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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Access to generic formal parameters in an generic package
 instantiation
Date: Sun, 29 Jul 2012 21:36:28 +0200
Organization: cbb software GmbH
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Date: 2012-07-29T21:36:28+02:00
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On Sun, 29 Jul 2012 20:16:00 +0200, Florian Weimer wrote:

> If G is a generic package and P is one of its formal parameters, it is
> legal to refer to G.P where G is visible?  Does this depend on the
> kind of entity, or whether the formal part uses <>?

Sometimes it works, sometimes it does not, I am not a language lawyer to
tell when GNAT is right or not. Facing this problem quite frequently, I
developed a custom to rename formal parameters within the declaration area,
e.g.

generic
   type Foo is ...;
   with package Bar is new Baz (<>);
package Boo is
   subtype My_Foo is Foo;
   package My_Bar is renames Bar;
   ...

Another method of making formal parameters visible is to have it a generic
child.

> that is, specify that two types in two formal packages are the same,
> without break down the formal packages to their components?

Yes. There several techniques to accomplish this.

1. You can to make that type a formal parameter and specify it among the
formal parameters of the instances:

   type T is ...
   with package P1 (T, <>);
   with package P2 (T, <>);

2. You can take the type from one package and constrain another with it:

   with package P1 (<>);
   with package P2 (P1.T, <>);

3. You can factor out a generic parameter package that defines T and other
common types and make it generic parent for the packages of interest.

> It seems that generic formal packages are unusual because GNAT shows
> some strange effects (and the occasional bug box).

There were always problems with generics in GNAT.

Though you likely won't believe me anyway, but using generics is a very bad
idea unless absolutely nothing else works. Here is an outlook what it
becomes in a relatively simple case:

   http://www.dmitry-kazakov.de/ada/fuzzy_packages.gif

Can you figure out a chain of instantiation of this for some given
floating-point type? How many instances it does? In a really complex case
there are multiple pages of such diagrams.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de