Re: Declaring private packages to instantiate generics?

[Adam Beneschan <adam@irvine.com>]

On Wednesday, May 16, 2012 7:21:24 AM UTC-7, Markus Schöpflin wrote:
> I just found myself wanting to write the following code, which Gnat doesn't like:
> 
> ---%<---
> package FOO is
>    procedure P;
> end FOO;
> 
> package body FOO is
>    package BAR is new SOME_GENERIC_PACKAGE;
>    procedure P is new BAR.P;
> end FOO;
> --->%---
> 
> Can I somehow provide the body of P without writing a procedure which just 
> forwards to an instantiation of BAR.P, IOW without having to write:
> 
> ---%<---
> package body FOO is
>    package BAR is new SOME_GENERIC_PACKAGE;
>    procedure BAR_P is new BAR.P;
>    procedure P is begin BAR_P; end;
> end FOO;
> --->%---
> 
> Regards,
> Markus

You can change the last to

   procedure P renames BAR_P;

But that's the best you can do.  If you have an incomplete declaration (i.e. the declaration of P in the *spec* of FOO), you can't complete it with a generic instantiation---that's the rule.  (I think he reason is that a generic instantiation is equivalent to a declaration of the instance followed by the body of the instance, and doing that here would mean you're declaring P twice, which isn't allowed.  It can be argued that the language rules should be relaxed in this case, since it's tripped up more than one person, and offhand I don't see any complications that would arise if it were allowed.)

                          -- Adam