public enum Either<L, R> {
    case left(L)
    case right(R)
}

extension Either {

    public var left: L? {
        switch self {
        case .left(let value): return value
        default: return nil
        }
    }

    public var right: R? {
        switch self {
        case .right(let value): return value
        default: return nil
        }
    }

    public func either<U>(ifLeft: (L) -> U, ifRight: (R) -> U ) -> U {
        switch self {
        case .left(let value): return ifLeft(value)
        case .right(let value): return ifRight(value)
        }
    }

}