42: Rise of the dependent types

Presented at the Scala eXchange 2014.

DEMO: https://github.com/folone/2014-scalaeXchange
Video: https://skillsmatter.com/skillscasts/5946-42-rise-of-the-dependent-types#video

1. @folone @xeno_by @adriaanm @retronym @extempore2
2. About me: @folone likes types* *same reasons why @propensive does in his “Batshit crazy algebra with types” talk
3. 12 hours of audio every minute 300 million people every month
4. Stop… Hackertime!
5. 42.type $ git clone git@github.com:folone/scala.git -b scalaexchange $ ant publish-local-opt -Dmaven.version.suffix="-typelevel"
6. 1. Ranged
7. 2. Residue
8. 3. Records
9. Consistency
10. Here’s what you can do in Scala scala> val x = "panda!" x: String = panda! scala> val t: x.type = x t: x.type = panda! scala> final val k = "panda!" k: String("panda!") = panda!
11. Here’s what you cannot scala> val t: "panda!".type = "panda!" <console>:1: error: identifier expected but string literal found. val t: "panda!".type = "panda!" ^
12. scala> val t: "panda!".type = "panda!" t: "panda!".type = panda! With 42.type
13. scala> val x = 42 x: Int = 42 scala> val t: x.type = 42 <console>:8: error: type mismatch; found : x.type (with underlying type Int) required: AnyRef val t: x.type = 42 ^ scala> val workaround = Witness(42) workaround: shapeless.Witness{type T = Int(42)} = fresh$macro$3$1@35b45d3f scala> val t: workaround.T = 42 t: workaround.T = 42 Even more
14. scala> val t: 42.type = 42 t: 42.type = 42 scala> val t: 42 = 42 t: 42.type = 42 Or even Solution with 42.type
15. SimpleType ::= Path ‘.’ type A singleton type is of the form p.type, where p is a path pointing to a value expected to conform to scala.AnyRef. Before
16. After | Literal [‘.’ type] SimpleType ::= Path ‘.’ type
17. History
18. Quiz Literal-based singleton types in scala have seen their first implementation in: 2013 2011 2008 2004
19. http://existentialtype.net/2008/07/21/literally-dependent-types/ Quiz Literal-based singleton types in scala have seen their first implementation in: 2013 2011 2008 2004
21. State of deptypes Scala “Scala vs Idris: Dependent Types, Now and in the Future”* *Edwin Brady & Miles Sabin, Strange Loop 2013
22. How can we make it better? Well, we can try. Scala + SMT-solver
23. You would do way better to implement the [typechecking] logic completely externally to the compiler and use it as a black box from the compiler, then you might have a chance.
24. scala> import z3.scala._, z3.scala.dsl._ import z3.scala._ import z3.scala.dsl._
25. scala> findAll((x: Val[Int]) => x > 23 && x < 42).toList res0: List[Int] = List(24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41)
26. ((x: Val[Int]) => x > 23 && x < 42)
27. ((x: Int) => x > 23 && x < 42)
28. ((x: Int) => x > 23 && x < 42).type
29. val magic: ((x: Int) => x > 23 && x < 42).type = 30
30. val magic: ((x: Int) => x > 23 && x < 42).type = 30 val x: 42 = 42 val x: ((x: Int) => x == 42).type = 42
31. val x: 42 = 42 val x: ((x: Int) => x == 42).type = 42 val x: Int = 42 val x: ((x: Int) => true).type = 42 val magic: ((x: Int) => x > 23 && x < 42).type = 30
32. But <: ((x: Int) => x > 10).type ((x: Int) => x > 23).type is
33. Liquid Types
34. L og i cally qu al i fie d Types
35. bit.ly/42_type
36. this thing would not be possible without: @xeno_by, @adriaanm, @retronym, and initial impl by @paulp Scala Z3 bindings: LARA@EPFL Z3: Microsoft research slides, illustrations: @killnicole Credits:
38. Thanks. Questions? folone@soundcloud.com