Introduction to Scala

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Introduction to Scala The “new” Java? Eivind Barstad Waaler BEKK/UiO

July 4, 2009

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Presentation Outline

Outline Introduction Concise Syntax Object Orientation Functional Programming Various Stuff  Conclusion

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Introducing Scala

What is Scala?

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100% Object-oriented

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Functional programming

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Typing: static, strong, inferred/implicits Java bytecode → JVM

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.NET (CLR) Also available

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Extensibility

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Java-like Java-like syntax → Scala syntax

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Current version 2.7.5

Eivind Barstad Waaler

→

2.8

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Introducing Scala

A first example class IntMath(val IntMath(val x: Int, Int, val y: Int) { def sum(): Int = { x + y; } def m u l = x * y } val test test = new IntMat IntMath(3 h(3, , 4) println(test.x) // ou outp tput ut: : 3 println(test.sum) // ou outp tput ut: : 7 println(test.mul()) // ou outp tput ut: : 12

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Introducing Scala

The Scala Interpreter

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An interactive “shell” for writing Scala expressions

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Automatically creates temporary result variables

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Simply type ’scala’ at the command prompt Using Maven2:

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scala:console’ Type ’  mvn scala:console Starts the Scala Interpreter with project classpath

Demo

Eivind Barstad Waaler

⇒

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Syntax Examples

Imports   

Like Java, but more features Can be anywhere Import from packages, classes or objects

import import import import

java.util._ // Al All l java.util.{ArrayList,HashSet} // Sel Select ected ed java.util.{ArrayList => _, _} // Al All l ex exce cept pt java.util.ArrayList._ // All from AL

val list list = new ArrayList import list._ // Al All l fr from om ob obje ject ct li list st import java.sql.{Date => SDate} // Ren Renami aming ng import java.util.Date Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Syntax Examples

Method syntax 

. and () can be omitted for binary/unary methods

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Methods can have any name

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Operators are simply methods

→

full operator overloading

2.max(5) // max of 2 and 5 2 max 5 // sa same me as abov above e class def } val x val y val z

MyNumber(val MyNumber(val num: num: Int) Int) { +(othe +(other: r: MyNumb MyNumber) er) = new MyNumb MyNumber( er(oth other. er.num num + num) num) = new MyNumber(5) = new MyNumber(6) = x + y

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Syntax Examples

Type inferrence/implicits   

If type is obvious – no need to write it Implicits can be defined Implicits heavily used in std lib (RichInt etc.)

// Ty Type pe in infe ferr rren ence ce val a = 4 2 // Type = Int val b = "he "hello llo wor world! ld!" " // Ty Type pe = St Stri ring ng def add(x: Int, y: Int) = x + y // Re Retu turn rn-t -typ ype e = In Int t val sum = add(3, 5) // Type of sum = Int // Imp Implic licits its class MyInt(val MyInt(val i: Int) { def doubleIt = i * 2 } implic imp licit it def from fromIn Int( t(i: i: Int) Int) = new MyInt(i) 5.doubleIt // = 10 Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

OO Introduction

Object Orientation

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Pure OO – everything is an object

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Classes – blueprints for objects (like Java)

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Singleton objects

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Traits – AOP like possibilities

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Semicolon inference

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Classes and Objects

Scala Class Hierarchy

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Classes and Objects

Classes  

Much like Java Contains fields and methods – can override eachother 

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override keyword mandatory

Can take parameters directly – constructor

class A(val A(val num: num: Int) Int) class B(num: B(num: Int, Int, val str: str: String String) ) extends A(nu A(num) m) { def calc() = num * num // calc is a method } class C(nu C(num: m: Int, Int, str: str: Stri String ng) ) extends B(nu B(num, m, str) str) { overri ove rride de val calc = 65 // ov over erri ride de me meth thod od wi with th va val l } val a = new A(35) // Type A val b = new B(32, "Eivind") "Eivind") // Type B - calc method val b2: B = new C(32, "Eivind" "Eivind") ) // Also type B - calc val Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Classes and Objects

Singleton Objects   

No static members in Scala → Singleton Objects Keyword object instead of  class Companion class/object – same name  

Factory methods Other unique/static behaviour

// Ar Arra ray y cl clas ass s de defi fini niti tion on final fin al cla class ss Array[A](_le Array[A](_length: ngth: Int) extends Array0[A] // Ar Arra ray y ob obje ject ct defi defini niti tion on wi with th me meth thod od object Arra Array y { def appl apply( y(xs xs: : Int* Int*): ): Arra Array[ y[In Int] t] = { ... ... } } // Cr Crea eate te arra array y wi with th fo four ur in inte tege gers rs val arr = Array(1, 2, 3, 4) Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Classes and Objects

The “magic” apply()-method  

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Scala provides special syntax for calling the apply() method In classes used to look up elements (for instance in Collections) In objects used to create class instances Functions in Scala are actually just apply() methods Make your classes/objects appear as built-in syntax

// Ar Arra ray y ob obje ject ct ha has s ap appl ply y me meth thod od fo for r cr crea eati ting ng ar arra rays ys def appl apply( y(xs xs: : Int* Int*): ): Arra Array[ y[In Int] t] = { ... ... } // Ar Arra ray y cl clas ass s ha has s ap appl ply y me meth thod od to ac acce cess ss th the e ar arra ray y def appl apply( y(i: i: Int) Int): : A // Sc Scal ala a sp spec ecia ial l sy synt ntax ax val arr = Array(4, 3, 2, 1) // Ar Arra ray. y.ap appl ply( y(4, 4, 3, 2, 1) val thre three e = arr( arr(1) 1) // arr arr.ap .apply ply(1) (1) Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Traits

Traits    

Encapsulates method and field definitions, much like classes A class can mix in any number of traits → multiple multiple inheritance inheritance Widen thin interfaces to rich ones Define stackable modifications

trait Hell Hello o { def hell hello o { prin printl tln( n("Hello!" "Hello!") ) } } trait Good Goodby bye e { def bye bye { prin printl tln( n("B "Bye ye by bye! e!" ") } } // Ob Obje ject ct wi with th bo both th he hell llo( o() ) an and d by bye( e() ) me meth thod ods. s.. . object A extends Hello with Goodbye Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Traits

Traits – Widen thin interface to rich  

Define one or a few abstract methods Define concrete methods implemented in terms of the abstract

trait Ordere Ordered[A d[A] ] { abstra abs tract ct def compar compare(t e(that hat: : A): Int def "s "str tr: : " + s case (a, (a, b) => "t "tup uple le: : " + a + b // Tu Tupl ple e pa patt tter ern n case _ => "unknown" // Wil Wildca dcard/ rd/def defaul ault t pat patter tern n } desc(8) // "i "int nt: : 8" desc("Scala" desc("Scala") ) // "s "str tr: : Sc Scal ala" a" desc(5) // "f "fiv ive" e" desc(("Eivind" desc(("Eivind", , 32)) 32)) // "tu "tuple ple: : Eiv Eivind ind32" 32" desc(4.0) // "un "unkno known" wn"

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Pattern Matching

Pattern matching – Case classes example  

Factory method + all params are val Methods toString, hashCode and equals added

case ca se cl clas ass s Person Person(na (name: me: String String, , age: age: Int) Int) def gree greet( t(x: x: Any) Any) = x match { case Person("Eivind" Person("Eivind", , 32) => "He "Hello llo cre creato ator!" r!" case Person Person(n, (n, a) if a < 3 2 => "H "Hel ello lo yo youn ung g " + n case Person Person(n, (n, _) => "H "Hel ello lo " + n case _ => "He "Hello llo wha whatev tever" er" } greet(Person("Ola" greet(Person("Ola", , 80)) 80)) // He Hell llo o Ol Ola a greet(Person("Ola" greet(Person("Ola", , 5)) // He Hell llo o yo youn ung g Ol Ola a greet("Eivind" greet("Eivind") ) // He Hell llo o wh what atev ever er greet(Person("Eivind" greet(Person("Eivind", , 32)) 32)) // He Hell llo o cr crea eato tor! r! Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Pattern Matching

Pattern matching – List example def desc desc(x (x: : Any) Any) = x match { case List List(_ (_: : Stri String ng, , _: Stri String ng) ) => "L "Lis ist t of tw two o st stri ring ngs" s" case List List(_ (_, , _) => "L "Lis ist t of tw two o el elem ems" s" case head :: _ => "L "Lis ist t st star arti ting ng wi with th " + head head case _ => "Whatever" } desc(L desc(List ist(1, (1, 2)) // List of two elems desc desc(L (Lis ist( t(1, 1, 2, 3)) 3)) // Li List st st star arti ting ng wi with th 1 desc(List("hello" desc(List("hello", , "world" "world")) )) // Li List st of tw two o st stri ring ngs s // No Note te! ! Tw Two o eq equi uiva vale lent nt de defs fs - "E "Err rror or: : un unre reac acha habl ble e co code de" " case head :: _ => "L "Lis ist t st star arts ts wi with th " + head head case List(h List(head ead, , _*) => "L "Lis ist t st star arts ts wi with th " + head head

Eivind Barstad Waaler

BEKK/UiO

Outline

Introduction

Concise Syntax

Object Orientation

Functional Programming

Various Stuff 

Conclusion

Other Functional Concepts

For expressions   

Generators, definitions and filters Can yield value – Range class A rewrite of methods map, flatMap and filter

for (l