flatMap Oslo 2019 presentation

1. Functors, in theory
and in practice !

2. Me, myself and I
●
●
●
●
●
●

3. Agenda
●
●
●
●
●

4. An intuition

5. An intuition
-

6. In practice ...

7. A common pattern
def isMagic(x: Int): Boolean = x == 42

8. A common pattern
val intO: Option[Int] = Some(42)
val res: Option[Boolean] = map(intO, isMagic)

9. A common pattern
val intL: List[Int] = List(1, 42, 99)
val res: List[Boolean] = map(intL, isMagic)

10. A common pattern
case class Container[A](value: A)
val intC: Container[Int] = Container(42)
val res: Container[Boolean] = map(intC, isMagic)

11. A common pattern
def map[F[_], A, B](ca: F[A], f: A => B): F[B]
F[A], (A => B) => F[B]

12. A common pattern

13. A type class - Definition

14. A type class - Definition

15. A type class - An exemple
trait Show[A] {
def print(a: A): String
}

16. A type class - An exemple
implicit val intShow: Show[Int] = new Show[Int] {
def show(a: Int): String = s"I'm an Int: $a"
}

17. A type class - Functor
trait Functor[F[_]] {
def map[A, B](fa: F[A], fab: A => B): F[B]
}

18. A type class - Type Constructors

19. A type class - Functor
trait Functor[F[_]] {
def map[A, B](fa: F[A], fab: A => B): F[B]
}

20. A type class - Functor
implicit val cFunctor = new Functor[Container] {
def map[A, B](fa: Container[A], fab: A => B): Container[B] =
Container(fab(fa.value))
}

21. A type class - Functor

22. … in theory ...

23. Category theory

24. Category theory

25. Category theory
-

26. Category theory

27. How does it relate to Scala ?

28. How does it relate to Scala ?

29. How does it relate to Scala ?
def identity[A](a: A): A = a

30. How does it relate to Scala ?

31. Back to our functors !

32. Back to our functors !

33. Back to our functors !
Objects dans S (Scal) Objects in S’ (Scal too)
A Option[A]
Int Option[Int]
String Option[String]

34. Back to our functors !

35. Back to our functors !
Morphisms of S (Scal) Morphisms of S’ (Scal too)
A → A (identity) Option[A] → Option[A]
A → B Option[A] → Option[B]
Int → String Option[Int] → Option[String]

36. … and back to
practice again !

37. What does it bring us ?
def cantDoMore[F[_]: Functor, A](fa: F[A]) = fa.map(...)

38. What does it bring us ?
-

39. Conclusion

40. Functors
-

41. Further reading

42. Thanks !
(easy) Questions ?