fp in scala EXERCISE 5.6 foldRightを用いたheadOptionの実装

StreamFunctionsViaFoldRight.scala

def foldRight[A, B](s: Stream[A])(z: => B)(f: (A, => B) => B): B = s match {
  case x #:: xs => f(x, foldRight(xs)(z)(f))
  case _ => z
}

def headOptionViaFoldRight[A](s: Stream[A]): Option[A] = foldRight(s)(None: Option[A]) {
  (e, acc) => Some(e)
}

def map[A, B](s: Stream[A])(f: A => B): Stream[B] = foldRight(s)(Stream.empty[B]) {
  (e, acc) => f(e) #:: acc
}

def filter[A](s: Stream[A])(f: A => Boolean): Stream[A] = foldRight(s)(Stream.empty[A]) {
  (e, acc) => if(f(e)) e #:: acc else acc
}

def append[A](s1: =>Stream[A], s2: =>Stream[A]): Stream[A] = foldRight(s1)(s2) {
  (e, acc) => e #:: acc
}

def flatMap[A, B](s: Stream[A])(f: A => Stream[B]): Stream[B] = foldRight(s)(Stream.empty[B]) {
  (e, acc) => append(f(e), acc)
}

def n(a: Int) = {println(a); a}
def s = n(1) #:: n(2) #:: n(3) #:: n(4) #:: Stream.empty
headOptionViaFoldRight(s)
map(s)(_ + 2)
filter(s)(_ % 2 == 0)
append(s, map(s)(_ * 3))
flatMap(s)(a => Stream(a, a))