First attempt at Functor protocol in Swift

gistfile1.swift

protocol Functor {
  func fmap <A, B> (Self<A>, A -> B) -> Self<B>
  // ^ ERROR: Cannot specialize non-generic type '`Self`'
}

enum Maybe<A> : Functor {
  case Nothing
  case Just(A)

  func fmap<B>(x: Maybe<A>, f: A -> B) -> Maybe<B> {
    switch x {
      case .Nothing: return .Nothing
      case .Just(let just): return f(just)
    }
  }
}