Re: Discriminant and type extensions

["Yannick Duchêne Hibou57" <yannick_duchene@yahoo.fr>]

Hi Bob,

On 5 oct, 22:42, Robert A Duff <bobd...@shell01.TheWorld.com> wrote:
> I'm not sure exactly what you mean, but you can do things like this:

An example will tell more (I've dropped any kind of genericity here) :



package P1 is

   type T_Type (Discriminant : Natural) is
      tagged
      private;

private

   type T_Type (Discriminant : Natural) is
      tagged
      null record;

end P1;



with P1;

package P2 is

   type Extension_Type is
      new P1.T_Type
      with private;

   -- Extension_Type as a discriminant, but it
   -- does not explicitly appears in this
   -- package specification.

private

   type Extension_Type is
      new P1.T_Type
      with null record;

end P2;



with P2;

procedure Usage is

   Entity :
      P2.Extension_Type
         (Discriminant => 0);

   -- Indeed, it really has a discriminant,
   -- although the package interface does
   -- not express it.

begin
   null;
end;



I was thinking in this context, it may be nice to be able to express
again the discriminant at the place Extension_Type is declared. To be
honest, it happens too I feel it would be nice to something which
would allow to recall about the underlying interface which is left
implicit. Something similar to what the keyword “ overriding ” does.
The keyword “ overriding ” recalls about what is the underlying stuff,
making things more explicit. I was thinking about something similar.

For procedures and functions, the Renames clause may do the thing
(explicitly re-introducing procedures and functions in the scope), but
for discriminants in this kind of context, I do not know a way.
Perhaps I do not know it, or perhaps this may be a candidate proposal.