Scala Days 2018 Berlin - A Pragmatic Introduction to Category Theory

Category Theory has become one of the hot topics in the community. Why is it so interesting for us? Why are the cool kids passionate about it?

This talk will introduce the general principles of Category Theory in a pragmatic, non-mathematical way. We will show practical examples of how this theory has managed to simplify and solve common challenges that we encounter in our code daily, such as nullable values, error handling, parallel and sequential operations and data validation. We will apply them to create our own category theory library from scratch using ScalaCheck as the only dependency.

Transcript

1. A PRAGMATIC INTRODUCTION TO CATEGORY THEORY @DANIELASFREGOLA SCALA DAYS 2018 - BERLIN github.com/DanielaSfregola/tutorial-cat
2. HELLOOOOO > ex Java Developer > OOP background > I am not a mathematician !
3. I AM NOT A MATHEMATICIAN
4. YOU DO NOT NEED TO KNOW CATEGORY THEORY TO WRITE GOOD CODE
5. YOU DO NOT NEED TO KNOW CATEGORY THEORY TO WRITE FUNCTIONAL CODE
6. CATEGORY THEORY DEEPER UNDERSTANDING ON OUR CODE
7. HOW DO WE REASON ?
8. COMPOSITION ABSTRACTION
9. CATEGORY THEORY HOW THINGS COMPOSE
10. ARROW THEORY CATEGORY THEORY HOW THINGS COMPOSE
11. WHAT IS A CATEGORY?
12. COMPOSITION LAW
13. IDENTITY LAW
14. COMPOSITION + ASSOCIATIVITY
15. CATEGORY'S RULES > Identity > Composition > Associativity
16. A PRACTICAL EXAMPLE
17. CATEGORY WITH 1 OBJECT
18. CATEGORY WITH 1 OBJECT = MONOID
19. MONOID'S RULES Identity n o id == id o n == n Composition forall x, y => x o y Associativity x o (y o z) == (x o y) o z
20. SCALACHECK FTW! MonoidSpec // n o id == id o n == n property("identity") = forAll { n: A => monoid.compose(n, id) == n && monoid.compose(id, n) == n } // forall x, y => x o y property("composition") = forAll { (x: A, y: A) => monoid.compose(x, y).isInstanceOf[A] } // x o (y o z) == (x o y) o z property("associativity") = forAll { (x: A, y: A, z: A) => val xY = monoid.compose(x,y) val yZ = monoid.compose(y,z) monoid.compose(xY, z) == monoid.compose(x, yZ) }
21. A PRACTICAL EXAMPLE
22. MONOID trait Monoid[A] { def identity: A def compose(x: A, y: A): A }
23. MONOID INSTANCES (1) implicit val intMonoid: Monoid[Int] = new Monoid[Int] { def compose(x: Int, y: Int): Int = x + y def identity: Int = 0 }
24. MONOID INSTANCES (2) implicit val stringMonoid: Monoid[String] = new Monoid[String] { def compose(x: String, y: String): String = x + y def identity: String = "" }
25. CATEGORY WITH 1+ OBJECT
26. CATEGORY IN A BOX
27. CATEGORY IN A BOX > Objects are in a Box > All the arrows are mapped
28. LIFTING: CONTEXT VS CONTENT
29. EXAMPLE OF BOXES > Option > Future > Try > List > Either
30. CATEGORY IN A BOX = FUNCTOR
31. FUNCTOR'S RULES Identity map(id) == id Composition map(g o f) == map(g) o map(f) Associativity map(h o g) o map(f) == map(h) o map(g o f)
32. SCALACHECK FTW! FunctorSpec // map_id == id property("identity") = forAll { box: Box[A] => map(box)(identity) == box } // map_(g o f) == (map_g) o (map_f) property("composition") = forAll { boxA: Box[A] => val fG = f andThen g val mapFG: Box[A] => Box[C] = map(_)(fG) mapFG(boxA) == (mapF andThen mapG)(boxA) } // map_(h o g) o map_f == map_h o map_(g o f) property("associativity") = forAll { boxA: Box[A] => val fG = f andThen g val mapFG: Box[A] => Box[C] = map(_)(fG) val gH = g andThen h val mapGH: Box[B] => Box[D] = map(_)(gH) (mapF andThen mapGH)(boxA) == (mapFG andThen mapH)(boxA) }
33. LIFTING: CONTEXT VS CONTENT
34. FUNCTOR class Functor[Box[_]] { def map[A, B](boxA: Box[A]) (f: A => B): Box[B] }
35. MAYBE sealed abstract class Maybe[+A] final case class Just[A](a: A) extends Maybe[A] case object Empty extends Maybe[Nothing]
36. FUNCTOR FOR MAYBE implicit val maybeFunctor: Functor[Maybe] = new Functor[Maybe] { override def map[A, B](boxA: Maybe[A]) (f: A => B): Maybe[B] = boxA match { case Just(a) => Just(f(a)) case Empty => Empty } }
37. BOX FUNCTION + BOX VALUES
38. COMBINE MORE BOXES INTO ONE = APPLICATIVE
39. APPLICATIVE'S RULES > Identity > Composition > Associativity > Homorphism > Interchange ...and more!
40. SCALACHECK FTW! ApplicativeSpec extends FunctorSpec // ap(id)(a) == a property("identity") = forAll { box: Box[A] => ap(pureIdentity)(box) == box } // ap(pure(f))(pure(a)) == pure(f(a)) property("homorphism") = forAll { a: A => ap(pureF)(pure(a)) == pure(f(a)) } // {x => pure(x)}(a) == pure(a) property("interchange") = forAll { a: A => toPureA(a) == pure(a) } // pure(h o g o f) == ap(pure(h o g))(pure(f(a))) property("composition") = forAll { a: A => val gH = g andThen h val fGH = f andThen gH val pureGH = pure(gH) val pureFA = pure(f(a)) pure(fGH(a)) == ap(pureGH)(pureFA) }
41. COMBINE BOXES TOGETHER > How to create a new box > How to combine their values together
42. APPLICATIVE class Applicative[Box[_]] extends Functor[Box] { def pure[A](a: A): Box[A] def ap[A, B](boxF: Box[A => B])(boxA: Box[A]): Box[B] /*************/ def map[A, B](boxA: Box[A])(f: A => B): Box[B] = ap[A, B](pure(f))(boxA) }
43. BOX FUNCTION + BOX VALUES
44. APPLICATIVE class Applicative[Box[_]] extends Functor[Box] { def pure[A](a: A): Box[A] def ap[A, B](boxF: Box[A => B])(value: Box[A]): Box[B] def ap2[A1, A2, B](boxF: Box[(A1, A2) => B]) (value1: Box[A1], value2: Box[A2]): Box[B] // up to 22 values! // same for map }
45. APPLICATIVE FOR MAYBE implicit val maybeApplicative: Applicative[Maybe] = new Applicative[Maybe] { def pure[A](a: A): Maybe[A] = Just(a) def ap[A, B](boxF: Maybe[A => B])(boxA: Maybe[A]): Maybe[B] = (boxF, boxA) match { case (Just(f), Just(a)) => pure(f(a)) case _ => Empty } }
46. BOX IN A BOX
47. BOX IN A BOX
48. FUSE TWO BOXES TOGETHER = MONAD
49. MONAD'S RULES > Identity > Composition > Associativity
50. SCALACHECK FTW! MonadSpec extends ApplicativeSpec // flatMap(pure(a))(f(a)) == f(a) property("left identity") = forAll { a: A => flatMap(pure(a))(toPureFa) == toPureFa(a) } // flatMap(pure(a))(f(a)) == f(a) property("right identity") = forAll { a: A => flatMap(toPureFa(a))(pure) == toPureFa(a) } // pure(h o g o f) == ap(pure(h o g))(pure(f(a))) property("associativity") = forAll { boxA: Box[A] => val left: Box[C] = flatMap(flatMap(boxA)(toPureFa))(toPureGb) val right: Box[C] = flatMap(boxA)(a => flatMap(toPureFa(a))(toPureGb)) left == right }
51. MONAD (AS FUNCTOR) class Monad[Box[_]] extends Functor[Box] { // map from Functor def flatten[A](bb: Box[Box[A]]): Box[A] /*******/ def flatMap[A, B](valueA: Box[A]) (f: A => Box[B]): Box[B] = { val bb: Box[Box[B]] = map(valueA)(f) bb.flatten } }
52. BOXES IN A SEQUENCE
53. FOR-COMPREHENSION val boxA: Box[A] def toBoxB: A => Box[B] def toBoxC: B => Box[C] def toBoxD: C => Box[D] for { a <- boxA b <- toBoxB(a) c <- toBoxC(b) d <- toBoxD(c) } yield d
54. MONAD (AS APPLICATIVE) trait Monad[Box[_]] extends Applicative[Box] { // pure from Applicative def flatMap[A, B](boxA: Box[A])(f: A => Box[B]): Box[B] /******/ def flatten[A](boxBoxA: Box[Box[A]]): Box[A] = flatMap(boxBoxA)(identity) def ap[A, B](boxF: Box[A => B])(boxA: Box[A]): Box[B] = flatMap(boxF)(f => map(boxA)(f)) def map[A, B](boxA: Box[A])(f: A => B): Box[B] = flatMap(boxA)(a => pure(f(a))) }
55. MONAD FOR MAYBE implicit val maybeMonad: Monad[Maybe] = new Monad[Maybe] { def flatMap[A, B](boxA: Maybe[A]) (f: (A) => Maybe[B]): Maybe[B] = boxA match { case Just(a) => f(a) case Empty => Empty } def pure[A](a: A): Maybe[A] = maybeApplicative.pure(a) }
56. FUNCTOR VS ENDOFUNCTOR
57. MONAD IS A MONOID IN THE CATEGORY OF ENDOFUNCTORS
58. MONAD MONOID => pure + flatten ENDOFUNCTORS => map
59. SUMMARY CATEGORY THEORY >> how things compose MONOID >> combining 2 values into 1 FUNCTOR >> values lifted to a context APPLICATIVE >> independent values applied to a function in a context MONAD >> ops in sequence in a context
60. Band BoundedSemilattice CommutativeGroup CommutativeMonoid CommutativeSemigroup Eq Group Semigroup Monoid Order PartialOrder Semilattice Alternative Applicative ApplicativeError Apply Bifoldable Bimonad Bitraverse Cartesian CoflatMap Comonad ContravariantCartesian FlatMap Foldable Functor Inject InvariantMonoidal Monad MonadError MonoidK NotNull Reducible SemigroupK Show ApplicativeAsk Bifunctor Contravariant Invariant Profunctor Strong Traverse Arrow Category Choice Compose Cats Type Classes kernel core/functor core/arrow core The highlighted type classes are the first ones you should learn. They’re well documented and well-known so it’s easy to get help. a |+| b a === b a =!= b a |@| b a *> b a <* b a <+> b a >>> b a <<< b a > b a >= b a < b a <= b Sync Async Effect LiftIO effect Some type classes introduce symbolic operators. NonEmptyTraverse InjectK CommutativeArrow CommutativeFlatMap CommutativeMonad ApplicativeLayer FunctorLayer ApplicativeLayerFunctor FunctorLayerFunctor ApplicativeLocal FunctorEmpty FunctorListen FunctorTell FunctorRaise MonadLayer MonadLayerFunctor MonadLayerControl MonadState TraverseEmpty Functor Applicative Monad Traverse mtl MTL type classes do not extend core type classes directly, but the effect is similar; the dashed line can be read “implies”.
61. FORGET ABOUT THE DETAILS FOCUS ON HOW THINGS COMPOSE
62. WANNA KNOW MORE? > Category Theory for the WH by @PhilipWadler > Category Theory by @BartoszMilewski > Cats-Infographics by Rob Norris - @tpolecat > Cats Documentation - Type Classes
63. THANK YOU! > Twitter: @DanielaSfregola > Blog: danielasfregola.com github.com/DanielaSfregola/tutorial-cat