Either.kt

sealed class Either<L, R> {
    class Left<L1, R1>(val left: L1) : Either<L1, R1>()
    class Right<L1, R1>(val right: R1) : Either<L1, R1>()
}

val left: Either<String, Throwable> = Either.Left("foo")
val right: Either<String, Throwable> = Either.Right(Throwable("bar"))
val leftOrRight = left

val x: String = when (leftOrRight) {
    is Either.Left ->  leftOrRight.left
//    is Either.Right -> leftOrRight.right.message
}