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What is the Result type?

Swift version: 5.0

Paul Hudson    @twostraws   

The Result type lets us encapsulate the success or failure of a method call in a single value, while also storing the contents of the successful return or the type of failure that occurred. More importantly, Result only stores one of these at a time: it will either be a success or a failure.

For example, we could use Result to handle networking. First we’d create a NetworkError` enum containing all the things that might go wrong:

enum NetworkError: Error {
    case badURL
}

Next we’d write a method that calls a completion handler with some sort of Result. In this instance we’re going to return the number of unread messages a user has in their inbox, or a NetworkError if something went wrong:

func fetchUnreadCount(from urlString: String, completionHandler: @escaping (Result<Int, NetworkError>) -> Void)  {
    guard let url = URL(string: urlString) else {
        completionHandler(.failure(.badURL))
        return
    }

    // complicated networking code here
    print("Fetching \(url.absoluteString)...")
    completionHandler(.success(5))
}

Tip: That code uses a completion handler rather than a simple return type because networking code will block the main thread – we want the method to return immediately and perform any complicated networking code in the background.

We can now call that method with a URL and evaluate what gets sent back. Remember. this will either be a success (sending us back the unread messages count), or a failure (sending us back what went wrong). Here’s the code:

fetchUnreadCount(from: "https://www.hackingwithswift.com") { result in
    switch result {
    case .success(let count):
        print("\(count) unread messages.")
    case .failure(let error):
        print(error.localizedDescription)
    }
}

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Available from iOS 8.0

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