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From: Mats Weber <Mats.Weber@elca-matrix.ch>
Subject: Re: Equality operator overloading in ADA 83
Date: 1997/04/29
Message-ID: <3365BC3E.54C5@elca-matrix.ch>#1/1
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Organization: ELCA Matrix SA
Reply-To: Mats.Weber@elca-matrix.ch
Newsgroups: comp.lang.ada
Date: 1997-04-29T00:00:00+00:00
List-Id: <comp.lang.ada>


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.