you wanted to do in the first place).
Then try this:

      module type IHelloWorld =
      sig
        type helloWorldType (* now abstract *)
        val hello : unit -> helloWorldType 
      end

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

        ...

Sebastian Egner.