Re: 'withing' problem

[Richard Riehle <richard@adaworks.com>]

Simon Wright wrote:

> If you are coming from a UML-based modelling environment,
> cross-dependency is entirely natural. It would be quite wrong to
> contort the model to cope with Ada's unfortunate limitations in this
> respect.

Funny.  I don't regard the current context clause (with) as a problem,
when one is designing in Ada.    It is most certainly not an "unfortunate
limitation."    Rather, the grumbling about it most often reflects a lack
of understanding, or acceptance, of the very strict visibility rules of
the language.   As to the potential for circular context clauses, these
need not exist when one correctly designs in Ada.

When I first learned Ada, what seems now many lifetimes ago, I was
annoyed by the recurring errors I would have due to visibility issues.
The compiler kept bothering me with nasty messages about this.  In
time, I came to appreciate that one of Ada's primary strengths is in
is separation scope and visibility.   Now I realize that those visibility
rules are essential in reasoning about good design.

More Ada programmers have problems with visibility rules than with
any other feature of the language.   Some will go for years, designing
and coding in Ada, and never come to terms with the visibility rules.
I am reminded of first learning to use a rotating brush floor buffer for
a part-time job in high-school.  The brushes rotate over the floor in
their own way.   When I struggled to overpower the rotating brushes,
I could not succeed.   Once I learned how to balance the brushes
lightly, and guide them across the floor according to their own
behavior, I could control the machine with one finger.   Struggling
with the context clause and visibility rules is much the same.  Learning
to use those rules, they make the Ada software design
process easy and fun.  Try to defeat them and the process is
hard and unpleasant.

Richard Riehle