Blogger - Scalaz Motivation - 14

implicit object TreeFoldLeft extends FoldLeft[Tree] {
  def foldLeft[A, B](ma: Tree[A], b: B, op: (B, A) => B) = {
    def toList(ma: Tree[A]): List[A] = ma match {
      // not the fastest implementation ever
      case Node(el, left, right) => toList(left) ::: (el :: toList(right))
      case Empty() => Nil
    }
    toList(ma).foldLeft(b)(op)
  }
}