<https://imgflip.com/i/6v3rs0> Evolving the Either API for Arrow 2.0, we want to reduce the API sur...

# arrow

[simon.vergauwen]

https://imgflip.com/i/6v3rs0

Evolving the Either API for Arrow 2.0, we want to reduce the API surface so that we can offer uniform APIs and simply the learning curve for everyone.

https://github.com/arrow-kt/arrow/pull/2830

[raulraja]

Looked over the files, all the deprecations and changes look good to me.
If foldMap is going away presumably because working with Monoid or type classes is a pain, should we also get rid of Monoid etc? or are those still useful for other things?

[simon.vergauwen]

TBH I don't think Monoid and Semigroup still offer a lot of value. Semigroup is a glorified (A, A) -> A function, and similar for Monoid.

Working with Iterable and data types is simplified a lot with the new APIs.

traverse is map + bind, and accumulating is mapAccumulating { either() }.

That would be outside of the scope of this PR, but absolutely something we can consider for 2.0 (and deprecation in 1.x.x)

[thanh]

Agree, with our current direction, Monoid and Semigroup don't add much value anymore. Removing them also reducing confusion for new comers.

[Satyam Agarwal]

I thought Semigroup gives a collection of left hand side when used with zip.
Thus deprecating Validate in favor of Either<List<E>, A> would be okay.
But is my understanding wrong ?

[raulraja]

in that case the Semigroup is just there in case you want to accumulate into a custom type that is not a NonEmptyList , but since by default error accumulation happens in a non-empty list and how to combine non-empty list (their semigroup) is known, we don't need that extra semigroup param or expose it to users.

[raulraja]

But is my understanding wrong ?

I think it's right, we are getting rid of Validated and error accumulation will happen naturally without having to provide a semigroup.

[simon.vergauwen]

The base signature will look like this (up to arity-9)

Either<E, A>.zip(other: Either<E, B>, combine: (E, E) -> E, transform: (A, B) -> C): Either<E, C>
Either<E, A>.zip(other: Either<E, B>, transform: (A, B) -> C): Either<Nel<E>, C>

Iterable<A>.mapAccumulating(combine: (E, E) -> E, transform: (A) -> Either<E, B>): Either<E, List<B>>
Iterable<A>.mapAccumulating(transform: (A) -> Either<E, B>): Either<Nel<E>, List<B>>

The new either DSL will be inline so no more need for eager or invoke and you can just add suspend wherever needed ad-hoc. Using the DSL will have the same cost as a single map, so it can be used everywhere without any performance hits.

[Satyam Agarwal]

nice