Sequencing Either

gistfile1.scala

package ophan

object Or {
  type Or[T] = Either[String, T]

  def traverse[A,B](seq: Seq[Or[A]])(f: A => B): Or[Seq[B]] = {
    seq.foldLeft[Or[Seq[B]]](Right(Nil)) { (acc, or) =>
      or match {
        case Left(s) => Left[String, Seq[B]](s)
        case Right(a) => acc.right.map(_ :+ f(a))
      }
    }
  }

  def sequence[T](seq: Seq[Or[T]]): Or[Seq[T]] = traverse(seq)(identity)
}