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
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()
}
如果您愿意，您可以将公共部分推送到超类:
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.