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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Generic child units
Date: 16 May 2003 21:00:00 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Georg Bauhaus <sb463ba@d2-hrz.uni-duisburg.de> writes:

> Stephen Leake <Stephe.Leake@nasa.gov> wrote:
> : Rodrigo Garc?a <rodrigo.garcia.ARROBA.epfl.ch> writes:
> : 
> :> I need some enlightment in this area... Let us suppose that I have two
> :> generic packages declared in separate files and one is the parent of
> :> the other:
> :> 
> :> generic
> :>     type Elem is private;
> :> package Parent is
> :>     type Vector is array (Integer range <>) of Elem;
> :> end Parent;
> :> 
> :> generic
> :> package Parent.Child is
> :>     subtype Vector2D is Vector (1 .. 2);
> :> end Parent.Child;
> :> 
> :> How can I instantiate the child package 
> : 
> : with Parent.Child;
> : procedure Foo is
> :   package Par is new Parent (Elem => Integer);
> :   package Chi is new Par.Child;
> : begin
> : ...
> : end Foo;
> : 
> : Note that the second instantiation is "Par.Child", not "Parent.Child".
> 
> I find it useful to compare to this variation of the packages, at least
> it has shed some light (I think :-/) as far as the instantiation sequence
> is concerned:

Yes, the following example shows what's going on.  A child is almost the
same thing as a nested unit.

> generic
>    type Elem is private;
> package Parent is
>    type Vector is array (Integer range <>) of Elem;
>    
>    generic
>    package Child is
>       subtype Vector2D is Vector (1 .. 2);
>    end Child;
> 
> end Parent;
> 
> So, elsewhere, you can't have an instance of child without an instance
> of parent.

Right.  Outside Parent, you can't say "new Parent.Child", but you can
say "new Par.Child" if Par is an instance of Parent.

> But you can have an instance of Child in Parent's body. I don't
> know why, exactely, though.

Because when you're inside the generic, you can see Child.  If you refer
to Parent inside Parent, you are not referring to a generic, you are
referring to a package -- whatever the "current instance" of Parent is.

> (No wonder that the editors next door tell me that programmers
> are funny people :-)

Indeed.

- Bob