Stream Examples

streams.scala

def fib(n: Int): Stream[Int] = n match {
  case 0 ⇒ 0 #:: fib(1)
  case 1 ⇒ 1 #:: fib(2)
  case _ ⇒ (fib(n - 1).head + fib(n - 2).head) #:: fib(n + 1)
}

fib(0) take 10 foreach println

// From http://www.scala-lang.org/api/current/#scala.collection.immutable.Stream
val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(fibs.tail).map { n => n._1 + n._2 }
fibs take 5 foreach println

/*
  tail is everything but the head. Zipping gives us:
  (Stream(0, ?), Stream(1, ?))
  map over that to add up the heads of the zipped stream with { n => n._1 + n._2 }
*/

// Finally, you can do this without creating two streams and zipping and thus creating lots of objects
// Notice that the call to loop now becomes tail recursive
lazy val fib: Stream[Int] = {
  def loop(h: Int, n: Int): Stream[Int] = h #:: loop(n, h + n)
  loop(0, 1)
}

// Factorial
lazy val factorial: Stream[Int] = {
  def loop(n: Int): Stream[Int] = (1 to n).product #:: loop(n + 1)
  loop(0)
}