A Fresh Look at Comparisons, Take 2

[Janice Caron]

I'm not clear on whether one needs partial ordering built into a
programming language. Certainly partially ordered sets exist in
mathematics. For example, consider the set of four points P = { a, b,
c, d }, defined such that

    a < b < d
    a < c < d

but where b cannot be compared with c (a diamond-shaped arrangement).
If D fully supported partially ordered types, then we would have

    (b < c) == false
    (b > c) == false
    (b <> c) == false
    (b == c) == false

and elements of type P could not be AA keys. The question is, does D
need to support this at the level of built-in comparisons? After all,
one could easily bolt that sort of thing on afterwards as a bunch of
plain functions, e.g.

    class P
    {
        bool partialLess(P p) {...}
        bool partialGreater(P p) {...}
    }

for those classes that need it. Library solutions such as
std.algorithm may be able to accept partially ordered sets where
appropriate. But I think that it's such a rare edge-case that having
it built into the language itself is probably unnecessary. After all,
we don't have that now, and I've never seen a complaint about its
absence.