The document discusses the introduction of lambdas in Scala 3, highlighting improvements in automatic eta-expansion compared to Scala 2, and provides various examples of lambda expressions and their definitions. It covers polymorphic function types and higher-kinded types, along with syntax updates related to lambda implementation. The content is aimed at software engineers looking to understand the changes in Scala 3's functional programming capabilities.

1. New Lambdas in Scala 3
Wiem Zine Elabidine, Software Engineer at LiveIntent

2. 2
Lambda expression
val identity: Int => Int = (a: Int) => a

3. 3
Lambda expression
val identity: Int => Int = (a: Int) => a
val identity: Function1[Int, Int] = new Function1[Int, Int] {
def apply(a: Int): Int = a
}

4. 4
Lambda expression
val identity: Int => Int = a => a
val listF = List(identity)

5. 5
Method
def identity(a: Int): Int = a

6. 6
Method
def identityMethod(a: Int): Int = a
val listF = List(identityMethod)
^
error: missing argument list for method identityMethod
Unapplied methods are only converted to functions when
a function type is expected.
You can make this conversion explicit by writing
`identityMethod _` or `identityMethod(_)` instead of
`identityMethod`.
In Scala 2

7. 7
Eta expansion in Scala 2
When we want to convert a def to a function,
Manual conversion:
def identityMethod(a: Int): Int = a
val identityFunction: Int => Int = identityMethod _
val listF = List(identityMethod _)

8. 8
Automatic eta-expansion in Scala 3
The eta expansion has been improved.
def identityMethod(a: Int): Int = a
val identityFunction: Int => Int = a => a
List(identityMethod) is similar to List(identityFunction)

9. 9
Automatic eta-expansion in Scala 3
The eta expansion has been improved.
def identityMethod(a: Int): Int = a
The identityMethod _ syntax is deprecated

10. 10
Automatic eta-expansion in Scala 3
The eta expansion has been improved.
def f() = println("hello")
List(f)
^
method f must be called with () argument

11. 11
Automatic eta-expansion in Scala 3
The eta expansion has been improved.
def f() = println("hello")
List(() => f())

12. 12
Typed lambda
identity: (a: A) => a

13. 13
Generic methods in Scala2
identity: (a: A) => a
def identity[A] = (a: A) => a
or
def identity[A](a: A): A= a

14. 14
Polymorphic function type in Scala3
identity: (a: A) => a
val identity = [A] => (a: A) => a
> val identity: PolyFunction{apply: [A](a: A): A} = <function1>

15. 15
Context function
A ?=> B

16. Type Lambdas

17. 17
Proper type
n: Int
Int

18. 18
Type constructor
A => List[A]
List[A]

19. 19
Type constructor
A => List[A]
List[A]
type F = [A] =>> List[A]

20. 20
Type constructor
A => List[A]
List[A]
type F = [A] =>> List[A]
type F[A] = List[A]

21. 21
Type constructor
List[A]
type F = [A] =>> List[A]
type F[A] = List[A]
val f= [A] => (a: A) => List[A]
def f[A]: List[A]

22. 22
Type constructor
(E, A) => Either[E, A]
Either[E, A]

23. 23
Type constructor
(E, A) => Either[E, A]
Either[E, A]
type F = [E, A] =>> Either[E, A]

24. 24
Type constructor
(E, A) => Either[E, A]
Either[E, A]
type F = [E, A] =>> Either[E, A]
type F[E, A] = Either[E, A]

25. 25
Higher kinded type
(A => F[A]) => Container[F]
trait Container[F[_]]

26. 26
Higher kinded type
type X = [F[_]] =>> Container[F]
(A => F[A]) => Container[F]
trait Container[F[_]]

27. 27
Higher kinded type
type X = [F[_]] =>> Container[F]
type X = [F[_]] =>> Container[A =>> F[A]]
(A => F[A]) => Container[F]
trait Container[F[_]]

28. 28
Higher kinded type
type X = [F[_]] =>> Container[F]
type X = [F[_]] =>> Container[A =>> F[A]]
type X[F[_]] = Container[F]
(A => F[A]) => Container[F]
trait Container[F[_]]

29. 29
type Lambdas
type ListContainer = Container[List]

30. 30
type Lambdas
type EitherContainer = Container[Either]
^^^^^^
Type argument Either does not have the same kind as its bound [_$1]

31. 31
Partial application
type EitherContainer = Container[Either]
^^^^^^
Type argument Either does not have the same kind as its bound [_$1]
val f: (A, B) => C
val f1: A => C = ???
val f2: B => C = ???

32. 32
Partial application
type EitherContainer = Container[Either]
^^^^^^
Type argument Either does not have the same kind as its bound [_$1]
val f: (A, B) => C
val f1: A => C = f(_, b)
val f2: B => C = f(a, _)

33. 33
type EitherContainer[A] = Container[({type T[E] = Either[E, A]})#T]
type Lambdas in Scala 2

34. 34
trait MyEither[A] {
type T[E] = Either[E, A]
}
type EitherContainer[A] = Container[MyEither[A]#T]
type Lambdas in Scala 2

35. 35
type EitherContainer[A] = Container[Either[*, A]]
type Lambdas using Kind projector plugin

36. 36
type EitherContainer[A] = Container[[E] =>> Either[E, A]]
type Lambdas using Scala 3

37. 37
type EitherF[A] = [E] =>> Either[E, A]
type EitherContainer = [A] =>> Container[EitherF[A]]
type Lambdas using Scala 3

38. 38
type EitherF[A] = [E] =>> Either[E, A]
type EitherContainer = [A] =>> Container[EitherF[A]]
type Lambdas using Scala 3
EitherF[Int][String]

39. Question

40. 40
Polymorphic function type in Scala3
val identity = [A] => (a: A) => a
> val identity: PolyFunction{apply: [A](a: A): A} = <function1>
Similar syntax as SAM

41. 41
Is there a plan to enable SAM to trait that has a
polymorphic functions?
trait Example[A]:
def f(a: A): A = a
val v: Example[Int] = (a: Int) => a
Example
trait Example:
def f[A](a: A): A = a
val v: Example = [A] => (a: A) => a
OR

42. 42
Follow me on twitter: @WiemZin
Thanks!

43. 43
Credits
◂ Presentation template by Slidesgo
◂ Icons by Flaticon
◂ Images & infographics by Freepik
◂ Author introduction slide photo created by freestockcenter - Freepik.com
◂ Text & Image slide photo created by tzido - Freepik.com
◂ Big image slide photo created by tzido - Freepik.com
Thank you!