Обзор языка Scala (ИСИ)

Обзор языка Scala, представленный на спецсеминаре ИСИ СО РАН.

Transcript

1. ОБЗОР ЯЗЫКА SCALA Владимир Парфиненко vladimir.parfinenko@gmail.com @cypok
2. SCALA & MARTIN ODERSKY Ученик Н. Вирта.
 Разрабатывал Scala с 2001 года в EPFL. Первая версия вышла в 2003 году.
3. ПОПУЛЯРНОСТЬ • 14 место – RedMonk Programming Language Rankings, популярность на Stack Overflow и GitHub • 41 место – TIOBE index, популярность поисковых запросов
4. ИДЕИ SCALA • Гибкость языка, мощный синтаксис • Безопасность и эффективность • Объектно-ориентированность • Функциональность
5. ГИБКОСТЬ ЯЗЫКА, МОЩНЫЙ СИНТАКСИС
6. HELLO WORLD! object HelloWorld extends App {! println("Hello, world!")! }
7. ОСНОВЫ def hi(name: String) {! println("Hi, " + name + "!")! }! ! val num = 10! var i = 0! while (i < num) {! hi("Scala")! i = i + 1! }
8. HELLO REPL! scala> val repl = Map('R' -> "Read", 'E' -> "Eval",! | 'P' -> "Print", 'L' -> "Loop")! ! scala> for ((k, v) <- repl) println(k + " is for " + v)! R is for Read! E is for Eval! P is for Print! L is for Loop
9. DOMAIN-SPECIFIC LANGUAGE class DominatorsSuite extends FunSuite with ShouldMatchers! with GraphBuilderDSL {! test("diamond") {! calcDominatorsOver(0 -> (1 || 2) -> 3)! idom(1) should be (0)! idom(2) should be (0)! idom(3) should be (0)! }! } 0 1 2 3
10. BASIC DSL object IfSample extends App with Baysick {! 10 LET (’a := 5)! 20 IF ‘a === 5 THEN 40! 30 PRINT "This will never execute"! 40 PRINT "They were equal!"! 50 IF ‘a === 6 THEN 70! 60 PRINT "This will execute 1st..."! 70 PRINT "...then this!"! 80 END! ! RUN! }
11. БЕЗОПАСНОСТЬ И ЭФФЕКТИВНОСТЬ
12. SCALA TO JAVA BYTECODE $ cat HelloWorld.scala! object HelloWorld extends App {! println("Hello, world!")! }! ! $ scalac HelloWorld.scala! ! $ scala HelloWorld! Hello, world!
13. СТАТИЧЕСКАЯ ТИПИЗАЦИЯ def foo() = "Some string"! def bar(x: Int) { ??? }! ! val s = foo()! println(s.length)! bar(s) // error: type mismatch;! // found : String! // required: Int
14. ОБЪЕКТНО-ОРИЕНТИРОВАННОСТЬ
15. BACK TO THE JAVA // Person.java! public class Person {! public final String name;! public final int age;! ! public Person(String name, int age) {! this.name = name;! this.age = age;! }! }! ! // IIS.scala! object IIS extends App {! val p = new Person("John", 20)! println(p.name + " is " + p.age + " years old")! }
16. SCALA STRIKES BACK ! class Person(val name: String, val age: Int)! ! ! ! ! ! ! ! ! ! ! object IIS extends App {! val p = new Person("John", 20)! println(p.name + " is " + p.age + " years old")! }
17. КЛАССЫ abstract class Animal {! def name: String! }! ! class Person(firstName: String, lastName: String)! extends Animal {! val name = firstName + " " + lastName! }! ! class Student(firstName: String, lastName: String, val year: Int)! extends Person(firstName, lastName)
18. TRAITS trait Ordered[A] {! def compare(that: A): Int! ! def < (that: A): Boolean = (this compare that) < 0! def > (that: A): Boolean = (this compare that) > 0! def <= (that: A): Boolean = (this compare that) <= 0! def >= (that: A): Boolean = (this compare that) >= 0! }! ! class Money extends Value with Ordered[Money] {! def compare(that: Money) = ...! }
19. ДИНАМИЧЕСКИЕ ТИПЫ class Duck {! def quack = println("Quaaaaaack!")! def feathers = println("The duck has white and gray feathers.")! }! ! class Person {! def quack = println("The person imitates a duck.")! def feathers = println("The person takes a feather! from the ground and shows it.")! }! ! def inTheForest(duck: { def quack; def feathers }) = {! duck.quack! duck.feathers! }
20. ДИНАМИЧЕСКИЕ ТИПЫ scala> inTheForest(new Duck)! Quaaaaaack!! The duck has white and gray feathers.! ! scala> inTheForest(new Person)! The person imitates a duck.! The person takes a feather from the ground and shows it.! ! ! scala> inTheForest("Duck")! error: type mismatch;! found : String("Duck")! required: AnyRef{def quack: Unit; def feathers: Unit}! inTheForest("Duck")
21. ФУНКЦИОНАЛЬНОСТЬ λ
22. ФУНКЦИИ def inc(x: Int): Int = {! x + 1! }! ! def inc(x: Int) = x + 1! ! val inc = { x: Int => x + 1 }! ! ! inc(3) // 4
23. ЕЩЕ ФУНКЦИИ def sum(x: Int, y: Int) = x + y! def inc(x: Int) = sum(1, x)! ! def sum(x: Int)(y: Int) = x + y! val inc = sum(1) _! ! def sum(x: Int) = {! def inner(y: Int) = {! x + y! }! inner! }
24. И ЕЩЕ ФУНКЦИИ Seq(1, 2, 3) map inc // Seq(2, 3, 4)! ! // 1 + 2 + 3! Seq(1, 2, 3) reduce { x, y => x + y }! Seq(1, 2, 3) reduce { _ + _ }
25. КОЛЛЕКЦИИ • Seq • IndexedSeq, Buffer, … • Set • HashSet, BitSet, … • Map • HashMap, TreeMap, … Traversable Iterable Seq Set Map TraversableOnce Iterator
26. КОЛЛЕКЦИИ • collect • count • exists • filter • find • flatMap • fold • forall • foreach • groupBy • map • max/min • partition • reduce • splitAt • take • to • …
27. КОЛЛЕКЦИИ import java.util.ArrayList;! // ...! Person[] people, minors, adults;! void foo() {! ArrayList<Person> minorsList = new ArrayList<Person>();! ArrayList<Person> adultsList = new ArrayList<Person>();! for (Person person : people)! (person.age < 18 ? minorsList : adultsList).! add(person);! minors = minorsList.toArray(new Person[minorsList.size()]);! adults = adultsList.toArray(new Person[adultsList.size()]);! }! ! ! val people: Array[Person]! val (minors, adults) = people partition { _.age < 18 } Java Scala
28. FOR val files: Seq[File]! ! for (file <- files) {! println(file.getName)! }
29. FOR def fileLines(f: File): Seq[String] = ???! ! for (file <- files) {! if (!file.getName.startsWith(".")) {! for (line <- fileLines(file)) {! if (line.nonEmpty) {! println(file + ": " + line)! }! }! }! }
30. FOR for {! file <- files! if !file.getName.startsWith(".")! line <- fileLines(file)! if line.nonEmpty! } println(file + ": " + line)! ! files withFilter { !_.getName.startsWith(".") } foreach { file =>! fileLines withFilter { _.nonEmpty } foreach { line =>! println(file + ": " + line)! }! }
31. PATTERN MATCHING var str: String = _! num match {! case 1 =>! str = "one"! case 2 =>! str = "two"! case _ =>! str = "many"! }! println(str) ! val str = num match {! case 1 => "one"! case 2 => "two"! case _ => "many"! }! println(str)
32. PATTERN MATCHING val str = anything match {! case x: Int if x > 0 => "positive integer"! case x: Float if x > 0 => "positive real"! case _: String => "string"! case _ => "unknown"! }
33. CASE CLASSES sealed class Element! ! case class Var(name: String) extends Element! case class Num(value: Int) extends Element! case class Neg(arg: Element) extends Element! case class Add(arg1: Element, arg2: Element) extends Element! ! def optimize(elem: Element): Element = elem match {! case Neg(Neg(x)) => optimize(x)! case Add(x, Num(0)) => optimize(x)! case Neg(Num(x)) => Num(-x)! case Add(x, Neg(y)) if x == y => Num(0)! case Add(Num(x), Num(y)) => Num(x + y)! case Neg(x) => Neg(optimize(x))! case Add(x, y) => Add(optimize(x), optimize(y))! case _ => elem! }
34. OPTION ! ! ! ! ! val set = Set(42, null, "IIS")! val found = set find { _ == null }! ! if (found != null) {! // huh, we've found it or not? ! println("Found " + found + "!") ! } else {! println("Not found.")! }
35. OPTION abstract class Option[+A]! ! case class Some[+A](x: A) extends Option[A]! case object None extends Option[Nothing]! ! val set = Set(42, null, "IIS")! val found = set find { _ == null }! ! found match {! case Some(elem) =>! println("Found " + elem + "!")! case None =>! println("Not found.")! }
36. И еще немного…
37. БОНУС: PARALLELISM import java.util.ArrayList;! // ...! Person[] people, minors, adults;! void foo() {! ArrayList<Person> minorsList = new ArrayList<Person>();! ArrayList<Person> adultsList = new ArrayList<Person>();! for (Person person : people)! (person.age < 18 ? minorsList : adultsList).! add(person);! minors = minorsList.toArray(new Person[minorsList.size()]);! adults = adultsList.toArray(new Person[adultsList.size()]);! }! ! ! val people: Array[Person]! val (minors, adults) = people partition { _.age < 18 } Java Scala
38. БОНУС: PARALLELISM ! ! ! ! ! ! ! ! ! ! ! ! ! ! val people: Array[Person]! val (minors, adults) = people.par partition { _.age < 18 } Java Scala ???
39. БОНУС: CONCURRENCY actor {! receive {! case people: Set[Person] =>! val (minors, adults) = people partition { _.age < 18 }! School ! minors! Work ! adults! }! }
40. БОНУС: FUTURES val f: Future[List[String]] = future {! session.getRecentPosts! }! ! f onFailure {! case t => println("An error has occured: " + t.getMessage)! }! ! f onSuccess {! case posts => posts foreach println! }
41. @tailrec! def find[A](xs: List[A], p: (A => Boolean)): Option[A] = {! xs match {! case Nil => None! case head :: tail =>! if (p(head)) Some(head) else find(tail, p)! }! } trait Provider {! def smth: Int! }! ! class LazyOne extends Provider {! lazy val smth = {! ???! }! } assert(assertion: Boolean, message: => Any): Unit! ! assert(true, { ??? })! assert(false, { ??? }) implicit class RockString(str: String) {! def rock() = ???! }! "foo".rock() package my! class Foo {! private[this] val x: Int = ???! private[Foo] val y: Int = ???! private[my] val z: Int = ???! } val end = s"And $much more!"
42. ЧТО ДАЛЬШЕ? Novosibirsk
 Scala
 Enthusiasts http://twitter.com/ScalaNsk http://www.meetup.com/ScalaNsk
43. РЕСУРСЫ • https://speakerdeck.com/cypok/obzor-iazyka-scala-isi – слайды • http://www.scala-lang.org • http://docs.scala-lang.org – guides & tutorials • Programming in Scala: Second Edition – книжка для начала • http://scala-ide.org – Scala IDE for Eclipse • http://plugins.intellij.net/plugin/?id=1347 – IntelliJ IDEA plugin