>
>       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


Hope this helps,
Julien Signoles
-- 
mailto:Julien.Signoles@lri.fr ; http://www.lri.fr/~signoles
"In theory, practice and theory are the same,
but in practice they are different" (Larry McVoy)