Algebraic Data Types in Kotlin

trait Tree
data object Empty : Tree
data class Leaf(val value: Int) : Tree
data class Node(val left: Tree, val right: Tree) : Tree

fun max(x:Int, y:Int):Int = if (x > y) x else y

fun depth(t: Tree): Int = when (t) {
    is Empty -> 0
    is Leaf  -> 1
    is Node  -> 1 + max(depth(t.left), depth(t.right))
    // unfortunately, the else branch is required
    else     -> throw NoWhenBranchMatchedException()
}


fun main(args: Array<String>) {
    println(depth(
            Node(Node(Leaf(1), Leaf(2)), Leaf(3))
    ))
}

FWIW, here's the relevant ticket in the Kotlin issue tracker: https://youtrack.jetbrains.com/issue/KT-1753

Thanks for the sample code and the link to the issue.