Re: Need advice re package organization.

[Richard Riehle <richard@adaworks.com>]

"Robert I. Eachus" wrote:

> chris wrote:
> > What is a private child package?
>
> private package Parent.Private_Child is... end Parent.Private_Child;
>
> See RM 10.1.1(12), and 10.1.2(8). Probably one of the best kept secrets
> in Ada 95.  A private child package can have a with clause for another
> private child of the same parent (or ancestor package), but the real use
> is that the body of the parent package or any sibling can with the
> private package.

One of the most ideas related to private children is the potential for
aspect-oriented programming (AOP).   The idea of cross-cutting
common facilities across a larger design while preserving encapsulation
is nicely accomplished with private chilren.

> So if you have common utility routines that you don't want to make part
> of the parent's package spec, put them in a private child.  For example,
> the parent package might export a private type, and the private child
> could include the inquiry functions and other operations that needed to
> see into the private part of the parent.

This allows us to promote any feature of a package body to a private
package specification.  What this means is that the package body
should only contain implementations in the specification.  Any other
functions or procedures should be declared in a private package
specification.  Once this is done,  those specifications are themselves
extensible.   Also, every set of subprograms is declared at the
specification level.  Voila, we have a model for AOP.

Of course, I do oversimplify in this description, but anyone who
explores this in depth will soon discover the validity of this
notion.

Richard Riehle