Typelevel's path to Scala 3

Scala 3 is just around the corner. Both EPFL and the broader community have put a lot of work into making this Scala release a success. In this talk, I will outline some of the history behind Scala 3 and what steps we took in the Typelevel ecosystem to get ready for Scala 3, and keep up with its development.

Transcript

1. T y p e l e v e l ’s
   p a t h t o
   S c a l a 3
   L a r s H u p e l
   S c a l a L o v e i n t h e C i t y
   2 0 2 1 - 0 2 - 1 3
   

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2. T y p e l e v e l
   

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3. View Slide

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5. T y p e l e v e l
   F o u n d e d i n 2 0 1 3 a t
   N o r t h e a s t S c a l a S y m p o s i u m
   

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6. T y p e l e v e l
   F o u n d e d i n 2 0 1 3 a t
   N o r t h e a s t S c a l a S y m p o s i u m
   T o d a y : 7 0 + p r o j e c t s ,
   v i b r a n t e c o s y s t e m
   

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7. T y p e l e v e l p r o j e c t s
   C e n t r a l t h e m e : S c a l a - i d i o m a t i c F u n c t i o n a l P r o g r a m m i n g
   

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8. T y p e l e v e l p r o j e c t s
   C e n t r a l t h e m e : S c a l a - i d i o m a t i c F u n c t i o n a l P r o g r a m m i n g
   • . . . w i t h a s l i t t l e h a s s l e a s p o s s i b l e
   

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9. T y p e l e v e l p r o j e c t s
   C e n t r a l t h e m e : S c a l a - i d i o m a t i c F u n c t i o n a l P r o g r a m m i n g
   • . . . w i t h a s l i t t l e h a s s l e a s p o s s i b l e
   • . . . w i t h a s l i t t l e r u n t i m e o v e r h e a d a s p o s s i b l e
   

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10. T y p e l e v e l p r o j e c t s
    C e n t r a l t h e m e : S c a l a - i d i o m a t i c F u n c t i o n a l P r o g r a m m i n g
    • . . . w i t h a s l i t t l e h a s s l e a s p o s s i b l e
    • . . . w i t h a s l i t t l e r u n t i m e o v e r h e a d a s p o s s i b l e
    • . . . a s s a f e a s p o s s i b l e
    

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11. T y p e c l a s s e s
    

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12. T y p e c l a s s e s
    S u p r e m e l y u s e f u l t o o l , p i o n e e r e d i n H a s k e l l
    

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13. T y p e c l a s s e s
    S u p r e m e l y u s e f u l t o o l , p i o n e e r e d i n H a s k e l l
    class Semigroup a => Monoid a where
    mempty :: a
    mconcat :: [a] -> a
    mconcat = foldr mappend mempty
    

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14. T y p e c l a s s e s
    S u p r e m e l y u s e f u l t o o l , p i o n e e r e d i n H a s k e l l
    class Semigroup a => Monoid a where
    mempty :: a
    mconcat :: [a] -> a
    mconcat = foldr mappend mempty
    I t J u s t W o r k s ™ !
    

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17. T y p e c l a s s e s i n S c a l a
    . . . n o w w e j u s t n e e d t o e n c o d e t h e m i n S c a l a
    

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18. T y p e c l a s s e s i n S c a l a
    . . . n o w w e j u s t n e e d t o e n c o d e t h e m i n S c a l a
    • m u l t i p l e i n h e r i t a n c e ?
    

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19. T y p e c l a s s e s i n S c a l a
    . . . n o w w e j u s t n e e d t o e n c o d e t h e m i n S c a l a
    • m u l t i p l e i n h e r i t a n c e ?
    • s y n t a x ? ?
    

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20. T y p e c l a s s e s i n S c a l a
    . . . n o w w e j u s t n e e d t o e n c o d e t h e m i n S c a l a
    • m u l t i p l e i n h e r i t a n c e ?
    • s y n t a x ? ?
    • g l o b a l c o n f l u e n c e ? ? ?
    

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21. T y p e c l a s s e s i n S c a l a
    . . . n o w w e j u s t n e e d t o e n c o d e t h e m i n S c a l a
    • m u l t i p l e i n h e r i t a n c e ?
    • s y n t a x ? ?
    • g l o b a l c o n f l u e n c e ? ? ?
    

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22. View Slide

23. T h e r i s e o f t h e m a c r o s
    

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24. T y p e c l a s s e s , e n c o d e d
    I n 2 0 1 5 , M i c h a e l P i l q u i s t s t a r t e d s i m u l a c r u m .
    G o a l : c o n s i s t e n t e n c o d i n g a c r o s s d i f f e r e n t p r o j e c t s , 0 b o i l e r p l a t e
    

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25. S i m u l a c r u m
    I n p u t
    import simulacrum._
    @typeclass trait Semigroup[A] {
    @op("|+|") def append(x: A, y: A): A
    }
    

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26. S i m u l a c r u m
    O u t p u t
    object Semigroup {
    def apply[A](implicit instance: Semigroup[A]): Semigroup[A] = instance
    // ...
    }
    

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27. S i m u l a c r u m
    M o r e o u t p u t
    object Semigroup {
    trait Ops[A] {
    def typeClassInstance: Semigroup[A]
    def self: A
    def |+|(y: A): A = typeClassInstance.append(self, y)
    }
    }
    

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28. S i m u l a c r u m
    E v e n m o r e o u t p u t
    object Semigroup {
    trait ToSemigroupOps {
    implicit def toSemigroupOps[A](target: A)(implicit tc: Semigroup[A]): Ops[A]
    val self = target
    val typeClassInstance = tc
    }
    }
    object nonInheritedOps extends ToSemigroupOps
    }
    

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29. S i m u l a c r u m
    Y e t m o r e o u t p u t
    object Semigroup {
    trait AllOps[A] extends Ops[A] {
    def typeClassInstance: Semigroup[A]
    }
    object ops {
    implicit def toAllSemigroupOps[A](target: A)(implicit tc: Semigroup[A]): AllO
    val self = target
    val typeClassInstance = tc
    }
    }
    }
    

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31. B u t i t w o r k s !
    S i m u l a c r u m s o l v e d a t o n o f i s s u e s
    W e c a n w r i t e x |+| y!
    

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32. B u t i t w o r k s !
    S i m u l a c r u m s o l v e d a t o n o f i s s u e s
    W e c a n w r i t e x |+| y!
    U s e d b y C a t s a n d t o n s o f t h i r d - p a r t y
    l i b r a r i e s
    

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33. W e ’ r e n o t d o n e y e t
    S i m u l a c r u m d i d n ’ t s o l v e t h e p e r f o r m a n c e i s s u e .
    

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34. W e ’ r e n o t d o n e y e t
    S i m u l a c r u m d i d n ’ t s o l v e t h e p e r f o r m a n c e i s s u e .
    I n p u t
    x |+| y
    

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35. W e ’ r e n o t d o n e y e t
    S i m u l a c r u m d i d n ’ t s o l v e t h e p e r f o r m a n c e i s s u e .
    O u t p u t
    Semigroup.ops.toAllSemigroupOps(x).|+|(y)
    

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36. E n t e r M a c h i n i s t
    S p l i t o u t o f S p i r e b y E r i k O s h e i m i n 2 0 1 4
    

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37. E n t e r M a c h i n i s t
    S p l i t o u t o f S p i r e b y E r i k O s h e i m i n 2 0 1 4
    N o w ( 2 0 2 0 ) a r c h i v e d a n d r e - i n c o r p o r a t e d i n t o S p i r e
    

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39. T y p e c o n s t r u c t o r s
    

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40. T y p e c o n s t r u c t o r p o l y m o r p h i s m
    case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
    // ...
    }
    

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41. T y p e c o n s t r u c t o r p o l y m o r p h i s m
    case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
    // ...
    }
    S c a l a i s t h e o n l y l a n g u a g e t h a t c a n d o t h a t ! *
    

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42. T y p e c o n s t r u c t o r p o l y m o r p h i s m
    L a n d e d i n S c a l a 2 . 5 b y A d r i a a n M o o r s ( 2 0 0 7 )
    

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43. T y p e c o n s t r u c t o r p o l y m o r p h i s m
    L a n d e d i n S c a l a 2 . 5 b y A d r i a a n M o o r s ( 2 0 0 7 )
    C o m p l e t e g a m e - c h a n g e r
    

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44. “ A s s o o n a s y o u g i v e S c a l a p r o g r a m m e r s a n e w t o y ,
    t h e y w i l l s t a r t a b u s i n g i t i n w a y s y o u c a n ’ t i m a g i n e .
    ”
    – a n c i e n t S c a l a p r o v e r b
    

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45. S c a l a i s n ’ t H a s k e l l
    H a s k e l l
    instance Monad (Either a) where
    {- ... -}
    

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46. S c a l a i s n ’ t H a s k e l l
    H a s k e l l
    instance Monad (Either a) where
    {- ... -}
    S c a l a
    implicit val eitherMonad[A]: Monad[({ type λ[β] = Either[A, β] })#λ] =
    // ...
    

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47. View Slide

48. k i n d - p r o j e c t o r
    

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49. k i n d - p r o j e c t o r
    E v e n m a c r o s c a n ’ t f i x m i s s i n g s y n t a x . *
    W e n e e d a c o m p i l e r p l u g i n !
    

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50. k i n d - p r o j e c t o r
    E v e n m a c r o s c a n ’ t f i x m i s s i n g s y n t a x . *
    W e n e e d a c o m p i l e r p l u g i n !
    implicit val eitherMonad[A]: Monad[Either[A, ?]] = // ...
    

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52. S I - 2 7 1 2
    def foo[F[_], A](fa: F[A]) = // ...
    // doesn't compile!
    // Either[_, _] is not an F[_]
    foo(x: Either[String, Int])
    

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53. U n a p p l y
    D e p e n d e n t m e t h o d t y p e s + t o n s o f b o i l e r p l a t e = S I - 2 7 1 2 h a c k
    

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54. U n a p p l y
    trait Unapply[TC[_[_]], MA] {
    type M[_]
    type A
    def TC: TC[M]
    def subst: MA => M[A]
    }
    // okay ...
    

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55. U n a p p l y
    implicit def unapply3MTLeft[TC[_[_]], F[_[_],_,_], AA[_], B, C]
    (implicit tc: TC[F[AA,?,C]]): Aux3MTLeft[TC,F[AA, B, C], F, AA, B, C] =
    new Unapply[TC, F[AA,B,C]] {
    type M[X] = F[AA, X, C]
    type A = B
    def TC: TC[F[AA, ?, C]] = tc
    def subst: F[AA, B, C] => M[A] = identity
    }
    // the what now?!?!
    

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57. S e v e n y e a r s l a t e r . . .
    M i l e s f i x e s i t f o r S c a l a 2 .1 2 !
    

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58. S e v e n y e a r s l a t e r . . .
    M i l e s f i x e s i t f o r S c a l a 2 .1 2 !
    ( . . . a n d f o r 2 .1 0 a n d 2 .1 1 w i t h a c o m p i l e r p l u g i n )
    

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59. S e v e n y e a r s l a t e r . . .
    M i l e s f i x e s i t f o r S c a l a 2 .1 2 !
    ( . . . a n d f o r 2 .1 0 a n d 2 .1 1 w i t h a c o m p i l e r p l u g i n )
    

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60. T y p e c o n s t r u c t o r s i n D o t t y
    T h e D o t t y t e a m h a s t a k e n g r e a t c a r e t o c o n s o l i d a t e t h e c u r r e n t “ h a c k s ”
    T y p e l a m b d a s n o w b u i l t i n !
    

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61. View Slide

62. View Slide

63. View Slide

64. M a c r o s
    

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65. S c a l a 2 m a c r o s
    L a n d e d i n S c a l a 2 .1 0 ( 2 0 1 2 )
    E n a b l e d l o t s o f i n n o v a t i o n a c r o s s t h e b o a r d
    

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66. S c a l a 2 m a c r o s
    L a n d e d i n S c a l a 2 .1 0 ( 2 0 1 2 )
    E n a b l e d l o t s o f i n n o v a t i o n a c r o s s t h e b o a r d
    S a d l y , n o t a v a i l a b l e a n y m o r e i n D o t t y
    

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67. View Slide

68. M i g r a t i o n o f s c o d e c
    F i r s t s u p p o r t e d v e r s i o n : D o t t y 0 . 2 2 . 0 *
    

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69. M i g r a t i o n o f s c o d e c
    F i r s t s u p p o r t e d v e r s i o n : D o t t y 0 . 2 2 . 0 *
    R e q u i r e s c o m p l e t e r e i m p l e m e n t a t i o n b e t w e e n S c a l a 2 a n d D o t t y
    

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70. W h a t ’s n e x t ?
    

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71. S c a l a 3 . 0 . 0 i s n e a r !
    l a n g u a g e f e a t u r e s
    s i m u l a c r u m
    m a n y m a j o r T y p e l e v e l p r o j e c t s
    r e l e a s e t r a i n
    

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72. S c a l a 3 . 0 . 0 i s n e a r !
    l a n g u a g e f e a t u r e s
    s i m u l a c r u m
    m a n y m a j o r T y p e l e v e l p r o j e c t s
    r e l e a s e t r a i n
    S t i l l l o t s t o d o ( M o n i x , h t t p 4 s , . . . )
    

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73. Q & A
    L a r s H u p e l
    �
    l a r s . h u p e l @ i n n o q . c o m
    � @ l a r s r _ h
    w w w . i n n o q . c o m
    

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74. L A R S H U P E L
    S e n i o r C o n s u l t a n t
    i n n o Q D e u t s c h l a n d G m b H
    L a r s i s k n o w n a s o n e o f t h e f o u n d e r s o f t h e T y p e -
    l e v e l i n i t i a t i v e w h i c h i s d e d i c a t e d t o p r o v i d i n g
    p r i n c i p l e d , t y p e - d r i v e n S c a l a l i b r a r i e s i n a f r i e n d l y ,
    w e l c o m i n g e n v i r o n m e n t . A f r e q u e n t c o n f e r e n c e
    s p e a k e r , t h e y a r e a c t i v e i n t h e o p e n s o u r c e c o m -
    m u n i t y , p a r t i c u l a r l y i n S c a l a .
    

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75. S o u r c e s
    • https://twitter.com/ohbadiah/status/299937487713878016
    • https://twitter.com/travisbrown/status/300015411125174273
    • https://secure.trifork.com/dl/techmesh-london-2012/slides/JohnHughes_and_PhilipWadler_and_
    SimonPeytonJones_KeynoteHaskellPracticalAsWellAsCool.pdf
    • https://www.reddit.com/r/shiba/comments/e6ec1e/angry_shiba/
    • https://pixabay.com/vectors/people-jump-silhouette-group-male-4894818/
    

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