Re: Declaring private packages to instantiate generics?

[Adam Beneschan <adam@irvine.com>]

On Wednesday, May 16, 2012 7:36:27 AM UTC-7, Adam Beneschan wrote:

> > 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.)

I just got reminded by Tucker Taft that there's a problem if P has parameters.  The parameters aren't listed in a generic_instantiation:

  package FOO is
     procedure P (Param : in Integer; Param2 : out Integer);
  end FOO;
 
  package body FOO is
     package BAR is new SOME_GENERIC_PACKAGE;
     procedure P is new BAR.P;
  end FOO;

This makes it less clear to the eye that the second P completes the first one.  (This would especially be a problem if there is more than one P with different parameter profiles in the specification.)  This makes it clearer:

  package FOO is
     procedure P (Param : in Integer; Param2 : out Integer);
  end FOO;
 
  package body FOO is
     package BAR is new SOME_GENERIC_PACKAGE;
     procedure New_P is new BAR.P;
     procedure P (Param : in Integer; Param2 : out Integer)
        renames New_P;
  end FOO;

So there's a good reason why the rules are the way they are.

                      -- Adam