ACCU 2011 Introduction to Scala: An Object Functional Programming Language

Blog: www.XeNoNiQUe.co.uk/blog/ACCU: Member since 1993Education: London South Bank UniversityLanguages: Scala, Groovy, JavaFX, JavaConferences: JavaOne, ACCU, QCon, Devoxx XeNoNiQUe.co.uk (c) 2011 4/14/2011 2

“If I were a pick a language to use today other than Java, it would be Scala” James Goslinghttp://www.adam‐bien.com/roller/abien/entry/java_net_javaone_which_programming XeNoNiQUe.co.uk (c) 2011 4/14/2011 3

Headline News “More and more programmers are going to need to understand functional programming at a productive level. Especially over the next 5 to 10 yeas” ‐ IBM, Jason Stidd, Functional Programming Blog, 2010 “Scala enjoys a singular advantage: performance. The language is compiled to optimised byte codes and runs as fast as native Java” Infoworld, 2010 XeNoNiQUe.co.uk (c) 2011 4/14/2011 4

Today’s Agenda Classes and Companion Objects Function Objects Case classes and Pattern matching Traits, Mix‐ins and Compositions Scala Collections Library Frameworks XeNoNiQUe.co.uk (c) 2011 4/14/2011 6

A Brief History of Scala Make Java Better 1996 – 2000: Pizza, GJ and javac Make a Better Java 2003 : The Scala 1.0 Experiment 2005 : Scala 2.0, written in Scala 2006‐2011: Industrial strength XeNoNiQUe.co.uk (c) 2011 4/14/2011 10

A Java Classpublic class Person { public final String firstName, lastName; public final float height; public final int age; public Person( String firstName, String lastName, float height, int age ) { this.firstName; this.lastName = lastName; this.height = height; this.age = age; }} XeNoNiQUe.co.uk (c) 2011 4/14/2011 11

Filtering Data with Javaimport java.util.ArrayList;Person[] people = getPeople();Person[] tallFolk;Person[] shortFolk;{ ArrayList<Person> tallList = new ArrayList<Person>(); ArrayList<Person> shortList = new ArrayList<Person>(); for ( int j=0; j<people.length; ++j) { (people[j].height < 5.0 ? shortList : tallList ) .add( people[j] ); } shortFolk = smallList.toArray( people ); tallFolk = tallList.toArray( people );} XeNoNiQUe.co.uk (c) 2011 4/14/2011 13

VariablesTwo Types of variables: val a immutable variable var a re‐assignable variableval s1 = “Purple Haze” // Type inference Stringvar s2 = “Barcelo”s1 = “Hotel” // Won’t compiles2 = “Electricona” // Ok! XeNoNiQUe.co.uk (c) 2011 4/14/2011 17

Variables Type Inferencesval s3: String = “Purple Haze”val f1: Double = 3.141596527val fruit = List(“orange”,”apple”) // scala.collection.immutable.List[String]val capitals = Map(“USA”-> “Washington”, ”Wales” -> “Cardiff”, “Germany” -> “Berlin” ) // scala.collection.immutable.Map[String, String] XeNoNiQUe.co.uk (c) 2011 4/14/2011 18

Scala IdentifiersFour forms of Scala identifiers Alphanumeric: A letter followed by a sequence of letters or digits _ count as a letter (but single _ is reserved) $ is reserved for system use Operator: One of more symbolic characters such + , *, /, ‐ , %, #, ! Mixed: gamma_! Quoted: any character sequence in back‐quotes: `yield` XeNoNiQUe.co.uk (c) 2011 4/14/2011 20

Basic Scala Literals Integer: 1, 882, ‐1 Boolean: true, false Double: 1.0, 1d, 1e3, 1.234e‐6 Long: 42L Float: 19.73f Characters: ‘6’, ‘@’, ‘?’, `K’ String: “Hello world” XeNoNiQUe.co.uk (c) 2011 4/14/2011 21

Expressions Initialise Valsval powerSeries = { var x = 2.0; var sum = 1.0 for ( c <- 1 to 7 ) yield { sum += x; x = x * x; sum }}// scala.collection.immutable.IndexedSeq[Double] = Vector(3.0, 7.0, 23.0, 279.0, 65815.0, 4.295033111E9, 1.8446744078004584E19) XeNoNiQUe.co.uk (c) 2011 4/14/2011 25

Functions Written Conciselydef max( x: Double, y: Double ): Double = if ( x > y) x else ydef min( x: Double, y: Double ): Double = if ( x < y) y else x// type inference on the return type: Doubledef cube( x: Double ) = x * x * x XeNoNiQUe.co.uk (c) 2011 4/14/2011 27

Procedures and The Unit Typedef sideEffect(): Unit = println(“Say hi!”)// sayHi: ()UnitsideEffect() // prints “Say hi!”• Unit has a value, written as ()• Unit roughly corresponds to void in Java XeNoNiQUe.co.uk (c) 2011 4/14/2011 28

The Primary Constructorclass Person ( val firstName: String val lastName: String, val height: Float, val age: Int ) { if ( age < 0 ) throw new Exception("age?") if ( height <= 0.5 ) throw new Exception("height?")} XeNoNiQUe.co.uk (c) 2011 4/14/2011 31

Class ‐ Better Styleclass Person ( val firstName: String, val lastName: String, val height: Float, val age: Int ) { require( age > 0, "age > 0" ) require( height >= 0.5, "height >= 0.5" ) override def toString = "%s %s %3d %5.2f (ft)". format( firstName, lastName, age, height )} XeNoNiQUe.co.uk (c) 2011 4/14/2011 32

Class ‐ Auxiliary Constructorsclass Complex( real: Float, imag: Float { def this() = this(0) def this( r: Float ) = this ( r, 0 ) def this( r: String ) = this ( java.lang.Float.parseFloat(r), 0 ); def this( r: String, i: String ) = this ( java.lang.Float.parseFloat(r), java.lang.Float.parseFloat(i) ) // ...} XeNoNiQUe.co.uk (c) 2011 4/14/2011 33

Built‐In Control Structures #2While ... Do Do ... Whilevar c1 = 0 var c2 = 0var y1 = "" var y2 = ""while ( c1 < 4 ) { do { y1 = y1 + "#" + c1 y2 = y2 + "#" + c2 c1 += 1 c2 += 1} }println(y2) while ( c2 < 4 ) println(y2) XeNoNiQUe.co.uk (c) 2011 4/14/2011 35

Scala Fixes Java MishapsScala Removes Scala Innovates Break and Continue Operator overloading Static members Function objects Primitive types Companion Objects Special treatment of Case classes interfaces User control abstractions Mix‐ins Pattern matching Implicit conversions XeNoNiQUe.co.uk (c) 2011 4/14/2011 36

Infix Operators in Classesclass Complex( val re:Float, val im: Float ) { def +( x:Complex ): Complex = { new Complex( this.re + x.re, this.im + x.im ) } def -( x:Complex ): Complex = { new Complex( this.re - x.re, this.im - x.im ) } // ... override def toString = re + (if (im < 0 ) "-" +(-im) else "+" +im )+"i"} XeNoNiQUe.co.uk (c) 2011 4/14/2011 37

Using Operator Overloadingval z1 = new Complex(3,4)val z2 = new Complex(1,2)val z3 = z1 + z2 // = z1.+(z2)val z4 = z1 * z2 // = z1.*(z2)val z5 = z1 - z2 // = z1.-(z2)val z6 = z1 / z2 // = z1./(z2) XeNoNiQUe.co.uk (c) 2011 4/14/2011 38

Infix Operator AssociativityNormally expressions associate from LEFT to RIGHTval x = 1 + 2 + 3 + 4val x = ((1 + 2) + 3 ) + 4Except for those operators that end with a colon(:) They associate from RIGHT to LEFTval list = 1 :: 2 :: 3 :: 4 :: Nilval list = 1 :: ( 2 :: ( 3 :: (4 :: Nil ))) XeNoNiQUe.co.uk (c) 2011 4/14/2011 39

Companion Objects• Represents a single instance of a class• Serves the analogous role of “static” in Java• Frequently defines two methods apply() and unapply() for factories and extractionobject Complex { val i = new Complex( 0, 1 ) def apply( re: Float, im: Float ) = new Complex( re, im )} XeNoNiQUe.co.uk (c) 2011 4/14/2011 40

For‐Filter Comprehensionsfor ( trade <- trades if trade.tradeType == "FX" && trade.amount >= 75000 && trade.counterparty.toUpperCase .startsWith("North")) processLayerDown( trade ) XeNoNiQUe.co.uk (c) 2011 4/14/2011 44

For‐Nested‐Filter Comprehensionfor ( trade <- trades if trade.tradeType == "MM" ; if trade.origin == "Summit"; settlement <- trade.settlements settlement.exists( _.name == Standard )) settleStandard( settlement ) XeNoNiQUe.co.uk (c) 2011 4/14/2011 45

Function Object ‐ Long Handval isEven = new Function1 [ Int, Boolean ] { def apply( k: Int ) = k % 2 == 0}Syntactic definition for function type that:• Accepts an Integer parameter• Returns a Boolean value• Defines a method called apply to invoke the function definition. XeNoNiQUe.co.uk (c) 2011 4/14/2011 47

First‐Class Functionsval isEven: (Int => Boolean) = (k: Int) => k % 2 == 0// The verbose short‐hand notionFunctions are values, values are objectErgo, functions are objects in Scala XeNoNiQUe.co.uk (c) 2011 4/14/2011 48

Filter Out Even Numbersval numbers = List(0,1,2,3,4,5,6,7)// List[Int] = List(0,1,2,3,4,5,6,7)numbers.filter( isEven )// List[Int] = List(0,2,4,6)Apply the function object isEven to all members of the sequence XeNoNiQUe.co.uk (c) 2011 4/14/2011 51

Real World Closuresvar extra = computeFudgeFactor()reactions += { case e:MouseClicked => plotXY( -1.0, 1.0, -2.0, 2.0, ( x: Float ) => extra + sin(pi * x) / (pi * x ) )}The variable extra defined in the enclosing scope is bounded to the lexical scope of the function object. XeNoNiQUe.co.uk (c) 2011 4/14/2011 57

Functions Can Return Functionsdef foo1( x: Float)(y: Float ) = x*y-2*xdef foo1( x: Float) = { (y: Float ) => x*y-2*x }val a1 = foo1( 3 ) ( 4 ) // a1 is 6.0val a2 = foo1( 3 ) // a2 is (Float) => Float = <function1>val a3 = a2(4) // a3 = 6.0 XeNoNiQUe.co.uk (c) 2011 4/14/2011 60

Parameterised Typesclass Holder [T] ( val value: T )val chipBox = new Holder( 555 )val z1 = chipBox.value // 555val fruitBox = new Holder( "apple" )val z2 = fruitBox.value // “apple”fruitBox.value = "orange" // re-assign errorchipBox.value = 73 // re-assign error XeNoNiQUe.co.uk (c) 2011 4/14/2011 61

Call‐By‐Value versus Call‐By‐NameCall By Value Call By Name Expression are evaluated Expressions are evaluated before being passed as a inside the function parameter to a function Computational nice Useful for library writers to create custom control abstractions def debug( s: String ):Unit def debug( s: => String ):Unit XeNoNiQUe.co.uk (c) 2011 4/14/2011 62

Custom Control Abstractionsdef using[ T <: { def close() }, A ] ( closable: T ) ( body: T => A ) = { try { body( closable ) } finally { if ( closable != null ) { try { closable.close() } catch { case e: Exception => // Log this error } } }} XeNoNiQUe.co.uk (c) 2011 4/14/2011 63

Closures As Control Abstractionimport java.io._val file = new File( "poem.txt" )using( new BufferedReader( new FileReader( file )) ) { r => println( r.readLine() )} XeNoNiQUe.co.uk (c) 2011 4/14/2011 64

Case Classes Convenience data types for pattern matching Scala compiler creates : Class with hashCode(), equalsTo(), copy() methods Nice toString() method Defaults to immutable data members Companion object Defines apply() and unapply() (extractor) methods XeNoNiQUe.co.uk (c) 2011 4/14/2011 66

Case Classescase class Stock( name: String, size: Int )val st1 = Stock( "EDH", 15000 )val st2 = Stock( “XNF", 40000 )val nameAndValue = st2.name + “ “ +st1.value // “EDH 15000”val st3 = st2.copy()val st4 = st3.copy( name = "AXA")assert( st2 == st3 ) // trueassert( st2 != st4 ) // true XeNoNiQUe.co.uk (c) 2011 4/14/2011 67

Pattern Matching on Typesval list = List( 1967, "Legacy", £, 51.50F )for ( e <- list ) { e match { case f: Float => println("A decimal "+f ) case s: String => println("A string "+s ) case c: Char => println("A character "+c ) case _ => println("I do not know “+e ) }} XeNoNiQUe.co.uk (c) 2011 4/14/2011 68

Pattern Matching on Sequencesval list = List( 1967, "Legacy", £, 51.50F )list match { case 1967 :: _ :: ‘£’ :: _ => println("got it") case _ => println("not found" )}list match { case List( 1967, "Legacy", _* ) => println("got two" ) case _ => println("not found")} XeNoNiQUe.co.uk (c) 2011 4/14/2011 69

Pattern Matching Algorithmsdef fib( n: Int ): Long = { n match { case 0 => 0 case 1 => 1 case n => fib(n-2) + fib(n-1) }}val z1 = fib(3) // 2val series = (0 to 25).map( fib _ ).mkString("n") XeNoNiQUe.co.uk (c) 2011 4/14/2011 70

Case Classes Abstract Data Typesabstract class BinTree[T]case object Empty extends BinTreecase class Node ( elem: T, left: BinTree[T], right: BinTree[T]) extends BinTree XeNoNiQUe.co.uk (c) 2011 4/14/2011 71

Pattern Matching Algorithm// Traverse tree in-order by algorithmic casesdef inOrder[T]( n: BinTree[T] ) : List[T] = { n match { case Empty => List() case BinTree(e,l,r) => inOrder(l) ::: List(e) ::: inOrder(r) }} XeNoNiQUe.co.uk (c) 2011 4/14/2011 72

Pattern Matching Algorithm// Recursively find the depth of the binary treedef depth[T]( n: BinTree[T] ): Int = { n match { case Empty => 0 case Node( _, Empty, r ) => 1 + depth(r) case Node( _, l, Empty ) => 1 + depth(l) case Node( _, l, r ) => Math.max( depth(l), depth(r)) + 1 }} XeNoNiQUe.co.uk (c) 2011 4/14/2011 73

Traits and Mix‐ins Trait is provides composition of behaviour type in Scala Traits can define abstract methods Traits can define implementation methods Traits can define data members Linearisation of behaviour is prioritised from left to right Traits can inherit from other traits XeNoNiQUe.co.uk (c) 2011 4/14/2011 75

Mix‐ins Refactored #2trait Vehicletrait Car extends Vehicle with HasWheels with Poweredtrait Bicycle extends Vehicle with HasWheelstrait Motorcycle extends Vehicle with HasWheels with Poweredtrait Bus extends Cartrait Truck extends Cartrait SUV extends Car XeNoNiQUe.co.uk (c) 2011 4/14/2011 79

Motorway Responsibilitydef motorway( x: Vehicle with Powered ) = println( "on the motorway: " +x.kickdown )val aCar = new Fordval aBike = new Choppermotorway( aCar ) // Okmotorway( aBike ) // Won’t compile! XeNoNiQUe.co.uk (c) 2011 4/14/2011 80

Higher Order Operationsval list = List(1,2,3,4,5)val z0 = list.last // Int 5val z1 = list.map( _ + 3 ) // List( 4,5,6,7,8 )val z2 = list.take( 2 ) // List(1,2)val z3 = list.drop( 2 ) // List(3,4,5)val z4 = list.slice( 2,4 ) // List(3,4)val z5 = list.reverse // List(5,4,3,2,1)val z7 = list.contains(3) // trueval z8 = list.filter( _ < 3 ) // List( 1,2 ) XeNoNiQUe.co.uk (c) 2011 4/14/2011 86

Sorting List Collectionsval xs = List( 11, 7, 2, 15, 9, 1 )val x0 = xs.sort( _ < _ ) // List[Int](1, 2, 7, 9, 11, 15)val x1 = xs.sort( _ > _ ) // List[Int](15, 11, 9, 7, 2, 1) XeNoNiQUe.co.uk (c) 2011 4/14/2011 87

More Higher Operationsval xs = List( "apple", “plum", "orange" )val z0 = xs.zipWithIndex // List((apple,0), (plum,1), (orange,2))val ys = List( "usa", "uk", "france" )val z1 = xs zip ys // List((apple,usa), (plum,uk), (orange,france))val zs = List( List(1,2,3), List(4,5), List(6))val z2 = zs flatten // List(1, 2, 3, 4, 5, 6) XeNoNiQUe.co.uk (c) 2011 4/14/2011 88

Map Collections Typesvar capitals = Map( "England" -> "London", "Scotland" -> "Edinburgh", "Wales" -> "Cardiff", "Northern Ireland" -> "Belfast" )capitals += ( "Brasil" -> "Brasilia" )val z0 = capitals( "Brasil“ ) // “Brasilia”val z1 = capitals.isEmpty // falseval z2 = capitals.size // 4for ( (k,v) <- capitals ) println("Capital of "+k+" is "+v ) XeNoNiQUe.co.uk (c) 2011 4/14/2011 89

Set Collections Typesimport scala.collection.immutable.SortedSetvar xs = SortedSet( 3,2,1 )println(xs) // TreeSet(1,2,3 )xs += 4xs += 3println(xs) // TreeSet(1,2,3,4 )xs -= 2println(xs) // TreeSet(1,3,4 )xs ++= List( 5,6)println(xs) // TreeSet(1,2,4,5,6 ) XeNoNiQUe.co.uk (c) 2011 4/14/2011 90

Implicit Conversions“Can you make an ADT from a library and make it look just like it is part of the language?”, Guy Steele, Benchmark of Languages Implicit informs the compiler about a custom default conversion method Power enabler for writing embedded DSL in Scala XeNoNiQUe.co.uk (c) 2011 4/14/2011 91

Declare Implicit Conversionsobject Complex { implicit def doubleToComplex( x: Double ) = new Complex( x, 0 ) val i = new Complex( 0, 1 ) def apply( re: Float, im: Float ) = new Complex( re, im )} XeNoNiQUe.co.uk (c) 2011 4/14/2011 92

Breaking The Mono‐Culture“Do not throw the baby out with the bath water by clinging to a Java‐only monoculture approach: it matters not that what current systems are written in, when running on the JVM, only what happens in the future” Dr. Russel Winder, Consultanthttp://groups.google.com/group/javaposseThread: “Your Experience of Alternative JVM Languages in Organisations, Divisions and Teams” XeNoNiQUe.co.uk (c) 2011 4/14/2011 99

Recommended References Programming in Scala by Martin Odersky, Lex Spoon, Bill Venners ; Artima; 2nd Edition 2010 http://www.artima.com/shop/programming_in_scala Programming Scala by Wampler, Payne; O’Reilly; 2010; http://programming‐ scala.labs.oreilly.com/ Beginning Scala by David Pollak; Apress; 2009; http://www.apress.com/book/view/1430219890 Steps in Scala by Loverdos , Syropoulos; Cambridge Univ Press; 2010; XeNoNiQUe.co.uk (c) 2011 4/14/2011 106

Popular Scala Frameworks Scala Test http://www.scalatest.org/ Scala Check https://www.ohloh.net/p/scalacheck Lift http://www.liftweb.net/ Akka http://akka.io/ Play! Scala module http://www.playframework.org/modules/scala Camel DSL http://camel.apache.org/scala‐dsl.html Squeryl http://squeryl.org/ Pinky https://github.com/pk11/pinky/wiki Sweet [Scala] http://code.google.com/p/sweetscala/ XeNoNiQUe.co.uk (c) 2011 4/14/2011 107

AttributionsAll images are Creative Commons (CC) License from Flickr.com– “You must attribute the source and you cannot change the content”Tim Ellis http://www.flickr.com/photos/tim_ellis/Lego Basics http://www.flickr.com/photos/tim_ellis/338755101/sizes/l/*PaysImaginaire* http://www.flickr.com/photos/nevrlndtink/Variable Plastic Bag http://www.flickr.com/photos/nevrlndtink/232906118/sizes/m/~Deiby http://www.flickr.com/photos/deiby/Expression http://www.flickr.com/photos/deiby/5489382677/sizes/l/Lisa Sinclair http://www.flickr.com/photos/plumandjello/fruit http://www.flickr.com/photos/plumandjello/2333263539/sizes/l/Nerovivo http://www.flickr.com/photos/dominik99/http://www.flickr.com/photos/dominik99/407716865/sizes/z/in/photostream/ XeNoNiQUe.co.uk (c) 2011 4/14/2011 108

AttributionsAll images are Creative Commons (CC) License from Flickr.com– “You must attribute the source and you cannot change the content”.Guilty http://www.flickr.com/photos/roll_initiative/Arbitrary Function Generator http://www.flickr.com/photos/roll_initiative/3278642272/Loop Oh Rupert Grazer http://www.flickr.com/photos/loop_oh/Pattern at the Senckenberg Museum in Frankfurt am Main / Germany. http://www.flickr.com/photos/loop_oh/4571485915/Lili Vieira de Carvalho, Vancouver, Canada http://www.flickr.com/people/lilivc/Composition of Bowls http://www.flickr.com/photos/lilivc/367582911/sizes/l/Mykl Roventine http://www.flickr.com/people/myklroventine/19/365 Game Over http://www.flickr.com/photos/myklroventine/3210068573/sizes/l/superciliousness / Bentley Smith http://www.flickr.com/people/superciliousness/200510 carpenters tools ‐ inside the workmans shed ‐ all his old tools http://www.flickr.com/photos/superciliousness/57486288/ XeNoNiQUe.co.uk (c) 2011 4/14/2011 109

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ACCU 2011 Introduction to Scala: An Object Functional Programming Language

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ACCU 2011 Introduction to Scala: An Object Functional Programming Language — Presentation Transcript