Seven at one blow – New & polished features in Scalaz 7

My talk at the Northeast Scala Symposium 2013, featuring: À-la-carte implicits, Unapply, Law checking, Type-level programming, and Community involvement.

Slides are prepared with LaTeX beamer/XeLaTeX, fonts: Roboto and DejaVu Sans Mono, code highlighting: minted/pygments.

Transcript

1. Seven at one blow New & polished features in Scalaz 7 Lars Hupel Februrary 8th, 2013
2. À-la-carte implicits Unapply Laws Type-level programming Community involvement About @larsr h ▶ studies Computer Science ▶ first contribution December 2011 ▶ committer since February 2012 ▶ active (co)maintainer since October 2012 ▶ did not invent any of the following Lars Hupel Seven at one blow Februrary 8th, 2013 2 / 35
3. À-la-carte implicits Unapply Laws Type-level programming Community involvement About Scalaz Scalaz is a Scala library for functional programming. ▶ purely functional data structures ▶ type classes known from Haskell ▶ current stable version: 6.0.4 ▶ successor under development for over a year compatible with 2.9.x, 2.10.x (and probably 2.11.x) Lars Hupel Seven at one blow Februrary 8th, 2013 3 / 35
4. À-la-carte implicits Unapply Laws Type-level programming Community involvement À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 4 / 35
5. À-la-carte implicits Unapply Laws Type-level programming Community involvement À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 5 / 35
6. À-la-carte implicits Unapply Laws Type-level programming Community involvement Previously, in Scalaz 6 all-you-can-eat object provides most of the implicits Lars Hupel Seven at one blow Februrary 8th, 2013 6 / 35
7. À-la-carte implicits Unapply Laws Type-level programming Community involvement Previously, in Scalaz 6 Example // import everything import scalaz._ import Scalaz._ // get all instances implicitly[Monad[Option]] // get all syntax (10.some |@| none) { (_: Int) |+| (_: Int) } Lars Hupel Seven at one blow Februrary 8th, 2013 7 / 35
8. À-la-carte implicits Unapply Laws Type-level programming Community involvement Now, in Scalaz 7 Distinction between different kinds of implicits 1. type class instances ▶ ... of our own data types ▶ ... of standard library types 2. syntax: infix operators ▶ ... for type class instances ▶ ... for standard library types 3. functions on our own data types Lars Hupel Seven at one blow Februrary 8th, 2013 8 / 35
9. À-la-carte implicits Unapply Laws Type-level programming Community involvement Now, in Scalaz 7 Example import scalaz._ { // import the instance import std.option._ Monad[Option] } Lars Hupel Seven at one blow Februrary 8th, 2013 9 / 35
10. À-la-carte implicits Unapply Laws Type-level programming Community involvement Now, in Scalaz 7 Example import scalaz._ { import std.option._, std.anyVal._ import syntax.apply._ import syntax.semigroup._ import syntax.std.option._ (10.some |@| none) { (_: Int) |+| (_: Int) } } Lars Hupel Seven at one blow Februrary 8th, 2013 9 / 35
11. À-la-carte implicits Unapply Laws Type-level programming Community involvement Now, in Scalaz 7 Example import scalaz._ { import std.option._, std.anyVal._ import syntax.all._, syntax.std.all._ (10.some |@| none) { (_: Int) |+| (_: Int) } } Lars Hupel Seven at one blow Februrary 8th, 2013 9 / 35
12. À-la-carte implicits Unapply Laws Type-level programming Community involvement Why is this better? Disadvantages ▶ more import statements ▶ makes compile-time debugging harder Lars Hupel Seven at one blow Februrary 8th, 2013 10 / 35
13. À-la-carte implicits Unapply Laws Type-level programming Community involvement Why is this better? Advantages ▶ you can use syntax, but you don’t have to ▶ improves readability of the code ▶ makes compile-time debugging easier ▶ increases compilation speed Lars Hupel Seven at one blow Februrary 8th, 2013 10 / 35
14. À-la-carte implicits Unapply Laws Type-level programming Community involvement À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 11 / 35
15. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem def f[M[_], A](x: M[A]): M[A] = x f { (x: Int) => x } Lars Hupel Seven at one blow Februrary 8th, 2013 12 / 35
16. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem def f[M[_], A](x: M[A]): M[A] = x f { (x: Int) => x } Prints a type error: found : Int => Int required: ?M[?A] Lars Hupel Seven at one blow Februrary 8th, 2013 12 / 35
17. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem Candidates ▶ M[X] = Int ⇒ X ▶ M[X] = X ⇒ Int ▶ M[X] = X ⇒ X ▶ M[X] = Int ⇒ Int ▶ ... This is a known limitation (SI-2712). Lars Hupel Seven at one blow Februrary 8th, 2013 13 / 35
18. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem Candidates ▶ M[X] = Int ⇒ X ▶ M[X] = X ⇒ Int ▶ M[X] = X ⇒ X ▶ M[X] = Int ⇒ Int ▶ ... This is a known limitation (SI-2712). Lars Hupel Seven at one blow Februrary 8th, 2013 13 / 35
19. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem Candidates ▶ M[X] = Int ⇒ X ▶ M[X] = X ⇒ Int ▶ M[X] = X ⇒ X ▶ M[X] = Int ⇒ Int ▶ ... This is a known limitation (SI-2712). Lars Hupel Seven at one blow Februrary 8th, 2013 13 / 35
20. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem Candidates ▶ M[X] = Int ⇒ X ▶ M[X] = X ⇒ Int ▶ M[X] = X ⇒ X ▶ M[X] = Int ⇒ Int ▶ ... This is a known limitation (SI-2712). Lars Hupel Seven at one blow Februrary 8th, 2013 13 / 35
21. À-la-carte implicits Unapply Laws Type-level programming Community involvement The problem Candidates ▶ M[X] = Int ⇒ X ▶ M[X] = X ⇒ Int ▶ M[X] = X ⇒ X ▶ M[X] = Int ⇒ Int ▶ ... This is a known limitation (SI-2712). Lars Hupel Seven at one blow Februrary 8th, 2013 13 / 35
22. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
23. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
24. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
25. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
26. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
27. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
28. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s use math! ▶ known as Higher-order unification ▶ Scala adds subtyping ▶ everything is subtype of Any ▶ Any is always well-kinded ▶ ambiguous ▶ undecidable Lars Hupel Seven at one blow Februrary 8th, 2013 14 / 35
29. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s approximate! def f[M[_], A](x: M[A]): M[A] = x f { (x: Int) => x } A type constructor without a type class is rarely useful. Lars Hupel Seven at one blow Februrary 8th, 2013 15 / 35
30. À-la-carte implicits Unapply Laws Type-level programming Community involvement Let’s approximate! def f[M[_] : Monad, A](x: M[A]): M[B] = ??? f { (x: Int) => x } Now: more information which can guide type inference Lars Hupel Seven at one blow Februrary 8th, 2013 15 / 35
31. À-la-carte implicits Unapply Laws Type-level programming Community involvement Dependent types to the rescue Define a proof object containing: ▶ the type constructor M[_] ▶ the inner type A ▶ the type class instance M[_] : Monad ... and have lots of implicits guided by: ▶ prioritisation ▶ availability of type class instances Lars Hupel Seven at one blow Februrary 8th, 2013 16 / 35
32. À-la-carte implicits Unapply Laws Type-level programming Community involvement Dependent types to the rescue Define a proof object containing: ▶ the type constructor M[_] ▶ the inner type A ▶ the type class instance M[_] : Monad ... and have lots of implicits guided by: ▶ prioritisation ▶ availability of type class instances Lars Hupel Seven at one blow Februrary 8th, 2013 16 / 35
33. Demo
34. À-la-carte implicits Unapply Laws Type-level programming Community involvement Does it work? Yes! ▶ Scalaz 6: conversions for each concrete type ▶ now: general mechanism for common shapes ▶ in many cases, type parameters can be omitted List(1, 2, 3) traverseU { a => State { x: Int => (x + 1, a) } } Lars Hupel Seven at one blow Februrary 8th, 2013 18 / 35
35. À-la-carte implicits Unapply Laws Type-level programming Community involvement Does it work? Well, sort of. ▶ 500 lines of boilerplate ▶ one implicit per type shape (there are infinitely many) ▶ compile errors can get a little funky ▶ needs dependent types Lars Hupel Seven at one blow Februrary 8th, 2013 18 / 35
36. À-la-carte implicits Unapply Laws Type-level programming Community involvement À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 19 / 35
37. À-la-carte implicits Unapply Laws Type-level programming Community involvement Contracts . . “ The equals method implements an equivalence relation on non-null object references: ▶ It is reflexive: for any non-null reference value x, x.equals(x) should return true. ▶ It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. ▶ ... Java API: Object#equals . . ” Lars Hupel Seven at one blow Februrary 8th, 2013 20 / 35
38. À-la-carte implicits Unapply Laws Type-level programming Community involvement Type classes & Lavs ▶ type classes abstract over types ▶ offer common functionality ▶ users expect similar behaviour ▶ ... which needs to be formalised Lars Hupel Seven at one blow Februrary 8th, 2013 21 / 35
39. À-la-carte implicits Unapply Laws Type-level programming Community involvement Type classes & Lavs ▶ type classes abstract over types ▶ offer common functionality ▶ users expect similar behaviour ▶ ... which needs to be formalised Lars Hupel Seven at one blow Februrary 8th, 2013 21 / 35
40. À-la-carte implicits Unapply Laws Type-level programming Community involvement Type classes & Lavs ▶ type classes abstract over types ▶ offer common functionality ▶ users expect similar behaviour ▶ ... which needs to be formalised Lars Hupel Seven at one blow Februrary 8th, 2013 21 / 35
41. À-la-carte implicits Unapply Laws Type-level programming Community involvement Legal problems . . “ I have a legal question: Is this a lawful monoid to you? Tony Morris . . ” Lars Hupel Seven at one blow Februrary 8th, 2013 22 / 35
42. À-la-carte implicits Unapply Laws Type-level programming Community involvement Laws in Scalaz ▶ embedded in type classes, mirroring their hierarchy ▶ one method per law ▶ ScalaCheck tests instances ▶ ... by producing random input ▶ Not a theorem prover! ▶ ... but better than usual unit tests ▶ our bindings can be used by other projects ▶ 332 occurences of checkAll in our tests Lars Hupel Seven at one blow Februrary 8th, 2013 23 / 35
43. Demo
44. À-la-carte implicits Unapply Laws Type-level programming Community involvement À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 25 / 35
45. À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 26 / 35
46. À-la-carte implicits Unapply Laws Type-level programming Community involvement It’s in the types Scalaz has heterogeneous lists too! ▶ ... but not as general as in Shapeless ▶ used for: ▶ Applicative syntax ▶ String formatting ▶ type class composition Lars Hupel Seven at one blow Februrary 8th, 2013 27 / 35
47. À-la-carte implicits Unapply Laws Type-level programming Community involvement Heterogeneous lists Everybody loves tuples. ▶ fixed arity, different element types ▶ maximum arity 22, one class per arity ▶ but: no useful manipulations (reverse, append, access, ...) Lars Hupel Seven at one blow Februrary 8th, 2013 28 / 35
48. Demo
49. À-la-carte implicits Unapply Laws Type-level programming Community involvement À-la-carte implicits Unapply Laws Type-level programming Community involvement Lars Hupel Seven at one blow Februrary 8th, 2013 30 / 35
50. À-la-carte implicits Unapply Laws Type-level programming Community involvement What happened in the past year? We got tons of contributions (≈ 200 issues & pull requests). ▶ folding non-empty data structures ▶ Lens families ▶ documentation & test fixes ▶ interoperability with other libraries ▶ new data types ▶ less lawless classes ▶ ... Lars Hupel Seven at one blow Februrary 8th, 2013 31 / 35
51. À-la-carte implicits Unapply Laws Type-level programming Community involvement Validation DSL check(”123456789012345”). checkThat(strLength(16, ”wrong length”)). checkThat(luhn(”wrong checksum”)). convertTo(int(”not an integer”)). toValidation // Result: // Failure(NonEmptyList(wrong length, wrong checksum)) Lars Hupel Seven at one blow Februrary 8th, 2013 32 / 35
52. À-la-carte implicits Unapply Laws Type-level programming Community involvement What can you do? ... if you’re using Scalaz 6 ▶ migrate to the latest milestone ▶ Tell us about it! ... if you’re not using Scalaz ▶ adopt Scalaz 7 ▶ Ask questions! Lars Hupel Seven at one blow Februrary 8th, 2013 33 / 35
53. À-la-carte implicits Unapply Laws Type-level programming Community involvement What are we going to do? ▶ release 7.0.0-final soon ... probably this year ▶ not break too much until then ▶ stay binary compatible in the 7.0.x series ▶ revamp our web site(s) Lars Hupel Seven at one blow Februrary 8th, 2013 34 / 35
54. Q & A larsrh.github.com @larsr h