Re: [Caml-list] implicit subtyping fails with recursive classes

[Leo White <lpw25@cam.ac.uk>]

I am not sure, but I think the problem is that you are using #foo before
it is fully defined, so it is becoming unified with the universal 'a and
then when #foo is generalised the universal is escaping its scope.

One way to avoid the problem is to expand out the type of #foo:

  # class type t = object method push : 'b. (< foo : t -> unit; ..> as 'b) -> unit end;;
  class type t = object method push : < foo : t -> unit; .. > -> unit end
  # class type foo = object method foo : t -> unit end;;
  class type foo = object method foo : t -> unit end

if foo has a lot of methods, and you want to avoid typing them out
multiple times, you could try:

  # class type ['a] foo' = object method foo : 'a -> unit end;;
  class type ['a] foo' = object method foo : 'a -> unit end
  # class type t = object method push : 'b. (t #foo' as 'b) -> unit end;;
  class type t = object method push : t #foo' -> unit end
  # class type foo = [t] foo';;
  class type foo = [t] foo'

Regards,

Leo

Goswin von Brederlow <goswin-v-b@web.de> writes:

> Hi,
>
> I'm trying to add implicit subtyping to a pair of recurisve classes:
>
> Without subtyping:
> ------------------
> class type foo = object method foo : t -> unit end
> and t = object method push : foo -> unit end
>
> With that I can call t#push (x :> foo) for any x that implements the
> foo interface.
>
>
> What I want:
> ------------
> # class type foo = object method foo : t -> unit end
> and t = object method push : 'a . (#foo as 'a) -> unit end;;
>                              ^^^^^^^^^^^^^^^^^^^^^^^^^
> Error: The universal type variable 'a cannot be generalized:
>        it escapes its scope.
>
>
>
> The problem seems to be the recursive definition with "and". Here is
> an example with and without "and":
>
> (* works *)
> class type foo = object method foo : string end;;
> class type bar = object inherit foo method bar : string end;;
> class type t = object method push : 'a . (#foo as 'a) -> unit end;;
>
> (* fails *)
> class type foo = object method foo : string end
> and bar = object inherit foo method bar : string end
> and t = object method push : 'a . (#foo as 'a) -> unit end;;
>
>
> So what am I doing wrong?
>
> MfG
> 	Goswin