foldLeft

foldLeft.scala

def flatMap[A](list: List[A], f: A => List[A]) =
  list.foldLeft(List.empty[A]) { (acc, x) => acc ++ f(x) }

assert(
  flatMap(1 :: 2 :: 3 :: Nil, (x: Int) => x :: x :: Nil) == 
  List(1, 1, 2, 2, 3, 3)
)

def max[A: Ordering](list: List[A]): Option[A] =
  list match {
    case Nil => None
    case hd :: tl => Some(tl.foldLeft(hd)(Ordering[A].max))
  }

assert(max((1 to 10).toList) == Some(10))