Simple Either implementation in Kotlin with fold method

either.kt

sealed class Either<L, R> {
    inline fun <B> fold(onLeft: (L) -> B, onRight: (R) -> B): B = when (this) {
        is Left -> onLeft(this.value)
        is Right -> onRight(this.value)
    }

    operator fun not() = fold(::id, ::id)
}

class Left<L, R>(val value: L) : Either<L, R>()
class Right<L, R>(val value: R) : Either<L, R>()

fun <T> id(id: T): T = id