How to reduce Seq[Either[A,B]] to Either[A,Seq[B]]?

Question

Given a sequence of eithers Seq[Either[String,A]] with Left being an error message. I want to obtain an Either[String,Seq[A]] where I

Answer 1

My answer is similar to @Garrett Rowe's: But it uses foldLeft (Also see: Why foldRight and reduceRight are NOT tail recursive?) and prepends to Seq rather than appending to Seq (See: Why is appending to a list bad?).

scala> :paste
// Entering paste mode (ctrl-D to finish)

def partitionEitherSeq[A,B](eitherSeq: Seq[Either[A,B]]): (Seq[A], Seq[B]) =
  eitherSeq.foldLeft(Seq.empty[A], Seq.empty[B]) { (acc, next) =>
  val (lefts, rights) = acc
  next.fold(error => (lefts :+ error, rights), result => (lefts, rights :+ result))
}

// Exiting paste mode, now interpreting.

partitionEitherSeq: [A, B](eitherSeq: Seq[Either[A,B]])(Seq[A], Seq[B])

scala> partitionEitherSeq(Seq(Right("Result1"), Left("Error1"), Right("Result2"), Right("Result3"), Left("Error2")))
res0: (Seq[java.lang.String], Seq[java.lang.String]) = (List(Error1, Error2),List(Result1, Result2, Result3))