Scalania11.scala

 def fib(a: Int, b: Int): Stream[Int] = a #:: fib(b, a + b)     

 fib(1, 1).take(10).toList // == List(1, 1, 2, 3, 5, 8, 13, 21, 34, 55)                                             
 fib(1, 1).takeWhile(_ < 4000000).filter(_ % 2 == 0).sum // == 4613732

 // --------------------------------------------------------------
 def euler1(l: Int) = 
   (1 until l).toStream.filter(x => x % 3 == 0 || x % 5 == 0).sum  

 def reverse1[T](ts: Seq[T]): Seq[T] =
    ts.foldLeft(Seq.empty[T]) { (a, e) => e +: a }

  def reverse2[T](ts: Seq[T]): Seq[T] = ts match {
    case Seq() => ts
    case h +: t => reverse2(t) :+ h
  }

  def reverse[T](ts: Seq[T]): Seq[T] = {
    @tailrec		  
    def go(ts: Seq[T], acc: Seq[T]): Seq[T] = ts match {
      case Seq() => acc
      case h +: t => go(t, h +: acc)
    }
    go(ts, Seq())
  }