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

Starting Scala 2.13, most collections are now provided with a partitionMap method which partitions elements based on a function which returns either Right or Left.

In our case, we don't even need a function that transforms our input into Right or Left to define the partitioning as we already have Rights and Lefts. Thus a simple use of identity:

val (lefts, rights) = List(Right(2), Left("a"), Left("b")).partitionMap(identity)
// lefts: List[String] = List(a, b)
// rights: List[Int] = List(2)

Answer 2

Well, in case it doesn't have to be a one-liner... then it can be a no-brainer.

def split[A,B](eithers : List[Either[A, B]]):(List[A],List[B]) = {
  val lefts = scala.collection.mutable.ListBuffer[A]()
  val rights = scala.collection.mutable.ListBuffer[B]()
  eithers.map {
    case Left(l) => lefts += l
    case Right(r) => rights += r
  }
  (lefts.toList, rights.toList)
}

But, to be honest, I'd prefer Marth's answer :)