Kotlin and discriminated unions (sum types)

Question

Does Kotlin have anything like discriminated unions (sum types)? What would be the idiomatic Kotlin translation of this (F#):

type OrderMessage =
    | New of Id: int * Quantity: int
    | Cancel of Id: int

let handleMessage msg = 
    match msg with
        | New(id, qty) -> handleNew id qty
        | Cancel(id) -> handleCxl id

Answer 1

The common way of implementing this kind of abstraction in an OO-language (e.g. Kotlin or Scala) would be to through inheritance:

open class OrderMessage private () { // private constructor to prevent creating more subclasses outside
    class New(val id: Int, val quantity: Int) : OrderMessage()
    class Cancel(val id: Int) : OrderMessage()
}

You can push the common part to the superclass, if you like:

open class OrderMessage private (val id: Int) { // private constructor to prevent creating more subclasses outside
    class New(id: Int, val quantity: Int) : OrderMessage(id)
    class Cancel(id: Int) : OrderMessage(id)
}

The type checker doesn't know that such a hierarchy is closed, so when you do a case-like match (when-expression) on it, it will complain that it is not exhaustive, but this will be fixed soon.

Update: while Kotlin does not support pattern matching, you can use when-expressions as smart casts to get almost the same behavior:

when (message) {
  is New -> println("new $id: $quantity")
  is Cancel -> println("cancel $id")
}

See more about smart casts here.