Re: Equality operator overloading in ADA 83

[mheaney@ni.net (Matthew Heaney)]

In article <dewar.862275693@merv>, dewar@merv.cs.nyu.edu (Robert Dewar) wrote:

>What frequently happens is that in the body, where a private type is
>visible, you don't really want the redefined equality at all, but you
>get it some of the time, and you have to convert etc to get rid of it,
>which of course is necessary in both Ada 83 and Ada 95 code. But if
>you extend the reach of this equality operator, then it will extend
>into these bodies -- and that easily cause incomaptibilities of the
>worst possible kind (i.e. Ada 83 code would compile fine, but give
>subtly different results). 
>

I don't accept this argument about "incompatibilities" in Ada 83 code,
because the BEHAVIOR of equality never changes, only the implementation. 
If a programmer overrides equality to have different behavior from
predefined equality, then of course that's an error, no different than
overriding addition to mean subtraction.

>I must say that I here a lot of sturm und drang with respect to this issue
>(equality not composing), but I must say I have never found this a 
>limitation in real code. Array and record equality is rare in any case,
>and is even rarer in cases where component equality has been redefined.

Perhaps our experience is different, as I see record comparisons ALL the
time.  One of the reasons one uses a type like Bounded_String is because he
needs a constrained string for a record component:

package String_80 is
   new Ada.String.Bounded.Generic_Bounded_Length (80);

type R is
   record
      S : String_80;
      ...;
   end record;

A programmer isn't thinking as he compares objects of type R, "Gee, I can't
assume equality composes."  He just compares them, and just assumes
equality always works as expected.

Direct array comparisons are admittedly less popular, but array comparisons
DO manifest themselves when an array is used to implement a bounded data
structure.  I see bounded buffers all the time, to implement I/O across an
external interface, for example.

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