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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Question about generic child packages
Date: Thu, 13 Oct 2016 19:19:54 +0200
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Date: 2016-10-13T19:19:54+02:00
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On 2016-10-13 19:01, Charly wrote:

> I have the following question:
> Is it possible to have a generic parent package Parent with two (or more)
> child packages and than use objects defined in one of the children and use
> it in the other one?
> Following is a very simple example to show my problem.
> In Child_A I defined a function Double and want to use it in Child_B.
> It would also be nice to use objects in Child_A, that are defined in B,
> not shown in following example.
> Of course I can make Child_B a subchild of Child_A, but that is
> unsatisfying because it is asymmetric and does not solve the second
> part with mutual usage.
>
> --------------------
> generic
>     type Data is digits <>;
> package Parent is
>
> end Parent;
>
> --------------------
> generic
> package Parent.Child_A is
>
>    function Double
>      (X : in Data)
>       return Data
>    is
>      (2.0 * X);
>
> end Parent.Child_A;
>
> --------------------
> with Parent.Child_A;
>
> generic
> package Parent.Child_B is
>
>    function Test
>      (X : in Data)
>       return Data
>    is
>       (2.0 * Parent.Child_A.Double (X));
>
>
> end Parent.Child_B;

It does not work this way because generic children are themselves 
generic. There is no cascade instantiation of generics in Ada. Therefore 
you must pass an instance of Parent.Child_A to Parent.Child_B.

with Parent.Child_A;
generic
    with package A is new Parent.Child_A;
package Parent.Child_B is
    function Test (X : in Data) return Data is (2.0 * A.Double (X));
end Parent.Child_B;

The only way of having a cascade is nested packages. Nested generic 
packages are proper packages.

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