fp-in-scala: ch05-stream

ch05-stream.scala

trait Stream[A] {
  def exists(f: A => Boolean): Boolean = foldRight(false)((h, t) => f(h) || t)

  def foldRight[B](z: => B)(f: (A, => B) => B): B = 
    uncons map { case (h ,t) => f(h, t.foldRight(z)(f)) } getOrElse z

  def forall(f: A => Boolean): Boolean = foldRight(true)((h, t) => f(h) && t)
  
  def take(n: Int): Stream[A] = 
    if (n == 0) Stream.empty
    else uncons map { case (hd, tl) => 
        lazy val rest = tl.take(n - 1)
        Stream.cons(hd, rest)
      } getOrElse Stream.empty

  def takeWhile(f: A => Boolean): Stream[A] = uncons map { case (hd, tl) => 
      if (f(hd)) {
        lazy val rest = tl.takeWhile(f)
        Stream.cons(hd, rest)
      } else Stream.empty[A]
    } getOrElse Stream.empty

  def toList: List[A] = uncons map { case (hd, tl) => hd +: tl.toList} getOrElse (List[A]())

  def uncons: Option[(A, Stream[A])]
}

object Stream {
  def empty[A]: Stream[A] = new Stream[A] {
    def uncons = None
  }

  def cons[A](hd: => A, tl: => Stream[A]): Stream[A] = new Stream[A] {
    lazy val uncons = Some((hd, tl))
  }

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