A Fresh Look at Comparisons, Take 2

[Yigal Chripun]

Steven Schveighoffer wrote:
> Huh?  I think maybe you meant C to inherit from B?  If so I see what you are 
> trying to explain :)
>
> I don't really agree that opEquals or opCmp should not be inherited.  I 
> think normal inheritance the way it is now is just fine.
>   
as Janice showed and I completely agree with her, this is a bad default.
We are talking about the D default behavior here and it can be
overridden. The programmer can define an opCmp that is not explicit and
than all derived classes will inherit that opCmp as well, so your
desired behavior is still available, it's just not the default anymore.
> You need to have opEquals in Object in order to support hashing.
>   
I'm not sure I fully understand you on this. the suggestion states that
unless opEquals is defined, the compiler defaults to identity
comparison.  This is equivalent to having an opEquals in Object that
does the same thing (which is the current implementation, I think). I
don't care if it's implemented inside the compiler, or via opEquals in
Object, what's important is the semantics. i.e. unless there's a user
defined opCmp for a class, (a == b) will be true iff (a is b) is true [
for two instances a and b of that class].
> The default opCmp should be redefined to throw an exception.  An alternative 
> is that opCmp is defined in an interface.  Or you could do both.  The 
> rationale is that opCmp doesn't have a default.  Some objects just cannot be 
> expected to have a well-defined order.
>
> -Steve 
>
>
>   
since D is relatively a new language, we can benefit by learning from
previous design errors in similar languages. Java implements that idea
of a method in Object that throw by default and that proved to be a very
fragile design. Object should provide only the methods that all objects
contain, and since not all classes are fully ordered, opCmp should not
be in Object.
I agree with you that providing a "Comparable" interface is a good
design. If D was only OOP like java, than this would have been my
favorite solution. indeed, Java contains such an interface and you are
expected to implement it if your class is comparable. if i remember
correctly, there are two interfaces in Java which facilitate the subtle
difference between an opEquals and opCmp comparison.
as Janice said, this interface is not strictly necessary with her
proposal, however such an interface can help document the fact that a
class is comparable, so it still could be added.

-- Yigal