nsnorth.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    var rootVC: UINavigationController? = nil

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        window = UIWindow(frame: UIScreen.main.bounds)
        window?.backgroundColor = .white
        let tableVC = ListsViewController(style: .plain)
        rootVC = UINavigationController(rootViewController: tableVC)
        window?.rootViewController = rootVC
        window?.makeKeyAndVisible()
        return true
    }

}

struct List: Codable {
    var name: String
    var id: Int
}

struct Todo: Codable {
    var title: String
    var done: Bool
}

let listsURL = URL(string: "http://localhost:8000/lists.json")!
func todosURL(listId: Int) -> URL {
    return URL(string: "http://localhost:8000/lists/\(listId)/todos.json")!
}

struct UnknownError: Error { }

extension URLSession {
    func dataTask(with url: URL, completionHandler: @escaping (Result<Data,Error>) -> Void) -> URLSessionDataTask {
        return dataTask(with: url, completionHandler: { (data, response, error) in
            guard let d = data else {
                completionHandler(.failure(error ?? UnknownError()))
                return
            }
            completionHandler(.success(d))
        })
    }
    
    func load<A>(_ resource: Resource<A>, completionHandler: @escaping (Result<A, Error>) -> ()) {
        dataTask(with: resource.url, completionHandler: { result in
            switch result {
            case let .failure(error):
                completionHandler(.failure(error))
            case let .success(data):
                completionHandler(resource.parse(data))
            }
        }).resume()
    }
}

// For videos on this, see https://talk.objc.io/collections/networking

struct Resource<A> {
    let url: URL
    let parse: (Data) -> Result<A, Error>
}

extension Resource where A: Codable {
    init(json url: URL) {
        self.url = url
        self.parse = { data in
            return Result { try JSONDecoder().decode(A.self, from: data) }
        }
    }
}

let listsResource = Resource<[List]>(json: listsURL)

final class ListsViewController: UITableViewController {
    var lists: [List] = [] {
        didSet {
            tableView.reloadData()
        }
    }
    
    override func viewDidLoad() {
        URLSession.shared.load(listsResource) { (result) in
            switch result {
            case let .failure(error):
                print("Got an error: \(error)")
            case let .success(lists):
                DispatchQueue.main.async {
                    self.lists = lists
                }
            }
        }
    }
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return lists.count
    }
    
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: .default, reuseIdentifier: nil)
        cell.textLabel?.text = lists[indexPath.row].name
        return cell
    }
}

// For more information, see the paper "Theorems for Free": https://people.mpi-sws.org/~dreyer/tor/papers/wadler.pdf

func foo<A, B>(_ x: A, _ y: B) -> (B, A) {
    return (y,x)
}