How to split a List[Either[A, B]]

Question

I want to split a List[Either[A, B]] in two lists.

Is there a better way ?

def lefts[A, B](eithers : List[Either[A, B]]) : List[A] = eit         


        

Answer 1

If you're going to bother to abstract the functionality, as in Marth's answer, then it may actually make more sense to use roterl's solution:

def splitEitherList[A,B](el: List[Either[A,B]]): (List[A], List[B]) =
  (el :\ (List[A](), List[B]()))((e, p) =>
    e.fold(l => (l :: p._1, p._2), r => (p._1, r :: p._2)))

val x = List(Left(1), Right(3), Left(2), Left(4), Right(8))
splitEitherList(x) // (List(1, 2, 4), List(3, 8))

This way awards more functional brownie points, but also may be more performant, as it makes use of a right fold to create the lists in one pass

But if you're doing it on the fly and/or find folds difficult to read, then by all means

el.partition(_.isLeft) match { case (lefts, rights) =>
  (lefts.map(_.left.get), rights.map(_.right.get)) }