Functional Swift Talk

AppDelegate.swift

import UIKit

struct Screen<A> {
    let run: (A -> ()) -> UIViewController
}

struct Step<A> {
    let build: (navigationController: UINavigationController, callback: A -> ()) -> UIViewController
}

extension Step {
    init(_ screen: Screen<A>) {
        build = { navigationController, callback in
            let vc = screen.run(callback)
            navigationController.pushViewController(vc, animated: true)
            return vc
        }
    }
    
    func run(callback: A -> ()) -> UIViewController {
        let nc = UINavigationController()
        build(navigationController: nc, callback: callback)
        return nc
    }
    
    func flatMap<B>(f: A -> Step<B>) -> Step<B> {
        return Step<B>(build: { (navigationController, callback) -> UIViewController in
            self.build(navigationController: navigationController, callback: { a in
                f(a).build(navigationController: navigationController, callback: callback)
            })
        })
    }
}

func question(text: String) -> Step<Int> {
    return Step(question(text))
}

func question(text: String) -> Screen<Int> {
    return Screen { callback in
        let storyBoard = UIStoryboard(name: "Main", bundle: NSBundle.mainBundle())
        let vc = storyBoard.instantiateViewControllerWithIdentifier("MyViewController") as! ViewController
        _ = vc.view // Force loading of the view
        vc.textLabel.text = text
        vc.onComplete = callback
        return vc
    }
}

class Game {
    let randomNumber: Int = Int(arc4random_uniform(100))
    var numberOfGuessesLeft = 3
    
    func turn(guess: Int) -> Step<Int> {
        numberOfGuessesLeft--
        if guess == randomNumber {
            return question("You won!")
        } else if numberOfGuessesLeft == 0 {
            return question("You lost! The number was \(randomNumber)")
        } else {
            let hint = "The number was \(randomNumber > guess ? "larger" : "smaller")."
            return question("\(hint) Wrong answer, try again. \(numberOfGuessesLeft) guesses left").flatMap(turn)
        }
    }
    
    func startGame() -> UIViewController {
        return question("Guess a number between 0 and 100").flatMap(turn).run { _ in () }
    }
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    
    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        window?.rootViewController = Game().startGame()
        return true
    }
}