Is there a way to make sealed classes generics? - kotlin

Question

How a generic result or error type could be defined in Kotlin? Something like this example from TypeScript
type Errorneous<E, R> =
{ is_error: true, error: E } | { is_error: false, result: R }
function calculate(): Errorneous<String, Number> {
return { is_error: false, result: 2 }
}
The problem is that Kotlin doesn't have generic sealed classes.
It's possible to define something like
data class Errorneous<E, R>(val error: E?, val result: R?)
But it not ideal as it allows wrong usage like
Errorneous<String, Int>(null, null)
Errorneous<String, Int>("", 2)
UPDATE
Possible (not compiling) Kotlin code
sealed class Errorneous
class Success<R>(val result: R) : Errorneous()
class Fail<R>(val error: R) : Errorneous()
fun calculate(): Errorneous {
return Success(2)
}
fun main() {
val result = calculate()
if (result is Success<*>) {
val r: Int = result.result // <= Problem here, no smart cast
}
}

Answer 1

You have to add generic parameters to the base class as well:
sealed class Errorneous<E,R>
class Error<E,R>(val error: E): Errorneous<E,R>()
class Success<E,R>(val result: R): Errorneous<E,R>()
fun calculate(): Errorneous<String, Int> {
return Success(2)
}
fun main() {
val result = calculate()
if (result is Success<*, Int>) {
val r: Int = result.result // <= smart cast
}
}