Either.kt

sealed class Either<out L, out R> {
    class Left<L>(val value: L) : Either<L, Nothing>()
    class Right<R>(val value: R) : Either<Nothing, R>()

    fun fold(
        onLeft: (L) -> Unit,
        onRight: (R) -> Unit,
    ) {
        when(this) {
            is Left -> onLeft(value)
            is Right -> onRight(value)
        }
    }
}

fun <R> right(value: R): Either<Nothing, R> = Either.Right(value)
fun <L> left(value: L): Either<L, Nothing> = Either.Left(value)