Scalaにおけるバリアント型の表現 ref: http://qiita.com/piyo7/items/e57cf08bbfd7557303d2

file0.ml

(* Binary tree with leaves car­rying an integer. *)
type tree = Leaf of int | Node of tree * tree

let rec exists_leaf test tree =
  match tree with
  | Leaf v -> test v
  | Node (left, right) ->
      exists_leaf test left
      || exists_leaf test right

let has_even_leaf tree =
  exists_leaf (fun n -> n mod 2 = 0) tree

file1.scala

sealed trait Tree[T]
case class Leaf[T](v: T) extends Tree[T]
case class Node[T](left: Tree[T], right: Tree[T]) extends Tree[T]

def exists_leaf[T](test: T => Boolean, tree: Tree[T]): Boolean = {
  tree match {
    case Leaf(v) => test(v)
    case Node(left, right) =>
      exists_leaf(test, left) ||
        exists_leaf(test, right)
  }
}

def has_even_leaf(tree: Tree[Int]): Boolean =
  exists_leaf[Int](_ % 2 == 0, tree)