On Saturday, 13 May 2017 at 12:53:33 UTC, Nicholas Wilson wrote:
For most types equality exists (i.e. are all members equal?),
but ordering is nonsensical.
How does this relate to what I wrote?
If you want to support equality only, implement opCmp with return
type Equality. (In Herb's proposal uses the spaceship operator
<=> instead of opCmp and strong_equality for Equality.)
to answer your why questions:
1) see above, we're still doing better than C++ (although in
the contexts of DLSs individual operator overloading is useful.
It's not about C++, it's about D.
If Herb's proposal is accepted (after being corrected) then -
indeed - C++ will be doing better than D. ;-)
2) see 1. you need only define both if opEquals is a
requirement for performance. defining opCmp suffices for
(in)equality. and you don't define opCmp unless your type has
sensible ordering.
What I was saying is: defining opCmp implies a definition of
equivalence which now needs to be specified redundantly in
opEquals. The developer now is responsible to ensure consistency
between opCmp and opEquals.
3) you can do that already. w.r.t sort you pass a predicate
(defaulting to "less") for which ordering is assumed to exist,
if it doesn't then you get a partition according to that
predicate.
Another misunderstanding. Currently, there is no means to express
that 'less' models a partial order vs. a linear order.
