Re: Equality operator overloading in ADA 83

[Mats Weber <Mats.Weber@elca-matrix.ch>]

Robert Dewar wrote:
> 
> Mats Weber said
> 
> <<> Composability should always be guaranteed by user-defined types, too.
> 
> There seems to be much disagreement on this question for all kinds of
> reasons. But I agree with you 100%.>>
> 
> It is helpful if you make clear whether you are making a pronouncement about
> the original Ada 83 design or the decision not to introduce upwards
> incompatibilities in the Ada 95 redesign (I trust you are NOT making a
> statement about Ada 95 implementations :-)

My pronouncement is about Ada 95; Ada 83 was "clean" with respect to
equality because of the strict rules on its redefinition (if we don't
count the unexpected possibility of redefining it for non-limited types)
and the fact that it does not automatically compose when some component
is of a limited type.

My view is that if you allow redefinition of "=", then you must make
sure it behaves (composes, does not reemerge) correctly, which was not
done in Ada 95 except for tagged types.

I also think that the upward incompatibilities that the non-reemergence
of predefined operations would have introduced are minimal compared to
how a programmer is betrayed when he writes

   type T is private;

   function "=" (L, R : T) return Boolean;

and the predefined "=" reemerges in some places with an unknown effect
since the type is private.