Scala overview

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S-Core Cloud Computing Lab
Data Solution Part
2013. 2. 18.

Contents

I. Scala란?
II. Scala는 간결하다.
III. Scala는 객체지향 언어이다.
IV. Scala는 함수형 언어이다.
V. Scala는 동적 언어이다.
VI. Scala는 정적으로 타입화된 언어이다.
VII. Scala는 많은 좋은 기능이 있다.
VIII.Scala를 어떻게 공부하지?
1

Scala 란?
object Hello {
def main(args:Array[String]) {
println(“Hello, World”)
}
}
2

Scala 정의

Scala [skah-lah]
= scalable language
The language is so named because it was designed to grow
with the demands of its users.
-Programming In Scala, p39

Scala is a modern multi-paradigm programming language
designed to express common programming patterns in a
concise, elegant, and type-safe way.
-scala-lang.com
3

Scala에 대한 의견들
If I were to pick a language to use on the JVM today other
than Java, it would be Scala.
-James Gosling, creater of Java
http://www.adam-bien.com/roller/abien/entry/java_net_javaone_which_programming

My tip thought for the long term replacement of javac is Scala.
I’m very impressed with it! I can honestly say if someone had
shown me the Programming in Scala book […] back in 2003.
I’d probably have never created Groovy.
-James Strachan, creator of Groovy
http://macstrac.blogspot.com/2009/04/scala-as-long-term-replacement-for.html

Scala, it must be stated, is the current heir apparent to the Java
throne. No other language on the JVM seems as capable of
being a “replacement for Java” as Scala, and the momentum
behind Scala is now uniquestionable.
- Charlies Nutter, JRuby lead
http://blog.headius.com/2009/04/future-part-one.html

4

Scalability in Programming Language

어플리케이션이 사용자의 요구에 맞추기 위해 크기나 용량을 변
경해도, 그 기능이 계속하여 잘 동작 할 수 있는 능력
-Terms

문제 영역의 크기나 복잡도가 증가하여도 유사한 방법으로 문제
해결이 가능한 것.

- 빌딩 블록의 수가 증가하면  추상화 / 축약
- 언어의 지원 / 라이브러리 / 프레임 워크

5

언어 확장의 예시 – 분수 (Rational)

규칙:
(a, b) 단, b != 0
(a, b) + (c, d) = (ad+bc, bd)
(a, b) * (c, d) = (ac, bd)

*기타
- Big Integer
- Complex number
- Vector
- Matrix …

6

BigInteger vs. BigInt
Java
Integer와
public BigInteger factorial(BigInteger x) { 유사하지 않음
if ( x == BigInteger.ZERO )
return BigInteger.ONE;
else
return x.multiply(
factorial (x.subtract(BigInteger.ONE)));
}

Scala

def factorial(x: BigInt):BigInt =
if (x == 0) 1 else x * factorial(x-1)

출처: http://www.artima.com/scalazine/articles/scalable-language.html 7

Google Trends

출처: http://www.google.com/trends/explore#q=scala,%20clojure,%20groovy 8

Job Trends

출처: http://www.indeed.com/jobtrends?q=scala%2C+clojure%2C+groovy&l= 9

Scala Today

출처: http://www.slideshare.net/Odersky/devoxx 10

Scala Best Practices

Twitter
– Kestrel: queuing
– FlockDB(and Gizzard): social graph store
– Hawkwind: people search
– Hosebird: streaming API
– Scalding: MapReduce Wrapper (build on top of Cascading)

Guardian: Open Platform
– 2009.11: java + guice + guice servlets + apache solr
– 2010.01: Integration tests with ScalaTest / maven scala plugin
– 2010.03: scala + guice + guice servlets + apache solr
– 2010.06: maven -> sbt
– 2011.06: scala + lift + apache solr ( 5k loc -> 3.5k)
출처: http://www.slideshare.net/mslinn/scala-adoption-by-enterprises 11

Scala 언어 가계도
Simula
Lisp objects Prolog
functional
C
Smalltalk
programming syntax
pattern
matching
ML
C++

Erlang
Haskell
Java
Actors

Scala
12

Scala를 멀리서 보기

함수형 객체 지향 동적
프로그래밍 프로그래밍 프로그래밍

강화된 타입 시스템

JVM/.Net
※동적 프로그래밍: 스칼라는 엄밀히 말하자면 동적 언어는 아니지만, 동적 언어에서 제공하는 많은 특징을 가지고 있다.
(간결함(Concise), REPL, 고수준함수, Duck Typing 등)
13

Scala 특징 1/3

Scala는 함수형과 객체지향형의 성격을 다
가지고 있다.
–Method를 포함한 모든 함수 (function)은 값(value)이다.
–모든 값(value)는 객체(object)이다.

Scala는 정적 타입시스템이다.
– 강력한 타입 추론 시스템:
in Java 1.5:
Pair p = new Pair<Integer, String>(1, "Scala");

in Scala:
val p = new MyPair(1, "scala");

14

Scala 특징 2/3
 가벼운 문법(lightweight syntax)
– Anonymous functions
– Higher-order functions
– Nested Functions
– Currying
 ML-style의 Pattern Matching
 XML과 통합
– Scala 프로그램에서 XML을 직접 쓸 수 있다.
– Scala 클래스 정의로 XML DTD로 변환할 수 있다.
 정규식 패턴
 DSLs
 Concurrency via actors

15

Scala 특징 3/3 – Scala는 실용적이다.

Java를 대체해서 사용이 가능함
 Java와 Scala를 혼합한 프로젝트가 가능

기존 Java 라이브러리를 활용가능

기존 Java 툴(Ant, Maven, Junit, etc…)을 활용가능

기존 IDE 환경을 지원 (Eclipse, NetBeans, IntelliJ)

16

Scala compared to Java
Scala adds Scala removes

+ a pure object system - static members

+ operator overloading - primitive Types

+ closures - break, continue

+ mixin composition with traits - special treatment of interfaces

+ existential types - wildcards

+ abstract types - raw types

+ pattern matching - enums

Modeled in libiaries:
assert, enums, properties, events, actors, using, queries, …

출처: http://www.slideshare.net/Odersky/fosdem-2009-1013261 17

Scala는 간결하다.
object Hello {
}
}
18

Type Interface

val sum = 1 + 2 + 3
>sum: Int = 6

val nums = List(1, 2, 3)
>nums: List[Int] = List(1, 2, 3)

val map = Map("abc" -> List(1,2,3))
>map:
scala.collection.immutable.Map[java.la
ng.String,List[Int]] = Map(abc ->
List(1, 2, 3))

19

Explicit Types

val sum: Int = 1 + 2 + 3

val nums: List[Int] = List(1, 2, 3)

val map: Map[String, List[Int]] = ...

20

Higher Level
// Java – Check if string has uppercase character
boolean hasUpperCase = false;
for(int i = 0; i < name.length(); i++) {
if(Character.isUpperCase(name.charAt(i))) {
hasUpperCase = true;
break;
}
}

// Scala
val hasUpperCase = name.exists(_.isUpperCase)

21

Less Boilerplate
Java Scala
public class Person { class Person(var name: String,
private String name; var age: Int)
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age; class Person(var name: String,
} private var _age: Int){
public String getName() {
def age = _age // Getter
return name;
def age_=(newAge:Int) { // Setter
}
public int getAge() { _age = newAge
return age; }
} }
public void setName(String name) {
this.name = name;
}
public void setAge(int age) {
this.age = age;
}
}
22

Variables and Values

// variable
var foo = "foo"
foo = "bar" // okay

// value
val bar = "bar"
bar = "foo" // nope

23

Scala는 객체지향 언어이다.
object Hello {
}
}
24

Class Hierarchy

출처: https://docs.google.com/document/pub?id=1kSNKKKwM8rjGhn9Gnw-6Q0VCImpwSRZ7_QzwNwXgMxM 25

Pure O.O.
// Every value is an object
1.toString

// Every operation is a method call
1 + 2 + 3  (1).+(2).+(3)

// Can omit . and ( )
"abc" charAt 1  "abc".charAt(1)

26

Classes

// Classes (and abstract classes) like Java
abstract class Language(val name:String) {
override def toString = name
}

// Example implementations
class Scala extends Language("Scala")

// Anonymous class
val scala = new Language("Scala") { /* empty */ }

27

Case Classes
case class Person(firstName: String = "Jamie", lastName: String = "Allen")
val jamieDoe = Person(lastName = "Doe") // Person(Jamie,Doe)

abstract class Tree
case class Sum(l: Tree, r: Tree) extends Tree
case class Var(n: String) extends Tree
case class Const(v: Int) extends Tree
…
type Environment = String => Int
def eval(t: Tree, env: Environment): Int = t match {
case Sum(l, r) => eval(l, env) + eval(r, env)
case Var(n) => env(n)
case Const(v) => v
}

 Data Transfer Objects (DTOs) done right
 By default, class arguments are immutable & public
 Should never be extended
 Provide equals(), copy(), hashCode() and toString() implementations
 Don’t have to use new keyword to create instances
 Named Parameters and Default arguments give us Builder pattern semantics
28

Traits
// Like interfaces in Java
trait Language {

val name:String

// But allow implementation
override def toString = name
}

29

Traits
trait JVM {
override def toString = super.toString+" runs on JVM" }
trait Static {
override def toString = super.toString+" is Static" }

// Traits are stackable
class Scala extends Language with JVM with Static {
val name = "Scala"
}

println(new Scala)  "Scala runs on JVM is Static"

30

Objects
object Bootstrapper extends App { Person.createJamieAllen }

object Person {
def createJamieAllen = new Person("Jamie", "Allen")
def createJamieDoe = new Person("Jamie", "Doe")
val aConstantValue = "A constant value”
}

class Person(val firstName: String, val lastName: String)

Singletons within a JVM process
No private constructor histrionics
Companion Objects, used for factories and constants

31

Tuples
def getNameAndAge: Tuple2[String, Int] = {
val name = “Steve”
val age = 30
(name, age)
}

val (name, age) = getNameAndAge
println(“Name: “+ name) // Name: Steve
println(“Age: “ + age) // Age: 30

val scalet = (“Scalet”, 27, “Seoul”) // scalet: (String, Int, String) = (“Scalet”, 27, “Seoul”)
println(“Address: “ + scalet._3) // Address: Seoul

Tuple is a generalization of that: a collection of two,
three, four, and more values
Tuple can hold objects with different types but they
are also immutable
32

Scala는 함수형 언어이다.
object Hello {
}
}
33

Imperative Style vs. Functional Style
Characteristic Imperative Functional

What information is
How to perform tasks
desired and what
Programmer focus (algorithms) and how to
transformations are
track changes in state.
required.

State Changes Important. Non-existent.

Order of execution Important. Low importance.

Loops, conditionals, and Function calls, including
Primary flow control
function (method) calls recursion.

Functions as first-class
Instances of structures or
Primary manipulation unit objects and data
classes.
collections.

출처: http://msdn.microsoft.com/en-us/library/bb669144.aspx 35

Function Type + Function Literal

// Type
(Int, Int)=>Int

// Literal
(x:Int, y:Int) => x + y

36

First Class Functions
// Lightweight anonymous functions
(x:Int, y:Int) => x + y

// Calling the anonymous function
val add = (x:Int, y:Int) => x + y
Function name Argument List Function expression

val add : (Int, Int) => Int = (x:Int, y:Int) => x + y
val add : (Int, Int) => Int = _ + _
val add = new Function2[Int, Int, Int] {
def apply(x:Int, y:Int):Int = x + y
}
어떤 함수가 다음 조건을 만족하면 일급 함수로 간주한다.

add(5,1)  6 1. 변수나 데이터 구조안에 담을 수 있다.
add.apply(5,1)  6 2.
3.
파라미터로 전달 할 수 있다.
반환값(return value)으로 사용할 수 있다.
4. 할당에 사용된 이름과 관계없이 고유한 구별이 가능하다.
출처: http://en.wikipedia.org/wiki/First-class_function 37

Closures
// plusFoo can reference any values/variables in scope
var foo = 1
val plusFoo = (x:Int) => x + foo

plusFoo(5)  6

// Changing foo changes the return value of plusFoo
foo = 5
plusFoo(5)  10

38

Pattern Matching
def what(any:Any) = any match {
case i:Int => "It's an Int"
case s:String => "It's a String"
case _ => "I don't know what it is"
}

what(123)  "It's an Int"
what("hello")  "It's a String"
what(false)  "I don't know what it is"

39

Pattern Matching
val nums = List(1,2,3)

// Pattern matching to create 3 vals
val List(a,b,c) = nums

a  1
b  2
c  3

40

Immutable Types
// Immutable types by default
var nums = Set(1,2,3)
nums += 4  nums = nums.+(4)

// Mutable types available
import scala.collection.mutable._

val nums = Set(1,2,3)
nums += 4  nums.+=(4)

41

Referential Transparency
// Transparent
val example1 = "jamie".reverse
val example2 = example1.reverse
println(example1 + example2) // eimajjamie

// Opaque
val example1 = new StringBuffer("Jamie").reverse
val example2 = example1.reverse
println(example1 append example2) // jamiejamie

 An expression is transparent if it can be replaced by its
VALUE without changing the behavior of the program
 In math, all functions are referentially transparent

42

Scala Collections

val myMap = Map(1 -> "one", 2 -> "two", 3 -> "three")
val mySet = Set(1, 4, 2, 8)
val myList = List(1, 2, 8, 3, 3, 4)
val myVector = Vector(1, 2, 3...)

You have the choice of mutable or immutable
collection instances, immutable by default
Rich implementations, extremely flexible

43

Rich Collection Functionality

val numbers = 1 to 20 // Range(1, 2, 3, ... 20)

numbers.head // Int = 1
numbers.tail // Range(2, 3, 4, ... 20)
numbers.take(5) // Range(1, 2, 3, 4, 5)
numbers.drop(5) // Range(6, 7, 8, ... 20)

There are many methods available to you in the
Scala collections library
Spend 5 minutes every day going over the
ScalaDoc for one collection class

44

scala.collection

45

scala.collection.immutable

46

scala.collection.mutable

47

Or Use Existing Java Collections
java.util
Apache Commons Collections
fastutil
Trove
Google Collections

scala.collection.JavaConversion available to convert to
and from java.util Interfaces

48

Higher Order Functions
val plusOne = (x:Int) => x + 1
val nums = List(1,2,3)

// map takes a function: Int => T
nums.map(plusOne)  List(2,3,4)

// Inline Anonymous
nums.map(x => x + 1)  List(2,3,4)

// Short form
nums.map(_ + 1)  List(2,3,4)

49

val nums = List(1,2,3,4)
// A few more examples for List class
nums.exists(_ == 2)  true
nums.find(_ == 2)  Some(2)
nums.indexWhere(_ == 2)  1
nums.reduceLeft(_ + _)  10
nums.foldLeft(100)(_ + _)  110
nums.filter(_ % 2 == 0)  List(2,4)

// Many more in collections library
flatMap, flatten, zip, fold, foldRight, reduce, reduceRight, groupBy
partition, slice, take, takeRight, takeWhile, drop, dropRight, dropWhile, splitAt
sortBy, sortWith, startsWith, endsWith, product, sum …

50


// functions as parameters
def call(f: Int => Int) = f(1)

call(plusOne)  2
call(x => x + 1)  2
call(_ + 1)  2

51

// functions as parameters
def each(xs: List[Int], fun: Int => Unit) {
if(!xs.isEmpty) {
fun(xs.head)
each(xs.tail, fun)
}
}

each(List(1,2,3), println)
 1
 2
 3
52

class List[+A] … {
def head: A
def tail: List[A]
…
}
// More complex example with generics & pattern matching
@tailrec
def each[T](xs: List[T], fun: T => Unit): Unit = xs match {
case Nil =>
case head :: tail => fun(head); each(tail, fun)
}
each(List(1,2), println)
 1
 2
each(List("foo", "bar"), println)
 foo
 bar

53

Parallel Collections
scala> 1 to 1000000
res0: scala.collection.immutable.Range.Inclusive = Range(1, 2, 3,...

scala> res0.par
res1: s.c.parallel.immutable.ParRange = ParRange(1, 2, 3,...

scala> res1 map(_ + 1)
res2: s.c.parallel.immutable.ParSeq[Int] = ParVector(2, 3, 4,...

scala> res2.seq
res3: s.c.immutable.Range = Range(2, 3, 4,...

 You can easily parallelize the application of a function literal to your collection by
calling the par() method on a collection instance
 Uses JSR166 under the covers to fork/join for you
 Use the seq() method on the parallel collection to return to a non-parallel
instance

54

Partial Functions
val sample = 1 to 10
val isEven: PartialFunction[Int, String] = {
case x if x % 2 == 0 => x+" is even"
}
val isEven= new PartialFunction[Int, String] {
override def apply(x:Int):String = x+" is even"
override def isDefinedAt(x:Int):Boolean = x % 2 == 0
}
// the method collect can use isDefinedAt to select which members to collect
val evenNumbers = sample collect isEven
> evenNumbers = Vector(2 is even, 4 is even, 6 is even, 8 is even, 10 is even)

val isOdd: PartialFunction[Int, String] = {
case x if x % 2 == 1 => x+" is odd"
}
// the method orElse allows chaining another partial function to handle
// input outside the declared domain
val numbers = sample map (isEven orElse isOdd)
> numbers = Vector(1 is odd, 2 is even, 3 is odd, 4 is even, 5 is odd, 6 is even, 7 is o
dd, 8 is even, 9 is odd, 10 is even)
55

Currying
// multiple arguments
def add(x:Int, y:Int) = x + y
add(1, 2) // 3
add(7, 3) // 10

// after currying, single arguments
def add(x:Int) = (y:Int) => x + y
def add(x:Int)(y:Int) = x + y
add(1)(2) // 3
add(7)(3) // 10
 In computer science, currying, invented by Moses Schönfinkel and
Gottlob Frege, is the technique of transforming a function that takes
multiple arguments into a function that takes a single argument (the
other arguments having been specified by the curry).

56

Partially Applied Functions
def justSum(a:Int,b:Int,c:Int) = a+b+c
val partial1 = justSum _
val partial2 = justSum(1, _:Int, 8)

def curriedSum(a:Int)(b:Int)(c:Int) = a+b+c

println("Partial 1: "+partial1(5,6,7)) // Partial 1: 18
println("Partial 2: "+partial2(4)) // Partial 2: 13

57

(엄밀히 말하자면 동적 언어는 아니지만, 동적 언어에서
제공하는 많은 특징을 가지고 있다.)

Scala는 동적 언어이다.
object Hello {
}
}
58

Scriptable

// HelloWorld.scala
println("Hello World")

bash$ scala HelloWorld.scala
Hello World

bash$ scala -e 'println("Hello World")'
Hello World

59

Read-Eval-Print Loop
bash$ scala
Welcome to Scala version 2.8.1.final (Java HotSpot(TM) 64-Bit Server VM,
Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class Foo { def bar = "baz" }
defined class Foo

scala> val f = new Foo
f: Foo = Foo@51707653

scala> f.bar
res2: java.lang.String = baz

60

Structural Typing – Type safe Duck Typing
// Duck Typing (other language)
def test(o)
log o.getName # Let's hope this will work
end

// Type safe Duck Typing
def doTalk(any:{def talk:String}) {
println(any.talk)
}
class Duck { def talk = "Quack" }
class Dog { def talk = "Bark" }

doTalk(new Duck)  "Quack"
doTalk(new Dog)  "Bark"

출처: http://beust.com/weblog/2008/02/11/structural-typing-vs-duck-typing/ 61

Implicit Conversions
// Extend existing classes in a type safe way

// Goal: Add isBlank method to String class
class RichString(s:String) {
def isBlank = null == s || "" == s.trim
}

implicit def toRichString(s:String) = new RichString(s)

// Our isBlank method is now available on Strings
" ".isBlank  true
"foo".isBlank  false

62

Implicit Conversions
// Does not type check
"abc".isBlank

// Search in-scope implicits defs that take a
// String & return a type with an isBlank method
implicit def toRichString(s:String):RichString

// Resulting code that type checks
new RichString("abc").isBlank

63

method_missing (Scala 2.9 Feature)
// Dynamic is a marker trait used by the compiler
class Foo extends Dynamic {
def typed[T] = error("not implemented")

def applyDynamic(name:String)(args:Any*) = {
println("called: "+name+"("+args.mkString(",")+")")
}

}

val f = new Foo
f.helloWorld  called: helloWorld()
f.hello("world")  called: hello(world)
f.bar(1,2,3)  called: bar(1,2,3)

64

Scala는 정적으로 타입화된 언어이다.
object Hello {
}
}
65

Generic Classes
class Stack[T] {
var elems: List[T] = Nil
def push(x: T) { elems = x :: elems }
def top: T = elems.head
def pop() { elems = elems.tail }
}
object GenericsTest extends Application {
val stack = new Stack[Int]
stack.push(1)
stack.push('a')
println(stack.top) // 97
stack.pop()
println(stack.top) // 1
}

출처: http://www.scala-lang.org/node/113 66

Variances
구분 Scala Java Description

Covariant +T ? extends T Covariant subclassing. E.g., List[Tsub] is a subtype of List[T].

Contravariant -T ? super T Contravariant subclassing. E.g., X[Tsup] is a subtype of X[T].

Invariant T T Invariant subclassing. E.g., Can’t substitute Y[Tsup]or Y[Tsub] for Y[T].

class Animal
class Dog extends Animal
class Vector[+T] { … }
class Contravariant[-T] {…}
class Array[T] {…}

val aCovariantVector: Vector[Animal] = Vector.empty[Animal] // ok
val aCovariantVector: Vector[Animal] = Vector.empty[Dog] // ok
val aCovariantVector: Vector[Dog] = Vector.empty[Animal] // not ok

val contravariantClass: Contravariant[Animal] = new Contravariant[Animal] // ok
val contravariantClass: Contravariant[Dog] = new Contravariant[Animal] // ok
val contravariantClass: Contravariant[Animal] = new Contravariant[Dog] // not ok

val anInvariantArray: Array[Animal] = Array.empty[Animal] // ok
val anInvariantArray: Array[Animal] = Array.empty[Dog] // not ok
val anInvariantArray: Array[Dog] = Array.empty[Animal] // not ok


Type Bounds (Upper, Lower)

Upper Bounds A <: B A extends B A must be a subtype of B

Lower Bounds A >: B - A must be a supertype of B

object Array {
...
def apply[A <: AnyRef](xs: A*): Array[A] = { // A must be a subtype of AnyRef
val array = new Array[A](xs.length)
var i = 0
for (x <- xs.elements) { array(i) = x; i += 1 }
array
}
...
}
class List[+A] { // A is covariant, List[+A] is subtype of List[A]
...
def ::[B >: A](x : B) : List[B] = new scala.::(x, this) // B must be a supertype of A
...
}
val languages = List("Scala", "Java", "Ruby", "C#", "C++", "Python") // languages:List[String] = List(…)
val list = 3.14 :: languages // list:List[Any] = List (…)
println(list) // List(3.14, “Scala”, “Java”, “Ruby”, “C#”, “C++”, “Python”)


Views
Implicit parameters and methods can also define implicit conversions called views.
The term view conveys the sense of having a view from one type (A) to another type (B).

A view is applied in two circumstances.
 When a type A is used in a context where another type B is expected and there is a view in scope that can convert A to B.
 When a non-existent member m of a type A is referenced, but there is an in-scope view that can convert A to a B that has
the m member.

def m [A <% B](arglist): R = ...
def m [A](arglist)(implicit viewAB: A => B): R = ...

class Container[A <% Int] { def addIt(x: A) = 123 + x }
(new Container[String]).addIt("123") // ok, 246
(new Container[Int]).addIt(123) // ok, 246
(new Container[Float]).addIt(123.2F) // not ok


View Bounds A <% B - A can be converted to B using a view


Inner Classes
class Graph { Use Companion Objects
class Node {
var connectedNodes: List[Node] = Nil object Graph {
def connectTo(node: Node) { class Node {
if (connectedNodes.find(node.equals).isEmpty) { var connectedNodes: List[Node] = Nil
connectedNodes = node :: connectedNodes def connectTo(node: Node) {
} if (connectedNodes.find(node.equals).isEmpty) {
} connectedNodes = node :: connectedNodes
} }
var nodes: List[Node] = Nil }
def newNode: Node = { }
val res = new Node }
nodes = res :: nodes class Graph {
res var nodes: List[Graph.Node] = Nil
} def newNode: Node = {
} val res = new Node
nodes = res :: nodes
object IllegalGraphTest extends Application { res
val g: Graph = new Graph }
val n1: g.Node = g.newNode }
val n2: g.Node = g.newNode
n1.connectTo(n2) // legal
val h: Graph = new Graph
val n3: h.Node = h.newNode Use Type Projection
n1.connectTo(n3) // illegal! class Graph {
} class Node {
var connectedNodes: List[Graph#Node] = Nil
def connectTo(node: Graph#Node) {
if (connectedNodes.find(node.equals).isEmpty) {
connectedNodes = node :: connectedNodes
}
}
}
1. Use Companion Objects: object Graph for class Graph …
2. Use Type Projection: Graph#Node is “a Node of any Graph” }

출처: http://www.scala-lang.org/node/115, http://weblogs.java.net/blog/cayhorstmann/archive/2011/08/05/inner-classes-scala-and-java 70

Abstract Types
abstract class Buffer[+T] {
val element: T
}

abstract class SeqBuffer[U, +T <: Seq[U]] extends Buffer[T] {
def length = element.length
}

object AbstractTypeTest2 extends Application {
def newIntSeqBuf(e1: Int, e2: Int): SeqBuffer[Int, Seq[Int]] =
new SeqBuffer[Int, List[Int]] {
val element = List(e1, e2)
}

val buf = newIntSeqBuf(7, 8)
println("length = " + buf.length) // length = 2
println("content = " + buf.element) // content = List(7, 8)
}


Explicitly Typed Self References
abstract class SubjectObserver {
type S <: Subject // S must be a subtype of Subject
type O <: Observer // O must be a subtype of Observer

trait Subject {
self: S => // #1. val self: SubjectObserver.this.S
private var observers = List[O]()
def addObserver(observer: O) = observers ::= observer
def notifyObservers = observers foreach (_.receiveUpdate(self)) // #2
}

trait Observer {
def receiveUpdate(subject: S)
}
}

- Comment #1 shows the self-type annotation, self: S =>. We can now use self as an alias for this, but whenever it appears, the
type will be assumed to be S, not Subject.

- Comment #2 shows that we pass self instead of this to receiveUpdate.


Polymorphic Methods
object PolyTest extends Application {
def dup[T](x: T, n: Int): List[T] =
if (n == 0) Nil
else x :: dup(x, n - 1)
println(dup[Int](3, 4)) // required parameters
println(dup("three", 3)) // not required to give actual type parameters explicitly
}

Method dup in object PolyTest is parameterized with type T and with the value parameters x:
T and n: Int


Scala는 많은 좋은 기능이 있다.
object Hello {
}
}
74

Default Parameter Values

def hello(foo:Int = 0, bar:Int = 0) {
println("foo: "+foo+" bar: "+bar)
}

hello()  foo: 0 bar: 0
hello(1)  foo: 1 bar: 0
hello(1,2)  foo: 1 bar: 2

75

Named Parameters

def hello(foo:Int = 0, bar:Int = 0) {
println("foo: "+foo+" bar: "+bar)
}

hello(bar=6)  foo: 0 bar: 6
hello(foo=7)  foo: 7 bar: 0
hello(foo=8,bar=9)  foo: 8 bar: 9

76

Everything Returns a Value
val a = if(true) "yes" else "no"

val b = try{
"foo"
} catch {
case _ => "error"
}

val c = {
println("hello")
"foo"
}
77

Lazy Definitions
// initialized on first access
lazy val foo = {
println("init")
"bar"
}

foo  init
foo 
foo 

78

Nested Functions
// Can nest multiple levels of functions
def outer() {
var msg = "foo"
def one() {
def two() {
def three() {
println(msg)
}
three()
}
two()
}
one()
}

79

By-Name Parameters
// msg parameter automatically wrapped in closure
def log(doLog:Boolean, msg: => String) {
if(doLog) {
msg // evaluates msg
msg // evaluates msg again!
}
}

def foo:String = {
println("in foo"); "Foo"
}

log(true, foo+" Bar") // foo called twice
 in foo
 in foo

log(false, foo+" Bar") // foo never called

80

Many More Features
 Annotations  @foo def hello = "world"
 Option
 Actors
 DSL
 For Comprehensions  for(i <- 1 to 5 if i % 2 == 0) yield i
 Implicit Conversion
 XML Literals  val node = <hello>world</hello>
 Reflections
 Macros (2.10)
 etc…

81

Scala를 어떻게 공부하지?
object Hello {
}
}
82

Scala 공부의 정석

83

Martin Ordersky 저자 강의

https://www.coursera.org/course/progfun 84

Typesafe

http://www.typesafe.com 85

Scala-Lang.org

http://www.scala-lang.org/ 86

Scala 공부관련 참고자료
 스칼라 입문자료
– Scala API Reference
– A Tour of Scala : 스칼라 공식 블로그에 올라온 스칼라 강좌
– Scala By Example : 마틴 오더스키가 작성한 100페이지짜리 스칼라 문서
– Scala School : 트위터가 작성한 스칼라 강좌
– Progammin in Scala 1st Edition : Programmin in Scala의 1st Edition은 온라인에서 무료로 볼
수 있음.
– Effective Scala : 트위터의 Marius Eriksen가 만든 스칼라 강좌

 스칼라 서적(영문)
– Programming in Scala : 마틴 오더스키가 참여한 스칼라 서적. 강추!!, Scala 2.8 기준.
– Programming Scala : 오렐리에서 나옴. 읽어본 사람 없어서 내용은 잘 모름.
– Programming Scala : Pragmatic에서 나온 스칼라 서적. 읽어볼 만은 하지만 빠진 내용이 많아
서 많이 추천하지는 않음.
– Scala in Depth : 매닝에서 나온 서적. 읽어본 사람이 없어서 내용은 잘 모름.

 그외 참고 자료
– Style Guide for Scala : 스칼라 코드 작성에 대한 스타일가이드 발표자료
– Scala Puzzlers : 스칼라 문제를 풀어볼 수 있는 사이트
– Scala Cheat Sheet : 간단한 스칼라문법을 참고할 수 있는 치트시트
– S99 Scala Problems: 99 개의 스칼라 프로그래밍 학습 예제

87

Scala overview

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