Kotlin and discriminated unions (sum types)

Kotlin’s sealed class approach to that problem is extremely similar to the Scala sealed class and sealed trait.

Example (taken from the linked Kotlin article):

sealed class Expr {
    class Const(val number: Double) : Expr()
    class Sum(val e1: Expr, val e2: Expr) : Expr()
    object NotANumber : Expr()
}