A generic Either structure.

Either.swift

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 var isLeft: Bool {
        switch self {
        case .left: return true
        case .right: return false
        }
    }

    public var isRight: Bool {
        switch self {
        case .left: return false
        case .right: return true
        }
    }

    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)
        }
    }
    
}

extension Either {

    public static func toLeft(value: L) -> Either {
        return .left(value)
    }

    public static func toRight(value: R) -> Either {
        return .right(value)
    }

    public static func toLeft(values: [L]) -> [Either] {
        return values.map(toLeft)
    }

    public static func toRight(values: [R]) -> [Either] {
        return values.map(toRight)
    }

    public static func lefts(_ array: [Either]) -> [L] {
        return array.flatMap { $0.left }
    }

    public static func rights(_ array: [Either]) -> [R] {
        return array.flatMap { $0.right }
    }

}