Declaring private packages to instantiate generics?

[Markus Schöpflin]

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

[Adam Beneschan]

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

[Markus Schöpflin]

Am 16.05.2012 16:36, schrieb Adam Beneschan:

[...]

>> ---%<---
>> 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;

Thanks for your answer. Not as nice as I had hoped, but basically what I was
looking for.

Markus

[Shark8]

Why is using RENAMES not nice?
It seems that the only difference between using RENAMES and what you showed (procedure P is new BAR.P;) is the syntax.

[Adam Beneschan]

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

[Markus Schöpflin]

Am 17.05.2012 05:37, schrieb Adam Beneschan:
> On Wednesday, May 16, 2012 7:36:27 AM UTC-7, Adam Beneschan wrote:

[...]

> 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.

You're right, thanks for the follow-up. And it's even the case in my case, I
have a whole bunch of Ps which are implemented as instantiations of the same
procedure.

Markus

PS: Sorry for rewrapping your part of the mail but my NNTP server started to
complain about lines longer than 79 characters when posting.

[Simon Wright]

Markus Schöpflin <no....@spam.spam> writes:

> PS: Sorry for rewrapping your part of the mail but my NNTP server
> started to complain about lines longer than 79 characters when
> posting.

I've done this for a long time as a matter of course. It's particularly
necessary now that Google Groups has stopped doing any line-wrap at all!
(and very easy using Emacs gnus - M-q).

I didn't think it was rude?