// *** before: ***
func fetchOld(completion: @escaping (String?, Error?) -> Void) { }

fetchOld { text, error in
    // two optionals give four states, and two of them cannot happen;
    // still every caller checked them
    if let error = error {
        print(error)
    } else if let text = text {
        print(text)
    }
}

// *** in version 5.0: ***
enum NetworkError: Error { case offline }

func fetch(completion: @escaping (Result<String, NetworkError>) -> Void) {
    completion(.success("text"))
}

fetch { result in
    switch result {
    case .success(let text): print(text)
    case .failure(let error): print(error)
    }
}

// a throwing call turns into a Result and back
let result = Result { try String(contentsOfFile: "/etc/hosts") }
let value = try? result.get()
let length = result.map { $0.count }