either.swift

enum Either<A, B> {
    case Left(A)
    case Right(B)
}

func isLeft<A,B>(it : Either<A,B>) -> Bool {
    switch it { case .Left: return true; case .Right: return false }
}

func isRight<A,B>(it : Either<A,B>) -> Bool {
    return !isLeft(it)
}

func either<A, L, R>(leftCase : (L -> A), rightCase : (R -> A), upon : Either<L, R>) -> A {
    switch upon {
        case let .Left(a): return leftCase(a)
        case let .Right(b) : return rightCase(b)
    }
}

func lefts<A,B>(it : Either<A,B>[]) -> A[] {
    return it.reduce([]) {
        switch $1 {
            case let .Left(a): return ($0 + [a])
            case _ : return $0
        }
    }
}