Hey fellas, I'm somewhat confused about the behaviour of Either. How do I retrieve the value out of ...

# arrow

[mat]

Hey fellas, I'm somewhat confused about the behaviour of Either. How do I retrieve the value out of a Left?

Basically, my code is this:

val results = someList.map(this::createsEitherResultOrErrorDescription)
if (results.any {it.isRight() }) return Right() // GET ALL RIGHTS HERE

return Left(consumesListOfResults(results.map {}) // GET ALL LEFTS HERE

I have found this super ugly hack here for retrieving the lefts:

val results = results.map { it.merge() as MyResult }

But it's not very nice.

For the Right case there is Either.getOrNull(), but I don't see a value for LEfts anywhere.

In the ideal case there would be these two extension functions:

List<Either<A,B>>.getAllLefts(): List<A>
List<Either<A,B>>.getAllRights(): List<B>

[mitch]

@mat you can leverage typesafe builders e.g

fun List<Either<L, R>>.getLefts() = 
  buildList {
    [e-mail] { either ->
      either.onLeft { add(it) }
    }
  }

[CLOVIS]

val lefts = someList.filterIsInstance<Either.Left<A>>().map { it.value }

val rights = someList.filterIsInstance<Either.Right<B>>().map { it.value }

[mat]

Ahh those two seem like usable options. Thanks

[simon.vergauwen]

val (lefts, rights) = someList.separateEither()

https://apidocs.arrow-kt.io/arrow-core/arrow.core/separate-either.html?query=fun%20%3C[…]3E.separateEither():%20Pair%3CList%3CA%3E,%20List%3CB%3E%3E

[mat]

Wow, that is perfect. Is there some way to mark a thread as answered or done in Slack?

[CLOVIS]

Not particularly. Some people use ✅ for that, but most people don't do anything 👍