Best way to turn a Lists of Eithers into an Either of Lists?

Starting in Scala 2.13, most collections are now provided with a partitionMap method which partitions elements based on a function returning 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 since we already have Rights and Lefts. Thus a simple use of identity!

Then it's just a matter of matching the resulting partitioned tuple of lefts and rights based on whether or not there are any lefts:

eithers.partitionMap(identity) match {
  case (Nil, rights) => Right(rights)
  case (lefts, _)    => Left(lefts)
}
// * List[Either[String, Int]] = List(Right(3), Left("error x"), Right(7))
//         => Either[List[String],List[Int]] = Left(List(error x))
// * List[Either[String, Int]] = List(Right(3), Right(7))
//         => Either[List[String],List[Int]] = Right(List(3, 7))

For the understanding of partitionMap here is the result of the intermediate step:

List(Right(3), Left("error x"), Right(7)).partitionMap(identity)
// (List[String], List[Int]) = (List(error x), List(3, 7))

data.partition(_.isLeft) match {                            
  case (Nil,  ints) => Right(for(Right(i) <- ints) yield i)        
  case (strings, _) => Left(for(Left(s) <- strings) yield s)
}

For one pass:

data.partition(_.isLeft) match {                            
  case (Nil,  ints) => Right(for(Right(i) <- ints.view) yield i)        
  case (strings, _) => Left(for(Left(s) <- strings.view) yield s)
}