caseClasses.scala

// If you define a Tree class like this:
sealed trait Tree[A]

case class Branch[A](l: Tree[A], r: Tree[A], v: A) extends Tree[A]
case class Leaf[A](v: A)                           extends Tree[A]

// Then summing up the tree can be done like this:
def sum(tree: Tree[Int]): Int = tree match {
  case Branch(l, r, v) => sum(l) + sum(r) + v
  case Leaf(v) => v
}