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Why Swift? A practical intro to thinking functionally

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Why Swift? We ❤ Objective C! • Optionals • Result • Deferred

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Optionals: they’re a PITA

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2015-04-02 12:28:56.882 Wallapop[3066:91913] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'

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As a physical manifestation of a logical system, computers are faced with the intractable problem of how to represent nothing with something Mattt Thompson

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Benefits of optionals • Reduces the cognitive load when dealing with objects and APIs you did not write - No more maintenance code like Assertions when an API is called incorrectly or an object is initialised incorrectly

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Drawbacks of optionals • PITA to write sometimes: use Undefined - https://github.com/weissi/swift-undefined • Interaction with Cocoa APIs is weird sometimes - Things are getting better with every release

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Objective C improvements • Apple introduced __nullable and __nonnull Released with Xcode 6.3

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Result: monads for the rest of us

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NSError *err = nil; CGFloat result = [NMArithmetic divide:2.5 by:3.0 error:&err]; if (err) { NSLog(@"%@", err) } else { [NMArithmetic doSomethingWithResult:result] }

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Result • Allows us to express with clarity what are the expected results from an function: enum Result { case Success(T) case Failure(ErrorType) } https://www.youtube.com/watch?v=LqwrUmuodyY

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func divide (dividend : Float, divisor : Float) -> Result let result = divide(dividend :2.5, divisor:3) switch result { case .Success(let quotient): doSomethingWithResult(quotient) case .Failure(let error): print(error) }

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Result: Chaining func map(f: T -> U) -> Result { switch self { case .Failure(let error): return .Failure(error) case .Success(let value): return .Success(f(value)) } }

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Result: Chaining func fetchImageData(url : NSURL) -> Result func processImageData(data : NSData) -> UIImage let imageResult = fetchImageData().map(processImageData)

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Result: Chaining

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Result: Chaining func bind(f: T -> Result) -> Result { switch self { case .Failure(let error): return .Failure(error) case .Success(let value): return f(value) } }

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func fetchWallData () -> Result { let mainBundle = NSBundle.mainBundle() guard let url = mainBundle.URLForResource("wall", withExtension: "json") else { return .Failure(NSError()) } guard let data = NSData(contentsOfURL: url) else { return .Failure(NSError()) } return .Success(data) }

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func parseWallData(data : NSData) -> Result { let jsonObject : AnyObject do { jsonObject = try NSJSONSerialization .JSONObjectWithData(data, options: .MutableContainers) } catch let error { return .Failure(error as NSError) } guard let jsonResult = jsonObject as? [String : AnyObject] else { return .Failure(NSError()) } guard let page = parseWallPage(jsonResult) else { return Result(NSError()) } return .Success(page) }

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Result: Chaining func fetchWallData () -> Result func parseWallData(data : NSData) -> Result let pageResult = fetchWallData().bind(parseWallData)

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Happy Path Programming!!!

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Deferred: functional concurrency

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What about the error?

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Deferred class Deferred { init() init(value:T) var isFilled: Bool func fill (value : T) func peek () -> T func upon (block : T -> ()) }

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Deferred extension Deferred { func map(f: T -> U) -> Deferred func bind(f: T -> Deferred) -> Deferred }

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Deferred • Can be used trivially whenever a completionBlock was being used. ✓ When calling upon, you can schedule a closure on any GCD queue, and, unlike completionBlocks in ObjC, multiple closures can be scheduled to be executed.

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Deferred func both(other: Deferred) -> Deferred<(T,U)> func all(deferreds: [Deferred]) -> Deferred<[T]> func any(deferreds: [Deferred]) -> Deferred>

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Deferred func fetchProductsInContext(context : NSManagedObjectContext) -> Deferred<[Product]> { let deferred = Deferred<[Product]>() dispatch_async(queue) { let products = fetchProductsInContextSync(context) deferred.fill(products) } return deferred }

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Result + Deferred: ≈> infix operator ≈> { associativity left precedence 160 } func ≈> (lhs : Deferred>, rhs : T -> Deferred>) -> Deferred>

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Deferred> func fetchWallData() -> Deferred> func parseWallPage(data:NSData) -> Deferred> func requestWallPage () -> Deferred> { return fetchWallData() ≈> parseWallPage }

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Deferred> let wallRequest = requestWallPage() wallRequest.upon(dispatch_get_main_queue()) { result in switch result { case .Success (let box) : //Populate collectionView case .Failure (let error) : //Show failure message } }

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Takeways: going forward

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⌘B > ⌘R

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• Thinking functionally makes things easier to test - More difficult to write, specially after years of OOP Takeaways

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Takeaways • Swift compiler is like an abusive boss

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• If your code compiles, chances are, it works - This is true for all Swift code, but even more if you use the types mentioned earlier Takeaways

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• Bridging to ObjC isn’t hard - It’s just boring Takeaways

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• It’s not as hard as it looks - Taking it one step at a time helps a lot Takeaways

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• OOP suits perfectly for UI implementations. • OOP suits fine for modeling data ‣ Functional is perfect for modeling the transformation of this data Takeaways

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• John Gallagher - Networking with Monads • Nick Lockwood - Thoughts on Swift 2 Errors • Andy Matuschak - Controlling Complexity in Swift • Wallapop iOS Team - WallaFoundation Resources

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