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Date: 13 Apr 92 22:13:07 GMT
From: terminator!caen!uvaarpa!software.org!smithd@gumby.wisc.edu  (Doug Smith)
Subject: Re: Ada 9X plans for Generic Formal Packages
Message-ID: <1992Apr13.221307.2127@software.org>
List-Id: <comp.lang.ada>

In article <1992Apr8.154108.2294@wdl.loral.com> mab@wdl39.wdl.loral.com (Mark A
 Biggar) writes:
> In article <1992Apr7.192922.24708@software.org> smithd@software.org (Doug Smi
th) writes:
> >A quick look at the March 92 Draft Ada 9X Project Report revealed
> >a new item:
> >    S.12.1.4 Generic Formal Packages [new]
> >However, the accompanying description and example leaves out a
> >lot of detail:
> 
[Several questions and helpful answers deleted]
> --
> Mark Biggar
> mab@wdl1.wdl.loral.com

Thank you (and I did find the Dec 91 Mapping Spec which helped).

And now to clarify the question I was hopping to get answered.  Can
a generic instantiate another generic that is part of a package
specified as a formal argument:

       generic
       package Template is
          generic
             with procedure To_Do;
          procedure Iterate; -- This generic will eventually be
                             -- instantiated indirectly
                             -- when Outer is Instantiated
       end Template;

       with Template;
       generic
          with package New_Template is
             new Template;
       package Outer is
          procedure To_Do; -- The essence of what this
                           -- package does.
          procedure Invoke is
             new New_Template.Iterate (To_Do); -- !!!!!!
       end Outer;

       -- The Instantiations:
       package Structure is new Template;
       package Algorithm is new Outer (Structure);

I think even if code like this example will be allowed in Ada9X,
it will not do what I envisioned.  Maybe someone can suggest
how Ada9X might implement a generic Sum utility that uses passive
iteration, without regard to where the iterator comes from:

       generic
          type Element is limited private;
          type List    is limited private;

          with generic
                  with procedure To_Do (E : in Element);
               procedure Iterate (L : in List);

          with procedure Add (Cur : in out Element;
                              Val : in     Element);

       procedure Sum (Result : in out Element;
                      L      : in     List);
          -- Result must be initialized before Sum is called.

       procedure Sum (Result : in out Element;
                      L      : in     List) is
          procedure Add_To_Result (E : in Element) is
          begin
             Add (Result, Val);
          end;

          procedure Composition is
             new Iterate (Add_To_Result);
       begin
          Composition (L);
       end Sum;

Let me know if there is a better place for this discussion.

        Doug