Hello,

> In ocaml I could hide all the additional objects in a module
> implementation. But so far it is not possible to hide methods via
> signature matching.

It is definitively not possible to hide methods in a class.  When a
method is redefined, it must keep the same type. The reason is that
other methods of the same class might invocate it, and thus expect it
to have a certain type. But if a method were hidden, its previous type
would also be hidden, and there would be no way to enforce a
redefinition of this method to have the right type.

> But I still don't understand the type error I got:
> 
>    Values do not match:
>      val newb : unit -> b
>    is not included in
>      val newb : unit -> < init : unit; .. >
> 
> If b does not match  < init : unit; .. >  then the type cast in 
> 
> #  let newb () : < init : unit; .. > = ((new b ()) : < init : unit; .. >)
> 
> should produce an error. On the other side if the type cast is
> valid, then the types should match.

Type < init : unit; .. > is more general than type
b = < init : unit; f : unit > (the ellipsis is an unamed type
variable). So, your type cast produces no error (these two types can
be unified), but you cannot expect a function of type unit -> b to be
considered as having the more general type
  unit -> < init : unit; .. >.

Regards,

Jerome