Why does `Either.bifoldMap` take a `Monoid` argument, when the monoid basically does no work, since ...

# arrow

[julian]

Why does Either.bifoldMap take a Monoid argument, when the monoid basically does no work, since bifoldMap always combines the mapped values of the Either with empty, where the result of that combination is know ahead of time to always be the same as the mapped values. In other words, why jump through the hoop of combining a value with empty, when we know the result will always be the same original value?

[julian]

The only reason I can think of is so that bifoldMap can build on top of bifoldLeft. Is that it?

[Scott Christopher]

I believe bifoldMap was previously part of a typeclass that has since been removed from Arrow, which needed the monoid instance to be general for other "bifoldable" types like tuples. Now that it is specialised to Either, I can't see any need for it.

[julian]

Ah, that makes sense. So maybe there are future plans to remove that dependency on Monad and simplify `bifoldMap`s signature. Thanks @Scott Christopher.

[raulraja]

Hi @julian @Scott Christopher, bifoldMap uses the Monoid.combine in addition to empty in bifoldmap. it’s kind of hidden because it’s achieved through run, but when below we use combine , that is syntax that is projected by the monoid receiver. I order to replace this monoid here we would have to provide both an empty operation and associate combine operation. https://github.com/arrow-kt/arrow/blob/b4714662adcd321cc8a03d01ff86da8fb187f406/ar[…]libs/core/arrow-core/src/commonMain/kotlin/arrow/core/Either.kt

I think it’s correct as it’s stated now

[julian]

Thanks @raulraja. But why use combine at all? Since we know ahead of time that for any type C, MN.empty().combine(f(a)) == f(a).

[raulraja]

yes makes sense, it should be doable just with the empty identity, it was probably like that because of the type classes as @Scott Christopher noted

[raulraja]

We should probably review all methods like that to see if there is others. I guess the combine here would only matter if the monoid is unlawful and empty is truly not an empty identity, otherwise as you are saying is essentially a no op in both branches

[julian]

Cool, thanks @raulraja!

[raulraja]

Thank you for finding this!

[julian]

😄 Sure thing!