Either.kt

/**
 * Represents a value of one of two possible types (a disjoint union).
 * Instances of [Either] are either an instance of [Left] or [Right].
 * FP Convention dictates that [Left] is used for "failure"
 * and [Right] is used for "success".
 *
 * @see Left
 * @see Right
 */
sealed class Either<out L, out R> {
    /** * Represents the left side of [Either] class which by convention is a "Failure". */
    data class Left<out L>(val left: L) : Either<L, Nothing>()

    /** * Represents the right side of [Either] class which by convention is a "Success". */
    data class Right<out R>(val right: R) : Either<Nothing, R>()

    val isRight get() = this is Right<R>
    val isLeft get() = this is Left<L>

    fun <L> left(left: L) = Left(left)
    fun <R> right(right: R) = Right(right)

    fun either(fnL: (L) -> Any, fnR: (R) -> Any): Any =
            when (this) {
                is Left -> fnL(left)
                is Right -> fnR(right)
            }
}

fun <A, B, C> ((A) -> B).c(f: (B) -> C): (A) -> C = {
    f(this(it))
}

fun <T, L, R> Either<L, R>.flatMap(fn: (R) -> Either<L, T>): Either<L, T> =
        when (this) {
            is Either.Left -> Either.Left(left)
            is Either.Right -> fn(right)
        }

fun <T, L, R> Either<L, R>.map(fn: (R) -> (T)): Either<L, T> = this.flatMap(fn.c(::right))