Algebraic data types in Kotlin

Question

I am trying to figure out how to use algebraic data types in Kotlin, so I\'m trying to implement a basic BinaryTree type the following way.

sealed class Tree<         


        

Answer 1

First you need to make T an out parameter. Then you can use Nothing as a type argument for Empty.

sealed class Tree<out T>{
  class Node<T>(val left: Tree<T>, val right: Tree<T>): Tree<T>()
  class Leaf<T>(val value: T): Tree<T>()
  object Empty: Tree<Nothing>()
}

Nothing is a special type in Kotlin, which cannot have an instance and is a subtype of all other types. So I would say it's opposite to Any in Kotlin type hierarchy.