dictionaryOptional.swift

func +=<K, V>(left: inout [K : V]?, right: [K : V]?) {
    right?.forEach { left?[$0] = $1 }
}

func +<K, V>(left: [K : V]?, right: [K : V]?) -> [K : V]? {
    guard let left = left else { return right }
    guard let right = right else { return left }
    return left.reduce(right) { dict, pair in
        var dict = dict
        dict[pair.0] = pair.1
        return dict
    }
}