Re: Real OO (was Choice of OO primitives in Ada95)

[donh@syd.csa.com.au (Don Harrison)]

Robert A Duff writes:

:In article <DnKnsz.C8C@assip.csasyd.oz>,
:Don Harrison <donh@syd.csa.com.au> wrote:
:>The only 'cost' is the effort of writing them in the first place but the benefit
:>is enormous because that correctness is propagated through reuse. When assertions
:>are used, there is nothing to fear from inheritance and no reason why all
:>abstractions should not be candidates for reuse by inheritance.
:
:Well, assertions are a nice feature of Eiffel, but they don't
:*guarantee* correctness.  For example, there are some things that can't
:be formalized, like "this procedure prints an error message that is
:easily understood by any competent user".  That's why Eiffel allows some
:assertions to be merely comments, and of course if it's just a comment,
:you can't know for sure that it's obeyed, unless you look at every
:overriding.

Granted. Not everything can be formalised. Of course, it's clearly valuable to 
exploit those aspects of a design that may be formalised.

:>In contrast, assertion-less inheritance propagates errors through reuse.
:>
:>'Frozen' is used when the designer knows the implementation will not change.
:>
:>: Of course, the counter-argument is that you don't always know ahead of
:>: time when you might need dispatching.
:>
:>Exactly.
:>
:>: I'm not sure I agree with Ray Toal, though, that requiring "tagged" is a
:>: Good Thing.  Even without that rule, Ada still gives you control over
:>: whether things dispatch, on a call-by-call basis.  So, I view "tagged"
:>: as simply an efficiency hack.  Efficiency is a Good Thing, too, though.
:>
:>Efficiency hack is a good way of describing it. 
:
:Would you say "frozen" is (merely) an efficiency hack?  I view it as
:partly an efficiency hack, and partly an aid to understanding the
:program.

The two are different in a couple of ways:

a) purpose - 'tagged' concerns dispatching whereas 'frozen' concerns redefinition

b) syntactic necessity
   - 'tagged' is not necessary if the language allows dispatching by default. 
   It is necessary, if dispatching is optional, as in Ada for the purpose of 
   efficiency.
   [I happen to think dispatching by default would be better because the human
   benefits of flexibility and simplicity would outweigh the cost of bigger and 
   better hardware].
 
   - 'frozen' is necessary to indicate whether an operation may be redefined 
   (which is not related to efficiency).

:- Bob

Don.