gistfile1.scala

  def unfold[A](a: A)(f: A => Option[A]): Stream[A] =
    f(a).map(a => a #:: unfold(a)(f)).getOrElse(Stream.empty)

  def hailstone(i: Int) = i #:: unfold(i) { n =>
    if (n == 1) None else Some(if (n % 2 == 0) n / 2 else n * 3 + 1)
  }

  def main(args: Array[String]) {
    println(hailstone(27).toList)
    val (n, len) = (1 to 100000).map(n => (n, hailstone(n).length)).maxBy(_._2)
    println("value=" + n + "  len=" + len)
  }

I would only put

n #:: {

on the following line, it's easily missed up there, but crucial