OptimusPrimeT

Code: https://github.com/larsrh/scalaio2014

Everybody knows monads by now, so a talk about monads would hardly be worthwhile. Let's take it to the next level: monad transformers. We'll learn what they are, how they naturally emerge in your code base and how to make good use of them in Scala – and maybe even how to create your own.

Transcript

1. Op musPrimeT Lars Hupel October 24th, 2014
4. (A ⊗ B) ⊗ TC αA,B,TC  tA⊗B,C // T((A ⊗ B) ⊗ C) T(αA,B,C )  A ⊗ (B ⊗ TC) A⊗tB,C // A ⊗ T(B ⊗ C) tA,B⊗C // T(A ⊗ (B ⊗ C))
7. Agenda 1 Functors, Applica ves, Monads 2 Composi on of Functors 3 Monad Transformers 7
8. Functors, Applica ves, Monads Everyone knows monads, right? But: there’s more. 8
9. If all you have are monads ... Monads have lots of use cases: ▶ error handling ▶ parser combinators ▶ value-oriented parallelism ▶ ... 9
10. Limita ons of monads Code using monads has both mathema cal and prac cal limita ons 10
11. Demo₁
12. Functors from the ground up Functors are about transforming data scala> List(1, 2, 3).map(x => x + 1) List(2, 3, 4) scala> Some(1).map(x => x + 1) Some(2) scala> Future { 1 }.map(x => x + 1) // something resembling 2 12
13. Functors from the ground up Functors are about transforming data scala> List(1, 2, 3).map(x => x + 1) List(2, 3, 4) scala> Some(1).map(x => x + 1) Some(2) scala> Future { 1 }.map(x => x + 1) // something resembling 2 12
14. The Functor class scalaz offers a generaliza on of that pa ern trait Functor[F[_]] { def map[A, B](f: A => B, fa: F[A]): F[B] } 13
15. More power: Applica ves Applica ves are about transforming independent pieces of data scala> List(1, 2) tuple List(3, 4) List((1, 3), (1, 4), (2, 3), (2, 4)) 14
16. The Applicative class trait Applicative[F[_]] { // from Functor def map[A, B] (f: A => B, fa: F[A]): F[B] def map2[A, B, C] (f: (A, B) => C, fa: F[A], fb: F[B]): F[C] } 15
17. Even more power: Monads Monads are about transforming dependent pieces of data def fileRequest(user: User, file: Path): Future[Int] = for { token <- db.getAccessToken(user) input <- db.readFile(token, file) _ <- HTTP.sendHeader(HTTP.OK) written <- HTTP.sendBody(input) } yield written 16
18. The Monad class trait Monad[F[_]] { def flatMap[A, B](f: A => F[B], fa: F[A]): F[B] } 17
19. The Monad class trait Monad[F[_]] { def flatMap[A, B](f: A => F[B], fa: F[A]): F[B] def map2[A, B, C] (f: (A, B) => C, fa: F[A], fb: F[B]) = ??? } 17
20. The Monad class trait Monad[F[_]] { def flatMap[A, B](f: A => F[B], fa: F[A]): F[B] def map2[A, B, C] (f: (A, B) => C, fa: F[A], fb: F[B]) = flatMap(a => flatMap(b => f(a, b), fb), fa) } 17
21. Monad vs Applicative revisited ¸ Demo₂ 18
22. Agenda 1 Functors, Applica ves, Monads 2 Composi on of Functors 3 Monad Transformers 19
23. Programming is ... Abstrac ng! But: non-composable abstrac ons are useless 20
24. Composing functors Input def intToString(x: Int): String val xss: List[List[Int]] Output val yss: List[List[String]] = ??? 21
25. Demo₃
26. But what about monads? Surely we can also compose monads. 23
27. But what about monads? Surely we can also compose monads. Except we can’t 23
28. But what about monads? Surely we can also compose monads. Except we can’t (in general) 23
29. Agenda 1 Functors, Applica ves, Monads 2 Composi on of Functors 3 Monad Transformers 24
30. Composing monads Our Tools // Monad[F] def flatMap[A, B](f: A => F[B], fa: F[A]): F[B] // Monad[G] def flatMap[A, B](f: A => F[B], ga: G[A]): G[B] Our Goal def flatMap[A, B] (f: A => F[G[B]], fga: F[G[A]]): F[G[B]] 25
31. Computer Science Interlude The Hal ng Problem It is undecidable whether a program will terminate on a given input. 26
32. Computer Science Interlude The Hal ng Problem It is undecidable whether a program will terminate on a given input. ... defeated? Yet there are programming languages which only accept termina ng programs. 26
33. Composing par cular monads Our Tools (refined) // Monad[F] def flatMap[A, B](f: A => F[B], fa: F[A]): F[B] Our Goal (refined) def flatMapForOption[A, B] (f: A => F[Option[B]], foa: F[Option[A]]): F[Option[B]] 27
34. Demo₄
35. When do monads transform? It’s difficult. 29
36. Recap ▶ Monads are awesome, but o en too powerful. ▶ Functors compose. ▶ Applica ves compose. ▶ Monads do not compose in general. ▶ Specific monads do compose. ▶ That actually comes up very o en. 30
39. Fin @larsr h larsrh.github.io typelevel.org/blog