def unfold[A,S](initial: S)(generateNext: S => Option[(A, S)]): Stream[A] =
  generateNext(initial) match {
    case Some((first, next)) => cons(first, unfold(next)(generateNext))
    case None => empty  
  }

val fibs =
  unfold((0,1)) {
    case (f0, f1) => Some((f0, (f1, f0+f1)))
  }

def from(n: Int) =
  unfold(n)(n => Some(n, n+1))

def constant[A](a: A) =
  unfold(a)(_ => Some((a, a)))