Contravariant functor instance for scala.math.Ordering

[durban]

There doesn't seem to be a Contravariant instance for scala.math.Ordering.

[LukaJCB]

There are some for cats.kernel.Order though. You can use fromOrdering to get an Order from Ordering :)

[durban]

Sure, but that doesn't really help me if I've got an Ordering[A], and I want to simply contramap to get an Ordering[B]. (Although for that to work, #1850 also need be fixed.)

[kailuowang]

@durban can you elaborate why #1850 is related?

[johnynek]

@durban you can call .on to go from Order[A] to Order[B] but I agree there should be an instance.

https://github.com/typelevel/cats/blob/master/kernel/src/main/scala/cats/kernel/Order.scala#L53

[non]

I'm opening a PR that addresses this lack (as well as PartialOrdering, Equiv, and Duration which I also noticed were missing).

[non]

#1878

[durban]

@kailuowang I was mistaken, it's not.