stree

stree.scala

sealed abstract class Tree[A]
case class Leaf[A](elem : A) extends Tree[A]
case class Node[A](left : Tree[A], right : Tree[A]) extends Tree[A]

val t = Node(Leaf(11), Node(Leaf(13),Leaf(15) ))

def sum(t: Tree[Int]):Int = t match {
    case Leaf(i) => i
    case Node(l,r) => sum(l) + sum(r)
    case _ => error("FUBAR tree")
}

println(sum(t))