try_sequence.scala

 import scala.util.{Try, Success, Failure}

  def sequence[T](s: Seq[Try[T]]): Try[Seq[T]] =
    (Try(Seq.empty[T]) /: s) {
      case (Success(s), Success(v)) => Success(v +: s)
      case (Failure(cause), _) => Failure(cause)
      case (_, Failure(cause)) => Failure(cause)
    } map (_.reverse)


  def sequenceOptimized[T](s: Seq[Try[T]]): Try[Seq[T]] = {
    def recSequence(s: Seq[Try[T]], acc: Seq[T]): Try[Seq[T]] =
      s.headOption map {
        case Failure(cause) => Failure(cause)
        case Success(v) => recSequence(s.tail, v +: acc)
      } getOrElse Success(acc reverse)
    recSequence(s, Seq())
  }