gistfile1.scala

  def parseProgram(s: String): Either[String, Program] = {
    // use Stream so we don't keep parsing past the first error
    val allLines: Stream[Either[String, Line]] =
      s.split("""\n""")
       .filter(_.nonEmpty)
       .toStream
       .map(parseLine)
    sequence(allLines).right.map{goodLines =>
      Program(goodLines.sortBy(_.number).toIndexedSeq)}
  }
  // returns the first Left, or all of the Rights
  def sequence[A, B](xs: Stream[Either[A, B]]): Either[A, Stream[B]] =
    xs.collectFirst{case Left(err) => err} match {
      case Some(err) =>
        Left(err)
      case None =>
        Right(xs.collect{case Right(x) => x})
    }