Add EitherOps#leftMapOrKeep and EitherOps#leftFlatMapOrKeep

[danicheg]

These are companions to Functor#mapOrKeep and cats.Monad#flatMapOrKeep, but for the left-hand side of Either[A, B].

[satorg]

Thank you!

[satorg]

I wonder if on a byte-code level this could be more performant:

case r : Right[B] => r

because there's no extractor to be called nor intermediate Option value to be created in opposite to the former case.

[danicheg]

Interestingly, I never thought that x @ Foo(_) might have any significant distinction from x: Foo.

[satorg]

It totally makes sense, thank you!

I've realized - perhaps it makes sense to add leftMapOrKeep to cats.Bifunctor. That way we could get it for many other bi-parameter types, not just Either.

Things seem more complicated for leftFlatMapOrKeep though. Looks like there's no appropriate type class where leftFlatMapOrKeep could be added.

In fact, there's Bimonad in Cats, but it has a completely different meaning there, which I find quite confusing.

[danicheg]

I've realized - perhaps it makes sense to add leftMapOrKeep to cats.Bifunctor. That way we could get it for many other bi-parameter types, not just Either.

Agreed. Are you arguing we should do it as a drop-in replacement instead of adding EitherOps#leftMapOrKeep? Apart from the existence of the Bifunctor[Either] instance, there are also EitherOps#leftMap and its companion EitherOps#leftFlatMap.

[satorg]

I think it wouldn't hurt if there were leftMapOrKeep in both Either syntax and Bifunctor, similar to leftMap.
Moreover, calling to leftMap through Bifunctor on Either incurs quite a bit of overhead currently. It could be alleviated a little if we had a specialized version of leftMap in the Bifunctor[Either] implementation. But for now it is not there and it is a completely different story.

Also I feel this PR would be better off if it stays focused on the Either syntax only. Bifunctor could be addressed in a separate work.

[danicheg]

@satorg wouldn't you mind submitting yet another review?

[satorg]

Thank you!

[danicheg]

@satorg thanks!