either_sequence.scala

def sequence[A, B](s: Seq[Either[A, B]]): Either[A, Seq[B]] =
  s.foldRight(Right(Nil): Either[A, List[B]]) {
    (e, acc) => for (xs <- acc.right; x <- e.right) yield x :: xs
  }

def sequence[A, B](s: Seq[Either[A, B]]): Either[A, Seq[B]] =
  s.foldLeft(Right(Seq.empty[B]): Either[A, Seq[B]]) { (acc, e) =>
    for (xs <- acc; x <- e) yield xs :+ x
  }