Typelevel programming in Scala (with lies)

Transcript

1. Hi! ! My name is @folone https://github.com/folone/london-scala
2. Type level programming in Scala 1 ! https://github.com/folone/london-scala 1 with lies
3. sealed trait Bool { type &&[B <: Bool] <: Bool type ||[B <: Bool] <: Bool type IfElse[T, F] <: Any } trait True extends Bool { type &&[B <: Bool] = B type ||[B <: Bool] = True type IfElse[T, F] = T } trait False extends Bool { type &&[B <: Bool] = False type ||[B <: Bool] = B type IfElse[T, F] = F }
4. // false || true == true // false.||(true) == true implicitly[False # `||` [True] =:= True] // if(true) String else Int // true.ifElse(string, int) == string implicitly[True # IfElse[String, Int] =:= String] /* if(true) { * if(false) Long else String * } else Int * * true.ifElse(false.ifElse(long, string), string) == string */ implicitly[True # IfElse[False # IfElse[Long, String], Int] =:= String]
5. https://github.com/folone/type-level-birds
6. There's a Smalltalk in your Scala — Stefan Zeiger - Type level Computations in Scala - ScalaIO - 2015
7. Peano numbers!
8. trait Nat trait Z extends Nat trait Succ[A <: Nat] extends Nat type _1 = Succ[Z] type _2 = Succ[_1] type _3 = Succ[_2]
9. trait MinusOne[A <: Nat] { type Res <: Nat } object MinusOne { implicit val baseCase: MinusOne[Z] { type Res = Z } = new MinusOne[Z] { type Res = Z } implicit def inductiveCase[A <: Nat]: MinusOne[Succ[A]] { type Res = A } = new MinusOne[Succ[A]] { type Res = A } }
10. trait MinusOne[A <: Nat] { type Res <: Nat } object MinusOne { type Aux[A <: Nat, Res1 <: Nat] = MinusOne[A] { type Res = Res1 } implicit val baseCase: Aux[Z, Z] = new MinusOne[Z] { type Res = Z } implicit def inductiveCase[A <: Nat]: Aux[Succ[A], A] = new MinusOne[Succ[A]] { type Res = A } }
11. trait Plus[A <: Nat, B <: Nat] { type Res <: Nat } object Plus { implicit def baseCase[A <: Nat]: Plus[A, Z] { type Res = A } = new Plus[A, Z] { type Res = A } implicit def inductiveCase[A <: Nat, B <: Nat, C <: Nat, D <: Nat] (implicit ev0: MinusOne[B] { type Res = C }, ev1: Plus[Succ[A], C] { type Res = D }): Plus[A, B] { type Res = D } = new Plus[A, B] { type Res = D } }
12. implicit def inductiveCase[A <: Nat, B <: Nat, C <: Nat, D <: Nat] (implicit ev0: MinusOne[B] { type Res = C }, ev1: Plus[Succ[A], C] { type Res = D }): Plus[A, B] { type Res = D } = new Plus[A, B] { type Res = D }
13. trait Plus[A <: Nat, B <: Nat] { type Res <: Nat } object Plus { type Aux[A <: Nat, B <: Nat, Res1 <: Nat] = Plus[A, B] { type Res = Res1 } implicit def baseCase[A <: Nat]: Aux[A, Z, A] = new Plus[A, Z] { type Res = A } implicit def inductiveCase[A <: Nat, B <: Nat, C <: Nat, D <: Nat] (implicit ev0: MinusOne.Aux[B, C], ev1: Plus.Aux[Succ[A], C, D]): Aux[A, B, D] = new Plus[A, B] { type Res = D } }
14. type _1 = Succ[Z] type _2 = Succ[_1] type _3 = Succ[_2] @ implicitly[Plus[_1, _2]] res0: Plus[_1, _2] { type Res = _3 } = Plus$$anon$2@f79a760
15. @ implicitly[Plus[_1, _2]] res0: Plus[_1, _2] = Plus$$anon$2@13c3c1e1 @ implicitly[Plus[_1, _2]#Res =:= _3] res1: =:=[Plus[_1, _2]#Res ,_3] = <function1>
16. @ implicitly[Plus.Aux[_1, _2, _3]] res0: Plus.Aux[_1, _2, _3] = Plus.Aux$$anon$2@3d08f3f5 @ implicitly[Plus.Aux[_1, _2, Z]] <console>:15: error: could not find implicit value for parameter e: Plus.Aux[_1, _2, Z] implicitly[Plus.Aux[_1, _2, Z]] ^
17. HLists!
18. trait Nat trait Z extends Nat trait Succ[A <: Nat] extends Nat trait HList trait HNil extends HList trait HCons[A, T <: HList] extends HList
19. trait Length[L <: HList] { type Res <: Nat } object Length { implicit val baseCase: Length[HNil.type] { type Res = Z } = new Length[HNil.type] { type Res = Z } implicit def inductiveCase[H, T <: HList, N <: Nat] (implicit ev0: Length[T] { type Res = N }) = new Length[HCons[H, T]] { type Res = Succ[N] } }
20. trait Length[L <: HList] { type Res <: Nat } object Length { type Aux[L <: HList, Res1 <: Nat] = Length[L] { type Res = Res1 } implicit val baseCase: Aux[HNil.type, Z] = new Length[HNil.type] { type Res = Z } implicit def inductiveCase[H, T <: HList, N <: Nat] (implicit ev0: Length.Aux[T, N]) = new Length[HCons[H, T]] { type Res = Succ[N] } }
21. Shapeless! https://github.com/milessabin/shapeless
22. @ import shapeless._ import shapeless._ @ val hlist = 1l :: "hello" :: HNil hlist: Long :: String :: HNil = 1 :: hello :: HNil
23. @ hlist(0) res7: Long = 1 @ hlist(1) res8: String = hello @ hlist(2) <console>:16: error: Implicit not found: Scary[Type].Please#Ignore You requested to access an element at the position TypelevelEncodingFor[2.type] but the HList Long :: String :: HNil is too short. hlist(2) ^ Compilation failed.
24. There's a Prolog in your Scala -- ScalaIO 2015
25. Json serialization
26. def json(o: Any): String
27. def json(o: Any): JsonAst
28. def json[A : Writes](o: A): JsonAst trait Writes[A] { def write(o: A): JsonAst }
29. @ import play.api.libs.json.{Json => PJson} import play.api.libs.json.{Json => PJson} @ case class Thing(id: Long, payload: String) defined class Thing @ PJson.writes[Thing] res0: Writes[Thing] = Writes$$anon$2@f79a760
30. @ case class Omg(_1: Int, _2: Int, _3: Int, _4: Int, _5: Int, _6: Int, _7: Int, _8: Int, _9: Int, _10: Int, _11: Int, _12: Int, _13: Int, _14: Int, _15: Int, _16: Int, _17: Int, _18: Int, _19: Int, _20: Int, _21: Int, _22: Int, _23: Int) defined class Omg @ PJson.writes[Omg] cmd9.sc:1: No unapply or unapplySeq function found for class Omg. val res9 = PJson.writes[Omg] ^ Compilation Failed
31. ! >700LOC of macro !
32. Thing(id: Long, payload: String)
33. (Long, String)
34. Long :: String :: HNil
35. 1. Case classes are essentially tuples (or HLists) with names. Can we transform one to another? 2. Define Writes for an HList: 1. How do we serialize an HNil? 2. How do we serialize an HCons? 3. !
36. Two (mandatory) building blocks • HLists • Generic2 2 lie: what we actually need is a LabelledGeneric
37. HList @ import shapeless._ import shapeless._ @ val hlist = 1l :: "hello" :: HNil hlist: Long :: String :: HNil = 1 :: hello :: HNil
38. @ hlist(0) res7: Long = 1 @ hlist(1) res8: String = hello @ hlist(2) <console>:16: error: Implicit not found: Scary[Type].Please#Ignore You requested to access an element at the position TypelevelEncodingFor[2.type] but the HList Long :: String :: HNil is too short. hlist(2) ^ Compilation failed.
39. Generic @ case class Thing(id: Long, payload: String) defined class Thing @ val generic = Generic[Thing] generic: shapeless.Generic[Thing]{type Repr = Long :: String :: HNil} = anon$macro$3$1@7f8f5e52
40. @ val representation = generic.to(Thing(1, "hello")) representation: res0.Repr = 1 :: hello :: HNil @ representation(0) res10: Long = 1 @ representation(1) res11: String = hello @ representation(2) <console>:19: error: Implicit not found: Scary[Type].Please#Ignore You requested to access an element at the position TypelevelEncodingFor[2.type] but the HList Long :: String :: HNil is too short. representation(2) ^
41. @ generic.from(hlist) res7: Thing = Thing(1L, "hello")
42. Putting this together 1. Let shapeless transform our case classes into some unified form (HLists) 2. Write down instances of the Writes typeclass for those forms in a generic way (HNil, HCons) 3. Let the magic flow !
43. object writes extends LabelledProductTypeClassCompanion[Writes] with DefaultWrites { object typeClass extends LabelledProductTypeClass[Writes] { override def emptyProduct: Writes[HNil] = Writes(_ => PlayJson.obj()) override def product[H, T <: HList](name: String, headEv: Writes[H], tailEv: Writes[T]) = Writes[H :: T] { case head :: tail => val h = headEv.writes(head) val t = tailEv.writes(tail) PlayJson.obj(name -> h) ++ t } override def project[F, G](instance: => Writes[G], to: F => G, from: G => F) = Writes[F](f => instance.writes(to(f))) } }
44. object writes extends LabelledProductTypeClassCompanion[Writes] with DefaultWrites { object typeClass extends LabelledProductTypeClass[Writes] { override def emptyProduct: Writes[HNil] = Writes(_ => PlayJson.obj()) override def product[H, T <: HList](name: String, headEv: Writes[H], tailEv: Writes[T]) = Writes[H :: T] { case head :: tail => val h = headEv.writes(head) val t = tailEv.writes(tail) PlayJson.obj(name -> h) ++ t } override def project[F, G](instance: => Writes[G], to: F => G, from: G => F) = Writes[F](f => instance.writes(to(f))) } }
45. object writes extends LabelledProductTypeClassCompanion[Writes] with DefaultWrites { object typeClass extends LabelledProductTypeClass[Writes] { override def emptyProduct: Writes[HNil] = Writes(_ => PlayJson.obj()) override def product[H, T <: HList](name: String, headEv: Writes[H], tailEv: Writes[T]) = Writes[H :: T] { case head :: tail => val h = headEv.writes(head) val t = tailEv.writes(tail) PlayJson.obj(name -> h) ++ t } override def project[F, G](instance: => Writes[G], to: F => G, from: G => F) = Writes[F](f => instance.writes(to(f))) } }
46. object writes extends LabelledProductTypeClassCompanion[Writes] with DefaultWrites { object typeClass extends LabelledProductTypeClass[Writes] { override def emptyProduct: Writes[HNil] = Writes(_ => PlayJson.obj()) override def product[H, T <: HList](name: String, headEv: Writes[H], tailEv: Writes[T]) = Writes[H :: T] { case head :: tail => val h = headEv.writes(head) val t = tailEv.writes(tail) PlayJson.obj(name -> h) ++ t } override def project[F, G](instance: => Writes[G], to: F => G, from: G => F) = Writes[F](f => instance.writes(to(f))) } }
47. object writes extends LabelledProductTypeClassCompanion[Writes] with DefaultWrites { object typeClass extends LabelledProductTypeClass[Writes] { override def emptyProduct: Writes[HNil] = Writes(_ => PlayJson.obj()) override def product[H, T <: HList](name: String, headEv: Writes[H], tailEv: Writes[T]) = Writes[H :: T] { case head :: tail => val h = headEv.writes(head) val t = tailEv.writes(tail) PlayJson.obj(name -> h) ++ t } override def project[F, G](instance: => Writes[G], to: F => G, from: G => F) = Writes[F](f => instance.writes(to(f))) } }
48. By @davegurnell
49. -? https://github.com/folone/london-scala @folone