val it: Iterator[Int] = Iterator.iterate((0, 1)) { case (a, b) => (b, a + b) }.map(_._1)

def fib(n: Int): Int = it.drop(n).next()

// example:
println(it.take(13).mkString(",")) // prints: 0,1,1,2,3,5,8,13,21,34,55,89,144