How to handle a response with `success` and `error` cases in only one completion block handler by using Swift?

Nowaday, there are a lot of solution to handle response which has multiple states.

func getUserDetail(completion: (User?, NSError?) -> Void) {...}

func getUserDetail(success: User -> Void, failure: NSError -> Void) {...}

I hate this, when ever I call that method I have to check error != nil

getUserDetail { user, error in
  if let error = error {
      //Do when failed
  } else if let user = user {
      //Do when success
  } else {
      //What should I do here
  }
}

or kind of prolix :) but simple right

getUserDetail({ user in
  //Do when success
}, failure: { error in
  //Do when failed
})

OK now I will do some think more complicated, but I think good for me (not recomended for everybody)

First, create an enum Response:

enum Response<Value> {
    case success(Value)
    case failure(NSError)
}

Time to for enum:

func getUserDetail(completion: Response<User> -> Void) {
  // if success
  completion(.success(user))
  //..
  // if failed
  completion(.failure(error))
}

Call your method:

getUserDetail { response in
  switch response {
  case .Success(let user):
      //Do when success
  case .Failure(let error):
      //Do when failed
  }
}

Let’s swifty! 🛀🛀🛀