Re: Interface/Implementation (was Re: Design by Contract)

[Samuel Mize <smize@link.com>]

Patrick Doyle wrote:

> In the cases where I simply said that what you were talking about
> is a design pattern, this means that special language features
> are not necessary to do these things.

Agreed.  In Ada, the design patterns you referred to are built up
from three very general facilities:
- the "package" (a programmer-defined grouping, not limited to the
  object-oriented concept of "class");
- the "generic" (loosely, a type-safe "template" facility);
- separation of package specification from implementation.

Eiffel has a different set of basic blocks, including (it appears
to me) more features directly supporting these patterns
(selective export, multiple inheritance from abstract classes).

The hidden agenda of my posting was to respond to some people
saying "we can do X in Eiffel, so it's better than Ada."  I
listed how to do all those X's, and more.  I certainly expected
Eiffel to support them, and I was interested to read how.  Thanks.


> Perhaps it's nicer to have
> the language help you out with these design patterns?  I don't
> know.

I think it's more a question of WHICH design patterns are directly
supported.  One can consider object-oriented programming a design
pattern.  Any language directly supports a set of design approaches,
and (if well designed) supports others with programmer discipline.

Ada pretty well supports building large, reliable software, to be
maintained over time, with a big team of developers, using
traditional methods (which now includes object-oriented methods,
but didn't in 1983).


> But I think there's a lot of value to Eiffel's simplicity
> and orthogonality which allows you to put together constructs
> in an intuitive manner which can achieve these things.

I would agree, and make exactly the same statement about Ada.

Note that programming can be divided orthogonally in various ways.
If you look at a basic capability supported by Eiffel, and see how
it's implemented using Ada, you may have to combine a number of
orthogonal Ada facilities.  The reverse is also true.

Ada does include more functionality in the language, and
represents a different design point.  However, the philosophy
of providing a set of orthogonal "building blocks" is the
same.  Indeed, maintaining this approach was one of Tucker
Taft's primary guidelines as he led the Ada 9X design.

Samuel Mize