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

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2011/05/23, at 16:50, Dmitry Grebeniuk wrote:

> 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 is indeed the expected behaviour.
Like a module signature, a class signature provides complete information.
If you declare a method as virtual in the signature, it will be made virtual by
the ascription (this is allowed, since virtual is a "supertype" of concrete).

In such a case you should rather use a class type:

  class type base = object method m : unit end

As suggested by others, if you don't want to suggest such a class
type you can also use the derived object type:

  class ext = object (_ : #base) method m = () end

These different approaches are described in the manual.

Jacques Garrigue