Re: Surprise in array concatenation

[<adaworks@sbcglobal.net>]

"Jeffrey Carter" <spam@spam.com> wrote in message
news:75KTe.5$1r.0@dfw-service2.ext.ray.com...
> adaworks@sbcglobal.net wrote:
> >
> > I have heard it expressed at some software-oriented conferences that
> > enumerated types represent an old-fashioned way of thinking about
> > software design.  Some in the OO community believe that, because
> > enumerated types are not extensible, they actually thwart good OO
> > design.
>
> How can an implementation technique thwart good design?
>
Enumerated types, as presently designed, are not extensible.  They
do no encapsulate their operations, and new operations, though
they can be added, do not bind in a way that conforms to conventional
OO design principles.

It is difficult, though not impossible, to create designs where the
enumerated type is not exposed in the public part of a package
specification.   For many OOP practitioners, enumerated types
are at the wrong level of abstraction for appropriate support
of object-oriented programming.

Too often, these non-extensible types are designed to be globally
visible, thus creating a dependency nightmare.   When they are
publicly visible, any change will ripple through the entire design,
and the compilation of every dependent library unit will be
involved.

This is not to say that enumerated types are evil.  I don't expect
anyone to write a letter to Ada Letters titled, "Enumerated Types
Considered Harmful."   However, they are easy to use incorrectly,
and in the hands of Ada novices, they often create as many
problems as they solve.

It would be very nice if the Ada language had a capability to include
abstract data types or extensible objects as members of the ordered
set.   This would be even more powerful if the enumerated type
were, itself, abstract and extensible.   I say nice, but realizing that
such a feature would make it nearly impossible to design this so
the underlying design goals of Ada would be satisfied.  I myself would
find the implementation of this design to be extraodinarily difficult.

Richard Riehle