1. Declare Either as a sealed class:

sealed class Either<out E, out V>

2. Add two subclasses of Either, representing Error and Value:

sealed class Either<out L, out R> {
    data class Left<out L>(val left: L) : Either<L, Nothing>()
    data class Right<out R>(val right: R) : Either<Nothing, R>()
}

3. Add factory functions for the convenient instantiating of Either:

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

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