Raheel Ahmad: Using Monads and other Functional Paradigms in Practice

Monads can be a great addition to your programming toolbox: they provide an abstraction and a design pattern that help in writing more composable and declarative code. The path to Monad Enlightenment, however, is either paved with "learn Haskell" or "it's just flatMap". This talk will help build an intuition for what Monads are in the context of Functional Programming, and also why (and if!) you should care about them.
Bio: Raheel currently works at PlanGrid on their iOS app. Previously he helped rewrite the Level Money app in Swift. He enjoys working on the iOS platform, but also indulge in web frameworks, Haskell, information visualization, and typography.
Twitter: https://twitter.com/raheel

Transcript

1. Raheel Ahmad 1 — Raheel Ahmad | PlanGrid
2. Monads in Practice Building Intuition 2 — Raheel Ahmad | PlanGrid
3. Outline Fundamentals of Functional Programming 3 — Raheel Ahmad | PlanGrid
4. Outline Fundamentals of Functional Programming [Functor, Monad].flatMap { Definition, Example, Intuition } 4 — Raheel Ahmad | PlanGrid
5. Fundamentals of FP · Immutable values · Pure functions · Higher-order functions · Declarative programming 5 — Raheel Ahmad | PlanGrid
6. let someDate: NSDate? let label: UILabel // label.text = formatted date 6 — Raheel Ahmad | PlanGrid
7. let someDate: NSDate? let label: UILabel func formatDate(date: NSDate) -> String { let formatter = NSDateFormatter() formatter.dateStyle = .ShortStyle formatter.timeStyle = .ShortStyle return formatter.stringFromDate(date) } 7 — Raheel Ahmad | PlanGrid
8. let someDate: NSDate? let label: UILabel func formatDate(date: NSDate) -> String { let formatter = NSDateFormatter() formatter.dateStyle = .ShortStyle formatter.timeStyle = .ShortStyle return formatter.stringFromDate(date) } // label.text = formatDate(someDate) ╳ 8 — Raheel Ahmad | PlanGrid
9. if let date = someDate { label.text = formatDate(date) } 9 — Raheel Ahmad | PlanGrid
10. label.text = someDate.map { date in let formatter = NSDateFormatter() formatter.dateStyle = .ShortStyle formatter.timeStyle = .ShortStyle return formatter.stringFromDate(date) } 10 — Raheel Ahmad | PlanGrid
11. label.text = someDate.map { formatDate($0) } 11 — Raheel Ahmad | PlanGrid
12. label.text = someDate.map(formatDate) 12 — Raheel Ahmad | PlanGrid
13. What is a Monad? 13 — Raheel Ahmad | PlanGrid
14. Functors Basic intuition · contain a value inside a context · provides an interface to transform/"map" the contained value 14 — Raheel Ahmad | PlanGrid
15. Surround a value inside a context struct Array<Element> { } enum Optional<Wrapped> { } enum Result<T, ErrorType> { } T -> U 15 — Raheel Ahmad | PlanGrid
16. Allow mapping the contained value extension Array { func map<U>(transform: Element -> U) -> [U] } enum Optional<Wrapped> { func map<U>(transform: Wrapped -> U) -> Optional<U> } enum Result<Value, ErrorType> { func map<U>(transform: Value -> U) -> Result<U, ErrorType> } 16 — Raheel Ahmad | PlanGrid
17. ... map for function func map<T, U, V>(f: T -> U, _ transform: U -> V) -> (T -> V) { return { t in transform(f(t)) } } 17 — Raheel Ahmad | PlanGrid
18. func formatDate(date: NSDate) -> String 18 — Raheel Ahmad | PlanGrid
19. map over an Optional 1. let someDate: Optional<NSDate> // NSDate? 19 — Raheel Ahmad | PlanGrid
20. map over an Optional 1. let someDate: Optional<NSDate> // NSDate? 2. someDate.map(formatDate) // Optional<String> 20 — Raheel Ahmad | PlanGrid
21. map over an Array 1. let dates = [ yesterday, today, tomorrow ] // [NSDate] 2. dates.map(formatDate) // [String] 21 — Raheel Ahmad | PlanGrid
22. map over a Result 1. let resultingDate: Result<NSDate, NSError> 2. resultingDate.map(formatDate) // Result<String, NSError> 22 — Raheel Ahmad | PlanGrid
23. map over a function let dateAtSection: Int -> Date map(dateAtSection, formatDate) // Int -> String 23 — Raheel Ahmad | PlanGrid
24. map li!s a regular function 24 — Raheel Ahmad | PlanGrid
25. Stay in the Functor! dateLabel.text = someDate.map(formatDate) // vs. if let date = someDate { dateLabel.text = formatDate(date) } 25 — Raheel Ahmad | PlanGrid
26. Contain transformations · not writing the map code everywhere · like we used to in Objective-C · simple for Array & Optional, more complex for other functors 26 — Raheel Ahmad | PlanGrid
27. Compose Transformations Functors compose transformations label.text = someDate .map(formatDate) .map(upper) · without "side-effect"s 27 — Raheel Ahmad | PlanGrid
28. Functors of your own · Intuitive for generic data structures, e.g. Tree · Possibly for custom data structure · Cache<T> → map → Cache<U> 28 — Raheel Ahmad | PlanGrid
29. Monads Basic intuition · surround a value inside a context · allow mapping and then flattening the contained value 29 — Raheel Ahmad | PlanGrid
30. Surround a value inside a context struct Array<Element> { } enum Optional<Wrapped> { } struct Result<T, ErrorType> { } class Signal<Event, ErrorType> { } 30 — Raheel Ahmad | PlanGrid
31. Allow flatMapping the contained value extension Array { func flatMap<U>(transform: Element -> [U]) -> [U] } enum Optional<Wrapped> { func flatMap<U>(transform: (Wrapped) -> U?) -> U? } struct Result<Value, E> { func flatMap<U>(transform: Value -> Result<U, E>) -> Result<U, E> } extension Signal { func flatMap<U>(transform: Value -> Signal<U, Error>) -> Signal<U, Error> } 31 — Raheel Ahmad | PlanGrid
32. map extension Array { func map<U>(transform: Element -> U) -> [U] } enum Optional<Wrapped> { func map<U>(transform: Wrapped -> U) -> Optional<U> } struct Result<Value, ErrorType> { func map<U>(transform: Value -> U) -> Result<U, ErrorType> } extension Signal { func map<U>(transform: Value -> U) -> Signal<U, Error> } 32 — Raheel Ahmad | PlanGrid
33. flatMap over an Array 33 — Raheel Ahmad | PlanGrid
34. struct User { } struct Issue { let users: [User] } 34 — Raheel Ahmad | PlanGrid
35. struct User { } struct Issue { let users: [User] } func issue(index: Int) -> Issue func collaborators(issue: Issue) -> [User] 35 — Raheel Ahmad | PlanGrid
36. struct User { ... } struct Issue { let users: [User] } func issue(index: Int) -> Issue func collaborators(issue: Issue) -> [User] let selectedRows: [Int] // selected collaborators 36 — Raheel Ahmad | PlanGrid
37. struct User { ... } struct Issue { let users: [User] } func issue(index: Int) -> Issue func collaborators(issue: Issue) -> [User] let selectedRows: [Int] // selected collaborators selectedRows .map(issue) // [Issue] .map(collaborators) // [ [User] ] 37 — Raheel Ahmad | PlanGrid
38. struct User { ... } struct Issue { let users: [User] } func issue(index: Int) -> Issue func collaborators(issue: Issue) -> [User] let selectedRows: [Int] // selected collaborators selectedRows .map(issue) // [Issue] .flatMap(collaborators) // [User] 38 — Raheel Ahmad | PlanGrid
39. Another flatMap over an array ... of functions dateStringsForSection: Int -> [String] durationStringsForSection: Int -> [String] 39 — Raheel Ahmad | PlanGrid
40. Another flatMap over an array ... of functions dateStringsForSection: Int -> [String] durationStringsForSection: Int -> [String] [dateStringsForSection, durationStringsForSection] .flatMap { $0(section) } 40 — Raheel Ahmad | PlanGrid
41. flatMap over an Optional func userForName(name: String) -> User? func avatarForUser(user: User) -> UIImage? let myName = "Raheel" // Compute the image for the given name 41 — Raheel Ahmad | PlanGrid
42. flatMap over an Optional func userForName(name: String) -> User? func avatarForUser(user: User) -> UIImage? let myName = "Raheel" userForName(myName).flatMap(avatarForUser) 42 — Raheel Ahmad | PlanGrid
43. flatMap over a Result func userForName(name: String) -> Result<User, NSError> func avatarForUser(user: User) -> Result<UIImage, NSError> let myName = "Raheel" userForName(myName).flatMap(avatarForUser) 43 — Raheel Ahmad | PlanGrid
44. Signal 44 — Raheel Ahmad | PlanGrid
45. Event values represent what a signal "emits" enum Event<Value, Error> { case Next(Value) case Failed(Error) case Completed case Interrupted } 45 — Raheel Ahmad | PlanGrid
46. Signal class Signal<Value, Error> { … func on(event: Event -> (), next: Value -> (), failed: Error -> (), completed: () -> (), interrupted: () -> ()) } 46 — Raheel Ahmad | PlanGrid
47. Using a Signal let numbersSignal: Signal<Int, SomeError> 47 — Raheel Ahmad | PlanGrid
48. Using a Signal let numbersSignal: Signal<Int, SomeError> numbersSignal.on( next: { print("Received \($0)") }, failed: { print("Error \($0)") } ) 48 — Raheel Ahmad | PlanGrid
49. flatMap over a Signal func userForPredicate(predicate: Predicate) -> Signal<User, NSError> func avatarForUser(user: User) -> Signal<UIImage, NSError> let predicate: Predicate 49 — Raheel Ahmad | PlanGrid
50. flatMap over a Signal extension Signal { func flatMap<U>(transform: Value -> Signal<U, Error>) -> Signal<U, Error> } 50 — Raheel Ahmad | PlanGrid
51. flatMap over a Signal func userForPredicate(predicate: Predicate) -> Signal<User, NSError> func avatarForUser(user: User) -> Signal<UIImage, NSError> let predicate: Predicate userForPredicate(predicate) // Signal<User, NSError> .flatMap(avatarForUser) // Signal<UIImage, NSError> 51 — Raheel Ahmad | PlanGrid
52. Providing a common pattern · recognize flatMapping across different · contexts · problems · disciplines 52 — Raheel Ahmad | PlanGrid
53. Transformations vs. Operations · Functor: chain transformations of contained value · Monad: chain operations on contained value 53 — Raheel Ahmad | PlanGrid
54. Theoretically Speaking 54 — Raheel Ahmad | PlanGrid
55. Monad requirements (Haskell-ish) Monads require a pure function func pure<T>(m: T) -> Optional<T> { return Optional<T>(m) } // let m: Int? = 23 · and satisfy basic Monadic laws · i.e. Set is not a Monad 55 — Raheel Ahmad | PlanGrid
56. Monads in Swift · No real Monad type (protocol) in Swi! · need Higher Kinded Types · see Scala & Scalaz · Methods vs. functions / PoP vs. Functional 56 — Raheel Ahmad | PlanGrid
57. Does it matter · maybe not if you are an app developer · maybe yes if you write libraries · pattern recognition matters · shared vocabulary matters · gateway to FP 57 — Raheel Ahmad | PlanGrid
58. THANK YOU! @raheel | PlanGrid.com Questions? 58 — Raheel Ahmad | PlanGrid