[Caml-list] ocaml, objects, classes, type ascription, virtual methods

[Dmitry Grebeniuk <gdsfh1@gmail.com>]

Hello.

  I have a base class with virtual methods and an extended classes
where these methods are implemented.
  When I'm ascribing type "base" to class "ext", I get an error:

=============
        OCaml version 3.13.0+dev3 (2011-03-07)

# class virtual base = object method virtual m : unit end;;
class virtual base : object method virtual m : unit end
# class ext : base = object method m = () end;;
Characters 6-43:
  class ext : base = object method m = () end;;
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This class should be virtual. The following methods are undefined : m
#
=============

  Is it the expected behaviour?
  It would be more useful to allow to constrain "class ext" by type "base"
while providing an implementation for virtual methods.
  For now, I'm omitting the type ascription, but then I have less guarantees
about extended classes' compatibility/interchangeability.