[Vala] Returning subclass objects from superclass, or type synonyms

[Maurizio Ferreira <ferreiramaurizio gmail com>]

I need to define a function in a superclass that should return  the same
type of object on which it is called

example : (constructors, functions and properties removed, for clarity)


class Tuple {
>   public Tuple add (Tuple another) {
>     // ...
>   }
> }
> class Point : Tuple {
>    // function 'add' inherited
> }


The class Point is 99% equal to Tuple, with only a couple of added methods

now the test:

Tuple t = new Tuple (...);
> Tuple t_other = new Tuple(...);
> Tuple t_result = t.add(t_other);  // ok, no problem
>
> Point p = new Point (...);
> Point p_other = new Point (...);
> Point p_result = p.add(p_other);    // compiler error



The compiler complains that the function return type is not assignable to a
variable of type Point (since it is a Tuple)
surely I can write

Tuple p_result = p.add(p_other);    // ok


But this make the program less readable.

I know that this behaviour is typical of several oo languages,
but in Go (and Pascal and Delphi ...) it is possible to define type
synonyms, like

type Point = Tuple;


So that Point and Tuple are actually identical types.

Is it possible to have them also in Vala, or is it there a way to bypass
the problem ?

Best regards.
Maurizio.