The Essence and Fundamentals of Scala

JUG Thüringen, Erfurt

Scala is one of the most popular alternative languages on the JVM. In this talk, we are going to look at what makes this language so interesting: A strong foundation in functional as well as object-oriented programming, combined with a powerful type system. We'll look at how these principles have shaped the language and how to put them to use in practice.

Transcript

1. The Essence and
   Fundamentals of
   Scala
   26,04.2018

   ERFURT / JAVA USER GROUP TH
   

   View Slide

2. About me
   • senior consultant at INNOQ
   • author of “The Neophyte’s Guide to Scala”
   • co-founder of ScalaBridge Berlin
   • Twitter: @kaffeecoder
   • website/blog: http://danielwestheide.com
   

   View Slide

3. Agenda
   • intro
   • basics
   • Scala’s object-oriented angle
   • Scala’s functional foundation
   • a rather powerful type system
   

   View Slide

4. Intro
   

   View Slide

5. Basics
   

   View Slide

6. Look ma, a value!
   scala> val screamedGreeting = "HELLO WORLD!!!!"
   screamedGreeting: String = HELLO WORLD!!!!
   

   View Slide

7. Look ma, a variable!
   scala> var screamedGreeting = "HELLO WORLD!!!!"
   screamedGreeting: String = HELLO WORLD!!!!
   

   View Slide

8. This looks familiar…
   scala> val firstBangPos = screamedGreeting.indexOf('!')
   firstBangPos: Int = 11
   scala> val bang = screamedGreeting.charAt(firstBangPos)
   bang: Char = !
   

   View Slide

9. Defining functions
   def areaOfRightTriangle(a: Int, b: Int): Double = a * b / 2.0
   

   View Slide

10. Defining functions
    def triangleArea(a: Int, b: Int, c: Int): Double = {
    val s = (a + b + c) / 2
    Math.sqrt(s * (s - a) * (s - b) * (s - c))
    }
    

    View Slide

11. Scala’s object-oriented
    angle
    

    View Slide

12. A simple class
    class Point(x: Int, y: Int) {
    require(x > 0 && y > 0)
    println(s"Point created with x: $x and y: $y")
    def toString: String = s"Point(x: $x, y: $y)"
    }
    

    View Slide

13. Uniform access principle
    class Point(x: Int, y: Int) {
    require(x > 0 && y > 0)
    println(s"Point created with x: $x and y: $y")
    val toString: String = s"Point(x: $x, y: $y)"
    }
    

    View Slide

14. Minimal boilerplate
    class Point(val x: Int, val y: Int) {
    require(x > 0 && y > 0)
    println(s"Point created with x: $x and y: $y")
    def toString: String = s"Point(x: $x, y: $y)"
    }
    

    View Slide

15. Singleton objects
    object Point {
    def create(x: Int, y: Int): Point = new Point(x, y)
    }
    

    View Slide

16. Everything is an object
    val x = 5 + 3 * 2
    val x = 5.+(3.*(2))
    

    View Slide

17. Scala’s functional
    foundation
    

    View Slide

18. Expressions everywhere
    def pluralize(singular: String, n: Int): String = {
    val text = if (n > 1) singular + "s" else singular
    n + " " + text
    }
    

    View Slide

19. Functions as values
    val isLong: (Email) => Boolean =
    (email: Email) => email.body.length >= 100
    

    View Slide

20. Functions as values
    val isLong: Email => Boolean = (email) => email.body.length >= 100
    

    View Slide

21. Functions as values
    val isLong: Email => Boolean = email => email.body.length >= 100
    

    View Slide

22. Functions as values
    val isLong: Email => Boolean = _.body.length >= 100
    

    View Slide

23. Functions as values
    val isLong = (email: Email) => email.body.length >= 100
    

    View Slide

24. Higher-order functions
    type EmailFilter = Email => Boolean
    def newMailsForUser(mails: Seq[Email],
    f: EmailFilter): Seq[Email] =
    mails.filter(f)
    newMailsForUser(myEmails, isLong)
    

    View Slide

25. Higher-order functions
    type EmailFilter = Email => Boolean
    def newMailsForUser(mails: Seq[Email],
    f: EmailFilter): Seq[Email] =
    mails.filter(f)
    newMailsForUser(myEmails, _.body.length >= 100)
    

    View Slide

26. Higher-order functions
    def minimumSize(n: Int): EmailFilter =
    email => email.body.length >= n
    def maximumSize(n: Int): EmailFilter = _.body.length <= n
    scala> val isLong = minimumSize(100)
    isLong: EmailFilter = $$Lambda$1285/789178034@1bee0085
    

    View Slide

27. Currying
    scala> val minimumSize: Int => Email => Boolean = size => email =>
    email.body.length >= size
    minimumSize: Int => (Email => Boolean) = $
    $Lambda$1305/559179052@8b670c0
    scala> minimumSize(100)(email)
    res0: Boolean = false
    scala> val isLong = minimumSize(100)
    isLong: Email => Boolean = $$Lambda$1308/1940974851@55061418
    

    View Slide

28. Function composition
    scala> val wordCount: Email => Int = _.body.split(' ').length
    wordCount: Email => Int = $$Lambda$1392/153337703@938e54a
    scala> def higherThan(threshold: Int): Int => Boolean = _ >
    threshold
    higherThan: (threshold: Int)Int => Boolean
    scala> val isLong = higherThan(100) compose wordCount
    isLong: Email => Boolean = scala.Function1$
    $Lambda$1395/2035659988@71c1e70f
    

    View Slide

29. Function composition
    scala> val wordCount: Email => Int = _.body.split(' ').length
    wordCount: Email => Int = $$Lambda$1392/153337703@938e54a
    scala> def higherThan(threshold: Int): Int => Boolean = _ >
    threshold
    higherThan: (threshold: Int)Int => Boolean
    scala> val isLong = wordCount andThen higherThan(100)
    isLong: Email => Boolean = scala.Function1$
    $Lambda$1374/1682100030@112f8f8f
    

    View Slide

30. Algebraic data types
    sealed trait TrafficLight
    case object Red extends TrafficLight
    case object Yellow extends TrafficLight
    case object Green extends TrafficLight
    

    View Slide

31. Algebraic data types
    def mayDrive(tl: TrafficLight): Boolean = tl match {
    case Green => true
    case Red | Yellow => false
    }
    

    View Slide

32. Algebraic data types
    scala> val me = ("Daniel", 37)
    me: (String, Int) = (Daniel,37)
    scala> val (name, age) = ("Daniel", 37)
    name: String = Daniel
    age: Int = 37
    

    View Slide

33. Algebraic data types
    final case class Person(name: String, age: Int)
    def greet(p: Person): String = p match {
    case Person("Daniel", _) =>
    "You look younger than ever!"
    case Person(_, age) if age > 30 =>
    "Yo, have you come here to die?"
    case _ =>
    “Hi!"
    }
    

    View Slide

34. Functional error handling
    sealed abstract class Option[+A]
    final case class Some[+A](value: A) extends Option[A]
    case object None extends Option[Nothing]
    

    View Slide

35. Functional error handling
    def greet(name: Option[String]): String = name match {
    case Some(name) => s"Nice to meet you, $name!"
    case None => "Hello, stranger!"
    }
    

    View Slide

36. Functional error handling
    final case class Person(name: String, age: Option[Int])
    def find(name: String): Option[Person] = ???
    scala> val name = find("Daniel").map(_.name).getOrElse("N/A")
    displayedName: String = N/A
    scala> val age = find("Daniel").flatMap(_.age)
    age: Option[Int] = None
    

    View Slide

37. For comprehensions
    for {
    user <- find("Daniel")
    age <- user.age
    } yield age
    

    View Slide

38. For comprehensions
    • Option
    • List and other collection types
    • Either
    • Try
    • Future
    

    View Slide

39. A rather powerful type
    system
    

    View Slide

40. Type safety
    val strings: Array[String] = Array("foo", "bar")
    val objects: Array[Any] = strings
    objects[1] = 42
    

    View Slide

41. Type safety
    final class Array[T]
    sealed abstract class List[+A]
    

    View Slide

42. Type safety
    • cheap value classes
    • phantom types
    • “there is a Prolog in the type system”
    • http://danielwestheide.com/blog/2015/06/28/put-
    your-writes-where-your-master-is-compile-time-
    restriction-of-slick-effect-types.html
    

    View Slide

43. Abstraction
    def foo[A, B](tuple: (A, B)): A = ???
    

    View Slide

44. Abstraction
    def higher[A <: Ordered[A]](x: A, y: A): A =
    if (x > y) x else y
    higher(5, 2)
    

    View Slide

45. Abstraction
    def higher[A](x: A, y: A, ordering: Ordering[A]): A =
    if (ordering.gt(x, y)) x else y
    higher(5, 2, Ordering.Int)
    

    View Slide

46. Abstraction
    def higher[A](x: A, y: A)(ordering: Ordering[A]): A =
    if (ordering.gt(x, y)) x else y
    higher(5, 2)(Ordering.Int)
    

    View Slide

47. Abstraction
    def higher[A](x: A, y: A)(implicit ordering: Ordering[A]): A =
    if (ordering.gt(x, y)) x else y
    higher(5, 2)
    

    View Slide

48. Abstraction
    object Ordering {
    implicit object Int extends IntOrdering
    }
    

    View Slide

49. Abstraction
    def higher[A: Ordering](x: A, y: A): A =
    if (implicitly[Ordering[A]].gt(x, y)) x else y
    higher(5, 2)
    

    View Slide

50. Abstraction
    def sequenceOption[A](as: List[Option[A]]): Option[List[A]] = ???
    def sequenceTry[A](as: List[Try[A]]): Try[List[A]] = ???
    def sequenceFuture[A](as: List[Future[A]]): Future[List[A]] = ???
    

    View Slide

51. Abstraction
    trait Traverse[F[_]] {
    def sequence[G[_]: Applicative, A](fga: F[G[A]]): G[F[A]]
    }
    val ageOptions: List[Option[Age]] = List(Some(Age(23)), None)
    val agesOption: Option[List[Age]] = ageOptions.sequence
    val responseFutures: List[Future[Response]] = List(svc1(), svc2())
    val responsesFuture: Future[List[Response]] =
    responseFutures.sequence
    

    View Slide

52. Thank you!
    Got any questions? :)
    

    View Slide