Re: Parent/child dependencies and internal library units

[Simon Wright <simon@pushface.org>]

Victor Porton <porton@narod.ru> writes:

> I noticed that child units depend on their parents.
>
> As such making parent dependent on a child makes a dependency loop.
>
> Before noting this feature of Ada language, I thought parent units could be 
> implemented based on their child units, but that does not work, because 
> making parents dependent on children is a circular dependency.
>
> So, my question:
>
> What is a good way to create "internal" or "implementation" units for a 
> unit? Should I create a *.Internal.* hierarchy of packages?

I dare say there are uses which could make this not work (possibly
involving initialisation of objects in specs via function calls, leading
to access-before-elaboration) but you can make a parent body depend on a
child spec.

If the implementation is meant to be hidden from users outside the
parent, consider using private child packages.

   package Hierarchy is
      procedure Tell;
   end Hierarchy;

   private package Hierarchy.Internal is
      procedure Tell;
   end Hierarchy.Internal;

   with Ada.Text_IO;
   package body Hierarchy.Internal is
      procedure Tell is
      begin
         Ada.Text_IO.Put_Line ("Hierarchy.Internal");
      end Tell;
   end Hierarchy.Internal;

   with Ada.Text_IO;
   with Hierarchy.Internal;
   package body Hierarchy is
      procedure Tell is
      begin
         Ada.Text_IO.Put_Line ("Hierarchy");
         Internal.Tell;
      end Tell;
   end Hierarchy;

   procedure Hierarchy.Main is
   begin
      Tell;  -- this is Hierarchy.Tell
   end Hierarchy.Main;