LazyList_Scala_2.13

Welcome to Scala 2.13.0-M4-pre-20d3c21 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_172).
Type in expressions for evaluation. Or try :help.

scala> class CallByNeed[A](a: => A){ lazy val value: A = a }
defined class CallByNeed

scala> def unfoldLazyList[A, S](init: S)(f: S => Option[(CallByNeed[A], S)]): LazyList[A] = {
     |   def loop(s: S): LazyList[A] = {
     |     f(s).fold(LazyList.empty[A]){ case (h, t) =>
     |       new LazyList.Cons(h.value, loop(t))
     |     }
     |   }
     |   loop(init)
     | }
unfoldLazyList: [A, S](init: S)(f: S => Option[(CallByNeed[A], S)])LazyList[A]

scala> val xs = unfoldLazyList(0){ s =>
     |   if (s <= 10)
     |     Some((new CallByNeed({println(s); s.toString}), s + 1))
     |   else
     |     None
     | }
xs: LazyList[String] = LazyList(_, ?)

scala> xs.size
res0: Int = 11

scala> xs(8)
8
res1: String = 8

scala> xs(3)
3
res2: String = 3