protocol Functor {
    associatedtype A
    associatedtype F: Functor = Self

    static func fmap<B> (_ f: @escaping (A) -> B) -> (Self) -> F where F.A == B
}

enum Maybe<A> {
    case Nothing
    case Just (A)
}

extension Maybe: Functor {
    static func fmap<B>(_ f: @escaping ((A) -> B)) -> (Maybe<A>) -> Maybe<B> {
        return { (functor: Maybe<A>) in
            switch functor {
            case let .Just (x): return .Just (f (x))
            case .Nothing: return .Nothing
            }
        }
    }
}

let test1: Maybe<Int>    = Maybe.Just (0)
  , test2: Maybe<String> = Maybe.fmap { String ($0) } (test1)

print (test2)

// func fmap