Re: Generic child units

[Stephen Leake <Stephe.Leake@nasa.gov>]

Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> Rodrigo Garc�a <rodrigo.garcia.ARROBA.epfl.ch> writes:
> 
> > Stephen Leake wrote:

> > > 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".
> > 
> >    Thanks, that is one of the things I was doing wrong.
> > 
> > >>within the declarative region of the parent package (as required by
> > >>RM 10.1(18))?
> > > There is no RM 10.1 (18), at least in the copy I have (came with
> > > GNAT). And I don't understand the rest of the sentence, either. So I
> > > don't know what you mean here.
> > 
> >    Sorry, I meant RM 10.1.1 (18) as David C. Hoos has remarked. I still
> > do not understand that requirement.
> 
> Paragraph 19.b in the AARM explains it.  You are instantiating the child
> *outside* the parent generic, so you want to instantiate the child of
> the instance of the parent, not the child of the parent.  Make sense?
> 
> Here's the AARM text:
> 
>  18    A child of a parent generic package shall be instantiated or renamed
>  only within the declarative region of the parent generic.

But in my code above, Chi is an instantiation of "a child of a parent
generic package", and it is _not_ "within the declarative region of
the parent generic".

>  19 For each declaration or renaming of a generic unit as a child of
>  some parent generic package, there is a corresponding declaration
>  nested immediately within each instance of the parent. [This
>  declaration is visible only within the scope of a with_clause that
>  mentions the child generic unit.]
> 
>      19.a Implementation Note: Within the child, like anything
>      nested in a generic unit, one can make up-level references to
>      the current instance of its parent, and thereby gain access to
>      the formal parameters of the parent, to the types declared in
>      the parent, etc. This ``nesting'' model applies even within the
>      generic_formal_part of the child, as it does for a generic
>      child of a nongeneric unit.
> 
>      19.b Ramification: Suppose P is a generic library package, and
>      P.C is a generic child of P. P.C can be instantiated inside the
>      declarative region of P. Outside P, P.C can be mentioned only
>      in a with_clause. Conceptually, an instance I of P is a package
>      that has a nested generic unit called I.C. 

I see. So I actually instantiated Par.Child (as the code actually says
:), which is _not_ a "child of a generic parent", even though I think
of it that way. 

That makes sense. As is sometimes the case, the ARM is not immediately
clear to us non-lawyers.

>      Mentioning P.C in a with_clause allows I.C to be instantiated.
>      I need not be a library unit, and the instantiation of I.C need
>      not be a library unit. If I is a library unit, and an instance
>      of I.C is a child of I, then this instance has to be called
>      something other than C.
> 
> - Bob

-- 
-- Stephe