Re: Visibility of private packages??

[Dale Stanbrough <dstanbro@bigpond.net.au>]

Peter.Coventry@gdcanada.com wrote:
> 
> >I am trying to get my head around the visibility rules applicable to private
> >packages.
> >
> >If I have the following package hierarchy:
> >
> >           A
> >        /      \
> >     A.B1   A.B2
> >    /
> > A.B1.C1
> >
> >A.B1 contains the interface that I wish to export to clients (i.e a type
> >plus routines for client manipulation).
> >A.B1.C1 contains additional routines for the type not meant for client use.
> >A.B2 contains some inner workings, that need to use the additional routines
> >in A.B1.C1.
> >
> >I want to make A.B1.C1 a private package to stop it from being misused by
> >clients, however the compiler disallows it, citing RM 10.1.2(8):
> >
> >" If a with_clause of a given compilation_unit mentions a private child of
> >some library unit, then the given compilation_unit shall be either the
> >declaration of a private descendant of that library unit or the body or
> >subunit of a (public or private) descendant of that library unit."
> >
> >Does this mean that if A.B1.C1 is private, it can only be withed by the
> >bodies of A.B1 and other descendants of A.B1, and specs of private packages
> >of descendants of A.B1?

Yes, that's correct. I think this is a case where the designers got it
wrong. The current rules prevent "compilation coupling", that is any
change to a private package will never result in the need to recompile
a package outside of the hierachy.

A laudible aim, but too restrictive IMHO. It would have been better 
to simply restrict the use of types defined in a private spec to
either the private part of a package spec, or the package body.
This would allow you to, for example, complete a private type 
declaration with a type derived from a private package, or a 
structure with a private component.

E.g.

   private package a.b is
      type hidden is <something>
   end a.b;

   with a.b;
   package a.c is
      type not_hidden is private;
   private
      type not_hidden is new a.b.hidden;
   end;

The change could be made, and it would not be incompatable with
existing programs.

Dale