protocol Functor {
    associatedtype A
    associatedtype F: Functor = Self

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

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

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

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

print (test2)

func fmap<A, B> (_ f: (A) -> B, _ functor: Functor) -> Functor { functor.fmap (f) }