Bifoldable

[ceedubs]

Bifoldable is a type class (currently not in cats) for a type that has two separate Foldables. For example, Or has a Foldable instance (via its Traverse instance) that folds over the RightOr case, but Or could equally fold over the LeftOr case.

One of my favorite methods from scalaz is MonadPlus.separate. This can do things like take a List[A Or B] and turn it into a (List[A], List[B]). Or similarly, take a List[(A, B)] and turn it into a (List[A], List[B]). In Cats, I believe we could implement the same sort of method on MonadFilter if we were to bring in a Bifoldable type class.

What do people think? Is this something that would belong in core?

[tpolecat]

Yep, I would like to see Bifoldable.

[mpilquist]

It's worth mentioning that Foldable and Bifoldable are lawless. We admitted Foldable anyway due to its usefulness for adhoc polymorphism. Bifoldable occurs much less frequently in practice, so we might want to carefully consider its inclusion.

[adelbertc]

Foldable kind of has laws: https://github.com/scalaz/scalaz/blob/series/7.2.x/scalacheck-binding/src/main/scala/scalaz/scalacheck/ScalazProperties.scala#L369

[mpilquist]

Those only require that foldMap, foldLeft, and foldRight are mutually consistent. They only exist in Scalaz because Foldable can be implemented in terms of foldMap or foldRight. If you pick one as the abstract method and just derive the other, the law simplifies to a unit test of the derived method.

[non]

Is there consensus here?

So far it seems like @ceedubs and @tpolecat would like to see this and @mpilquist is skeptical?

[coltfred]

I personally use Bifoldable from Scalaz on a regular basis and would like to have it.

Plus it gives rise to wildly useful things like separate on MonadPlus.

👍

[adelbertc]

Re: Laws some quick Googling turned up

https://github.com/scalaprops/scalaprops/blob/master/scalazlaws/src/main/scala/scalaprops/scalazlaws/bifoldable.scala /cc @xuwei-k

and

https://github.com/ekmett/bifunctors/blob/master/tests/BifunctorSpec.hs#L169

[mpilquist]

Re-reading this just now, I'm not skeptical of adding Bifoldable. 👍