ACCU 2011 Introduction to Scala: An Object Functional Programming Language

INTRODUCTION INTO SCALA THE OBJECT‐FUNCTIONAL PROGRAMMING LANGUAGE

Peter PilgrimOracle Java Champion

Blog: www.XeNoNiQUe.co.uk/blog/ACCU: Member since 1993Education: London South Bank UniversityLanguages: Scala, Groovy, JavaFX, JavaConferences: JavaOne, ACCU, QCon,Devoxx

“If I were a pick a language to use today other than Java, it would be Scala”

James Gosling

http://www.adam‐bien.com/roller/abien/entry/java_net_javaone_which_programming

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 years” ‐ 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

Today’s Agenda

Classes and Companion ObjectsFunction ObjectsCase classes and Pattern matchingTraits, Mix‐ins and CompositionsScala CollectionsLibrary Frameworks

The Basics

Scalable Language

Very Bright Future

Purely Object OrientedStatically Typed

Functional

JVM Language

Martin Odersky ‐ Language Designer

A Brief History of Scala

Make Java Better 1996 – 2000: Pizza, GJ and javac

Make a Better Java2003 : The Scala 1.0 Experiment2005 : Scala 2.0, written in Scala2006‐2011: Industrial strength

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;

Equivalent Scala class

class Person (

val firstName: String

val lastName: String,

val height: Float,

val age: Int ) { }

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 );

And in Scalaval people: Array[Person] = getPeople()

// Type inference

val ( smallFolk, bigFolk ) =

people partition ( _.height < 5.0 )

Semicolon is optional

Valsand Vars

VariablesTwo Types of variables:val a immutable variablevar a re‐assignable variable

val s1 = “Purple Haze” // Type inference String

var s2 = “Barcelo”

s1 = “Hotel” // Won’t compile

s2 = “Electricona” // Ok!

Variables Type Inferencesval s3: String = “Purple Haze”

val f1: Double = 3.141596527

val fruit = List(“orange”,”apple”)

// scala.collection.immutable.List[String]

val capitals = Map(“USA”-> “Washington”,

”Wales” -> “Cardiff”, “Germany” -> “Berlin” )

// scala.collection.immutable.Map[String, String]

Scopeval s1 = “Purple Haze”

val s1 = “Day Tripper”

println( s1 ) // "Day Tripper"

println( s1 ) // "Purple Haze"

Scala Identifiers

Four forms of Scala identifiersAlphanumeric: 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`

Basic Scala Literals

Integer: 1, 882, ‐1Boolean: true, falseDouble: 1.0, 1d, 1e3, 1.234e‐6Long: 42LFloat: 19.73fCharacters: ‘6’, ‘@’, ‘?’, `K’String: “Hello world”

ExpressYourself

Expressions == Resultsval msg = “Macro Economics”

val hasA =

if ( msg contains ‘a’)

“yes”

“no”

println(hasA) // “yes”

Expressions On Everythingval z1 =

“play havoc”

finally {

“yes”

// z1 is “yes”

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)

How To Declare Functions

def volumePrice( price: Float, vol: Int ): Float = {

if ( vol >= 1000 )

price * 0.90

else if ( vol >= 100 )

price * 0.95

Functions Written Concisely

def max( x: Double, y: Double ): Double

= if ( x > y) x else y

def min( x: Double, y: Double ): Double

= if ( x < y) y else x

// type inference on the return type: Double

def cube( x: Double ) = x * x * x

Procedures and The Unit Type

def sideEffect(): Unit = println(“Say hi!”)

// sayHi: ()Unit

sideEffect() // prints “Say hi!”

• Unit has a value, written as ()

• Unit roughly corresponds to void in Java

Classes

How To Declare Classes

class Foo

val f1 = new Foo()

val f2 = new Foo

class Bar ( val name: String )

val b1 = new Bar(“Twix”)

val b2 = new Bar(“Curly-Wurly”)

The Primary Constructor

class 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?")

Class ‐ Better Style

class 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 )

Class ‐ Auxiliary Constructors

class 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) )

// ...

Built‐In Control Structures #1

var x1 = 3.0

var r1 = if ( x1 >= 0 )

"positive" else "negative"

x1 = -9.25

println( if ( x1 < 0 )

"negative" else "positive" )

Built‐In Control Structures #2

While ... Do Do ... While

var c1 = 0

var y1 = ""

while ( c1 < 4 ) {

y1 = y1 + "#" + c1

c1 += 1

println(y2)

var c2 = 0

var y2 = ""

y2 = y2 + "#" + c2

c2 += 1

while ( c2 < 4 )

println(y2)

Scala Fixes Java Mishaps

Scala Removes Scala Innovates

Break and Continue

Static members

Primitive types

Special treatment of interfaces

Operator overloading

Function objects

Companion Objects

Case classes

User control abstractions

Mix‐ins

Pattern matching

Implicit conversions

Infix Operators in Classes

class 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"

Using Operator Overloading

val 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)

Infix Operator Associativity

Normally expressions associate from LEFT to RIGHT

val x = 1 + 2 + 3 + 4

val x = ((1 + 2) + 3 ) + 4

Except for those operators that end with a colon(:)

They associate from RIGHT to LEFT

val list = 1 :: 2 :: 3 :: 4 :: Nil

val list = 1 :: ( 2 :: ( 3 :: (4 :: Nil )))

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 extraction

object Complex {

val i = new Complex( 0, 1 )

def apply( re: Float, im: Float ) =

new Complex( re, im )

Comprehensions

For‐Loop Expressions

for ( j <- 1 to 4 )

println( "Iteration "+j )

// Iteration 1 .. 4

for ( j <- 1 until 4 )

println( "Iteration "+j )

// Iteration 1 .. 3

Looping Over Collections

val fruits = List("apple", "orange","pear",

"kiwi", "plum" )

for ( fruit <- fruits )

println( fruit )

For‐Filter Comprehensions

for ( trade <- trades

if trade.tradeType == "FX" &&

trade.amount >= 75000 &&

trade.counterparty.toUpperCase

.startsWith("North")

processLayerDown( trade )

For‐Nested‐Filter Comprehension

for ( trade <- trades

if trade.tradeType == "MM" ;

if trade.origin == "Summit";

settlement <- trade.settlements

settlement.exists( _.name == Standard )

settleStandard( settlement )

Function Objects

Function Object ‐ Long Hand

val 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.

First‐Class Functions

val isEven: (Int => Boolean) = (k: Int) => k % 2 == 0

// The verbose short‐hand notion

Functions are values, values are objectErgo, functions are objects in Scala

First‐Class Functions

val isEven = (k: Int ) => k % 2 == 0

// The short‐hand notion // Scala infers the function type for the LHS

Calling Function Objects

isEven.apply(24) // true

isEven(11) // false

Filter Out Even Numbers

val 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

numbers.filter( (k: Int) => k % 2 == 0 )

// in-lined function value:

// List[Int] = List(0,2,4,6)

numbers.filter( k => k % 2 == 0 )

// 0,2,4,6

numbers.filter( _ % 2 == 0 )

// 0,2,4,6

Function Object Summary

( x: Float ) => sin(pi * x) / (pi * x )

Closures

( x: Float ) => extra + sin(pi * x) / (pi * x )

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.

Closure Lexical Scope

var extra = 4

val addExtra = (x: Int) => x + extra

addExtra(2) // 6

extra = 7

addExtra(2) // 9

Curried Functions

( x: Float )( y: Float ) => y * cos ( sin (pi * x ) )

Functions Can Return Functions

def foo1( x: Float)(y: Float ) = x*y-2*x

def foo1( x: Float) = { (y: Float ) => x*y-2*x }

val a1 = foo1( 3 ) ( 4 ) // a1 is 6.0

val a2 = foo1( 3 )

// a2 is (Float) => Float = <function1>

val a3 = a2(4) // a3 = 6.0

Parameterised Types

class Holder [T] ( val value: T )

val chipBox = new Holder( 555 )

val z1 = chipBox.value // 555

val fruitBox = new Holder( "apple" )

val z2 = fruitBox.value // “apple”

fruitBox.value = "orange" // re-assign error

chipBox.value = 73 // re-assign error

Call‐By‐Value versus Call‐By‐Name

Call By Value Call By Name

Expression are evaluated before being passed as a parameter to a function

def debug( s: String ):Unit

Expressions are evaluated inside the function

Computational niceUseful for library writers to create custom control abstractions

def debug( s: => String ):Unit

Custom Control Abstractionsdef using[ T <: { def close() }, A ]

( closable: T ) ( body: T => A ) = {

body( closable )

} finally {

if ( closable != null ) {

try { closable.close() } catch {

case e: Exception => // Log this error

Closures As Control Abstraction

import java.io._

val file = new File( "poem.txt" )

using(

new BufferedReader(

new FileReader( file )) ) {

r => println( r.readLine() )

Pattern Matching

Case Classes

Convenience data types for pattern matchingScala compiler creates :

Class with hashCode(), equalsTo(), copy() methodsNice toString() methodDefaults to immutable data membersCompanion object Defines apply() and unapply() (extractor) methods

Case Classes

case 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 ) // true

assert( st2 != st4 ) // true

Pattern Matching on Types

val 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 )

Pattern Matching on Sequences

val 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")

Pattern Matching Algorithms

def fib( n: Int ): Long = {

n match {

case 0 => 0

case 1 => 1

case n => fib(n-2) + fib(n-1)

val z1 = fib(3) // 2

val series = (0 to 25).map( fib _ ).mkString("\n")

Case Classes Abstract Data Types

abstract class BinTree[T]

case object Empty extends BinTree

case class Node (

elem: T, left: BinTree[T], right: BinTree[T]

) extends BinTree

Pattern Matching Algorithm

// Traverse tree in-order by algorithmic cases

def inOrder[T]( n: BinTree[T] ) : List[T] = {

n match {

case Empty => List()

case BinTree(e,l,r) =>

inOrder(l) ::: List(e) ::: inOrder(r)

Pattern Matching Algorithm

// Recursively find the depth of the binary tree

def 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

Composition

Traits and Mix‐ins

Trait is provides composition of behaviour type in ScalaTraits can define abstract methodsTraits can define implementation methodsTraits can define data membersLinearisation of behaviour is prioritised from left to rightTraits can inherit from other traits

Mix‐Ins and Traits

trait Vehicle

trait Car extends Vehicle

trait Bicycle extends Vehicle

trait Motorcycle extends Vehicle

trait Powered

trait HasWheels

Mix‐In Composition

class Ford extends Car with Powered with HasWheels

class Chopper extends Bicycle with HasWheels

val myCar = new Ford

val myBike = new Chopper

Mix‐Ins Refactored

trait Powered {

def kickdown() = "Vrroom!!"

trait HasWheels {

def turn() = "Wheels are turning"

Mix‐ins Refactored #2

trait Vehicle

trait Car extends Vehicle with HasWheels

with Powered

trait Bicycle extends Vehicle with HasWheels

trait Motorcycle extends Vehicle with HasWheels

with Powered

trait Bus extends Car

trait Truck extends Car

trait SUV extends Car

Motorway Responsibility

def motorway( x: Vehicle with Powered ) =

println( "on the motorway: " +x.kickdown )

val aCar = new Ford

val aBike = new Chopper

motorway( aCar ) // Ok

motorway( aBike ) // Won’t compile!

Collections

Immutable Listsscala.collection.immutable.List

• Insertion at front O(1)• Insertion at end O(n)

Creating List Collections

import scala.collection.immutable.List

val xs0 = List(1,2,3,4)

val xs1 = 1 :: 2 :: 3 :: 4 :: Nil

Creating Nested Lists

val xs2 = ( 1 :: 2 :: 3 :: Nil) ::

( 4 :: 5 :: Nil ) ::

( 6 :: Nil ) ::

// xs2: List[List[Int]] =

List(List(1, 2, 3), List(4, 5), List(6))

List Functions

val list = List(1,2,3,4,5)

val z0 = list.head // 1

val z1 = list.tail // List(2,3,4)

val z2 = list.isEmpty // false

val z3 = list.size // 4

Higher Order Operations

val list = List(1,2,3,4,5)

val z0 = list.last // Int 5

val 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) // true

val z8 = list.filter( _ < 3 ) // List( 1,2 )

Sorting List Collections

val 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)

More Higher Operations

val 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)

Map Collections Types

var capitals = Map( "England" -> "London",

"Scotland" -> "Edinburgh", "Wales" -> "Cardiff", "Northern Ireland" -> "Belfast" )

capitals += ( "Brasil" -> "Brasilia" )

val z0 = capitals( "Brasil“ ) // “Brasilia”

val z1 = capitals.isEmpty // false

val z2 = capitals.size // 4

for ( (k,v) <- capitals )

println("Capital of "+k+" is "+v )

Set Collections Types

import scala.collection.immutable.SortedSet

var xs = SortedSet( 3,2,1 )

println(xs) // TreeSet(1,2,3 )

xs += 4

xs += 3

println(xs) // TreeSet(1,2,3,4 )

xs -= 2

println(xs) // TreeSet(1,3,4 )

xs ++= List( 5,6)

println(xs) // TreeSet(1,2,4,5,6 )

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 methodPower enabler for writing embedded DSL in Scala

Declare Implicit Conversions

object 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 )

Using Implicit Conversion

import xenonique.numerics.Complex

val z1 = new Complex( 4.7, -2.3 )

val z4 = 2 + c1

val z2 = 2.50 * c1

val z3 = (2.50).*(c1)

Builders

Scala – Launcher / REPLScalac ‐CompilerScaladoc – Documentation GeneratorSBT ‐ Simple Build Tool Maven Scala Plug‐inApache Ant Plug‐InGradle

Tools ‐ IDEs

Intellij IDEA v10.2Eclipse Helios v3.6Net Beans v7.0EmacsJEdit, NotePad++, TextPad, TextMate

Tools ‐ Frameworks

Testing Domain Specific

Scala Test SpecsScala Check

Lift AkkaPlay!Camel DSLWicket

Squeryl, Pinky, Sweet

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, Consultant

http://groups.google.com/group/javaposseThread: “Your Experience of Alternative JVM Languages in Organisations, Divisions and Teams”

Beyond Java Today

Beyond Java Future 2016?

Scalable Language

Powerful

Deep LanguageResponsible

Comprehensive

Expressive Concise

Learn more

http://xenonique.co.uk/blog/peter.pilgrim@gmail.comhttp://scala‐lang.org/twitter:peter_pilgrim

Professional Services

peter.pilgrim@gmail.comScala Adoption in EnterprisesContractingTraining with ConsultancyTechnical Leadership

Introduction to ScalaThe Object‐Functional Programming Language

Peter PilgrimJava Champion ‐Java EE specialist w/ ScalaenthusiasmXeNoNiQUe.co.uk/blog

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;

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/

Attributions

All 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/

Attributions

All 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 carpenter's tools ‐ inside the workman's shed ‐ all his old tools http://www.flickr.com/photos/superciliousness/57486288/

ACCU 2011 Introduction to Scala: An Object Functional Programming Language

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