A fresh look at comparisons

[Janice Caron]

On 16/04/2008, Yigal Chripun <yigal100 at gmail.com> wrote:
> Someone on this thread made a very good observation: opCmp should be a
>  multi-method.
>  This is an excellent idea and applies to more than just opCmp.
>
>  given the following classes:
>
> class A {}
>  class B : A {}
>
> class C : A {}
>
>  I'd suggest the following syntax for multi-methods: (taken from the
>  article linked bellow)
>  void func(virtual  A a1, virtual  A a2);
>  so both a1 and a2 can also be of any subtype of A.
>
>  let's look at some of the comparison variations:
>  opCmp(A a1, A a2); // compare only instances of A
>  opCmp(virtual A a1, virtual A a2); // will work on all subtypes
>  opCmp(virtual B b1, virtual B b2); // this overrides the above more
>  general function
>  etc...
>  I think this can provide all possible combinations needed.

But it still won't work. Watch.

    class A {}
    class B : A {}
    class C : A {}

    opCmp(A a1, A a2) { ... } /* not virtual */

    B b = new B
    C c = new C

    if (b < c) ...

This will /still/ cast both b and c to A's, and then do A's comparison
test. That's because B and C will both implicitly cast to A.

The only difference "virtual" will make is in the following situation

    class A {}
    class B : A {}
    class C : A {}

    opCmp(virtual A a1, virtual A a2) { ... }
    opCmp(B a1, C a2) { ... }

    A b = new B
    A c = new C

    if (b < c)...

now opCmp(B,C) would be called, because it exists. But if opCmp(B,C)
did /not/ exist, then opCmp(A,A) would be called, whether it was
virtual or not. That's because "virtual" only enables polymorphism; it
doesn't disable inheritance.

The desired goal is for opCmp(A,A) /not/ to be called if opCmp(B,C)
doesn't exist. So far as I can see, mine is the only suggestion on
this thead which achieves that goal.