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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: A proposal for formal packages matching
Date: Mon, 15 Dec 2008 04:13:47 -0800 (PST)
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Date: 2008-12-15T04:13:47-08:00
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On Dec 15, 11:21 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> There seems one overlooked thing in the rules controlling matching formal
> packages. An instance of generic child package does not match as an
> instance of the generic parent.
>
> I mean the following:
>
> generic
> package Generic_A is
> end Generic_A;
>
> package A is new Generic_A;
>
> generic
> package Generic_A.Generic_B is
> end Generic_A.Generic_B;
>
> package AB is new A.Generic_B;
>
> generic
>    with package A is new Generic_A (<>);
> package Generic_Foo is
> end Generic_Foo;
>
> package Foo is new Generic_Foo (AB);
>    -- Error: AB is not an instance of Generic_A
>
> Semantically, AB being an extension of Generic_A can be considered an
> instance of.

No, because semantically, packages are not types. They are units of
visibility. And they are orthogonal to type extension, dynamic
dispatching and object-oriented programming if that's what you have in
mind.

--
Ludovic Brenta.