Scala 2.11までのEitherっぽいEither(right-biasedでないEither)

Either.swift

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

extension Either {
    var rightProjection: RightProjection<L, R> {
        return RightProjection(either: self)
    }

    var leftProjection: LeftProjection<L, R> {
        return LeftProjection(either: self)
    }
}

struct RightProjection<L, R> {
    let either: Either<L, R>

    init(either: Either<L, R>) {
        self.either = either
    }
    
    func map<R2>(_ f: (R) -> R2) -> Either<L, R2> {
        switch self.either {
        case .left(let l):
            return .left(l)
        case .right(let r):
            return .right(f(r))
        }
    }

    func flatMap<R2>(_ f: (R) -> Either<L, R2>) -> Either<L, R2> {
        switch self.either {
        case .left(let l):
            return .left(l)
        case .right(let r):
            return f(r)
        }       
    }
}

struct LeftProjection<L, R> {
    let either: Either<L, R>

    init(either: Either<L, R>) {
        self.either = either
    }
    
    func map<L2>(_ f: (L) -> L2) -> Either<L2, R> {
        switch self.either {
        case .left(let l):
            return .left(f(l))
        case .right(let r):
            return .right(r)
        }
    }

    func flatMap<L2>(_ f: (L) -> Either<L2, R>) -> Either<L2, R> {
        switch self.either {
        case .left(let l):
            return f(l)
        case .right(let r):
            return .right(r)
        }       
    }
}