Stream.scala

package laziness

sealed trait Stream[+A] {
  import Stream._

  def toList: List[A] = ???

  def take(n: Int): Stream[A] = ???

  //This can be tailrecursive. Uncomment below to let the compiler check
  //@annotation.tailrec
  def drop(n: Int): Stream[A] = ???


  // Methods shown during lecture

  def headOption: Option[A] = this match {
    case Cons(h, _) => Some(h())
    case Empty => None
  }

  def exists(p: A => Boolean): Boolean = this match {
    case Cons(x, xs) => p(x()) || xs().exists(p)
    case Empty => false
  }

  def foldRight[B](z: => B)(f: (A, => B) => B): B = this match {
    case Cons(x, xs) => f(x(), xs().foldRight(z)(f))
    case Empty => z
  }
}

case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]

object Stream { // companion object

  def fibs: Stream[Int] = {
    // tip: write a recursive def here and call it with some start values
    ???
  }

  def unfold[A, S](z: S)(f: S => Option[(A, S)]): Stream[A] = ???

  def fibsViaUnfold: Stream[Int] = ???


  // Methods shown during Lecture

  def cons[A](h: => A, t: => Stream[A]): Stream[A] = {
    lazy val head = h
    lazy val tail = t
    Cons(() => head, () => tail)
  }

  def empty[A]: Stream[A] = Empty

  def apply[A](as: A*): Stream[A] =
    if (as.isEmpty) empty
    else cons(as.head, apply(as.tail: _*))


  val ones: Stream[Int] = cons(1, ones)
}