A bit about Scala

НЕМНОГО О SCALA
Владимир Парфиненко
vladimir.parﬁnenko@gmail.com
@cypok

NEW

SCALA
Martin Odersky
разрабатывал Scala с 2001
года в École Polytechnique
Fédérale de Lausanne,
релиз состоялся в 2003 году.

ПОПУЛЯРНОСТЬ

• 11
место – RedMonk Programming Language Rankings,
популярность на Stack Overﬂow и GitHub

• 36 место – TIOBE index, популярность поисковых запросов

ИДЕИ SCALA

• Безопасность и эффективность

• Гибкость языка, мощный синтаксис

• Объектно-ориентированность

• Функциональность

БЕЗОПАСНОСТЬ И ЭФФЕКТИВНОСТЬ

HELLO WORLD!

$ cat > HelloWorld.scala
object HelloWorld extends App {
println("Hello, world!")
}

$ scalac HelloWorld.scala

$ scala HelloWorld
Hello, world!

STATIC TYPING

var i = 37
i = 42
i = "Foo" // error: type mismatch;
// found : java.lang.String("Foo")
// required: Int

ГИБКОСТЬ ЯЗЫКА, МОЩНЫЙ СИНТАКСИС

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

DSL

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) 0
idom(3) should be (0)
}
}
1 2

3

ОБЪЕКТНО-ОРИЕНТИРОВАННОСТЬ

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;
}
}

// Mainstreamless.scala
object Mainstreamless extends App {
val p = new Person("John", 20)
println(p.name + " is " + p.age + " years old")
}

SCALA STRIKES BACK
class Person(val name: String, val age: Int)

object Mainstreamless extends App {
val p = new Person("John", 20)
println(p.name + " is " + p.age + " years old")
}

OOP: CLASSES
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)

OOP: 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
def compareTo(that: A): Int = compare(that)
}

class Money extends Ordered[Money] with SomeOtherTrait {
def compare(that: Money) = ...
}

OOP: TYPES
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
}

OOP: TYPES
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 : java.lang.String("Duck")
required: AnyRef{def quack: Unit; def feathers: Unit}
inTheForest("Duck")

λ
ФУНКЦИОНАЛЬНОСТЬ

FUNCTIONS
def inc(x: Int): Int = {
x + 1
}

def inc(x: Int) = x + 1

val inc = { x: Int => x + 1 }

inc(3) // 4

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 { _ + _ }

SCALA COLLECTIONS

• Seq
TraversableOnce
• IndexedSeq, Buffer, …
Iterator Traversable
• Set
Iterable
• HashSet, BitSet, …
Seq Set Map
• Map

• HashMap, TreeMap, …

SCALA COLLECTIONS

• collect • fold • partition

• count • forall • reduce

• exists • foreach • splitAt

• filter • groupBy • take

• find • map • to

• flatMap • max/min •…

DEMO
import java.util.ArrayList;
// ...
Person[] people, minors, adults;
void foo() {
ArrayList<Person> minorsList = new ArrayList<Person>();
Java

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()]);
}
Scala

val people: Array[Person]
val (minors, adults) = people partition { _.age < 18 }

PATTERN MATCHING

val str = num match {
case 1 => "one"
case 2 => "two"
case _ => "many"
}

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"
}

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
}

BONUS: FUNCTIONAL

def modN(n: Int)(x: Int) = ((x % n) == 0)

val nums = Seq(1, 2, 3, 4, 5, 6, 7, 8)

nums filter modN(2) // Seq(2, 4, 6, 8)
nums filter modN(3) // Seq(3, 6)

BONUS: CONCURRENCY

actor {
receive {
case people: Set[Person] =>
School ! minors
Work ! adults
}
}

BONUS: PARALLELISM

val people: Array[Person]


val (minors, adults) = people.par partition { _.age < 18 }

ag ic!
M

BONUS: 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
}

ЗАДАЧКА

val expr = Div(Add(Var("a"), Num(37)), Num(2))
expr.draw()

a + 37
------
2

РЕСУРСЫ

• http://github.com/cypok/mainstreamless – условие задачи

• http://www.scala-lang.org

• http://docs.scala-lang.org – guides & tutorials

• Programming in Scala: Second Edition – good book

• http://scala-ide.org – Scala IDE for Eclipse

• http://plugins.intellij.net/plugin/?id=1347 – IntelliJ IDEA plugin

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