Result type in Swift 5

Raúl Ferrer
April 6, 2018

Result type

In Swift 5 several new features have been introduced. One of them is the Result type. The Result type in swift 5 is an enum that contains two cases: success and failure.

public enum Result<Success, Failure: Error> {
    case success(Success), failure(Failure)
}

Result allows to handle errors for asynchronous APIs in an automatic way. For example, to make a call to a server that returns a list of users (struct User), we can implement the following method:

func fetchUsers(url: URL, completion: @escaping (Result<[User], Error>) -> Void) {
    URLSession.shared.dataTask(with: url) { data, response, error in
        guard let data = data, error == nil else {
            if let error = error as Error? {
                completion(.failure(error))
            }
            return
        }

        do {
            let users = try JSONDecoder().decode([User].self, from: data)
            completion(.success(users))
        } catch let error as? Error {
            completion(.failure(error))
        }

    }.resume()
}

Now, we can call the fetchUsers method as follows:

let  url = 'FETCH_USERS_URL'

fetchUsers(url: url) { result in
    switch result {
        case .success(let users):
            // Code to process user information.
        case .failure(error):
            // Code to handle error cases.
    }
}

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