Scala. The scalabale Language. May 30, Department of Computer Sciences University of Salzburg. Functional Features Object oriented Features

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1 Functional Features The scalabale Language Stefan Hinterkörner Andreas Rottmann Department of Computer Sciences University of Salzburg May 30, 2010

2 Overview Functional Features 1 Functional Features 2

3 Functional Features Why can be considered a functional language? Primarily... Functions as first-class values Closures Expression-based syntax But also... Tail recursion (though not full tail call optimization) Currying

4 Functional Features Declaring functions Methods You ve seen those already. Local functions You can declare a function inside another one. Function literals Functions are first class values, and there s syntax for writing function values: (x: Int) => x + 3 val add3 = (x: Int) => x + 3 add3(4) // evaluates to 7

5 Functional Features First-class functions Functions, like any other value, can be stored in data structures or returned from functions. Example import scala.collection.mutable type BinaryOperators = mutable.map[string, (Int, Int) => Int] def makebinaryoperators(): BinaryOperators = { val ops = mutable.map.empty[string, (Int, Int) => Int] ops("+") = (x: Int, y: Int) => x + y ops("-") = (x: Int, y: Int) => x - y ops("*") = (x: Int, y: Int) => x * y ops("/") = (x: Int, y: Int) => x / y ops val ops = makebinaryoperators() ops("*")(6, 7) // evaluates to 42

6 Closures Functional Features Functions can use variables from enclosing scopes: Example def makeadder(n: Int): (Int) => Int = { def add(x) = x + n add val add3 = makeadder(3) add3(4) // evaluates to 7

7 Functional Features Closures (contd.) They can also change them: Example def makecounter(start: Int): () => Int = { var count = start () => { val result = count count = count + 1 result val c1 = makecounter(0) val c2 = makecounter(5) for (i <- 1 to 3) { println("c1() = " + c1()) println("c2() = " + c2())

8 Functional Features Abstract classes Example cannot be instantiated contain only abstract members Exception: parameterless methods abstract class Element { def contents: Array[String] def height: Int = contents.length - Example from: Programming in, Odersky et al 2008

9 Functional Features Extending classes Example like in Java with "extends" AnyRef no Inheritance for private and overwritten members class ArrayElement(conts: Array[String]) extends Element { def contents: Array[String] = conts val elem: Element = new ArrayElement(Array("hello"))

10 Overriding Functional Features super override this Example class StrikeLabel(text:String) extends JLabel(text) { def this() = this("") override def paintcomponent(g:graphics):unit = { super.paintcomponent(g) g.setcolor(color.red) g.drawline(1, getheight() / 2, getwidth() - 1, getheight() / 2) - Example from: May 2010

11 Traits Functional Features Java Example with Interfaces I public abstract class Vehicle { public abstract Engine getengine(); public interface ICar { public void drive(); public interface IPlane { public void fly(); - Original example from:

12 Traits Functional Features Java Example with Interfaces II public class Car extends Vehicle implements ICar { private Engine carengine; public Car(TyreType type) {... public Engine getengine() { return carengine; public void drive() {... public class Plane extends Vehicle implements IPlane { private Engine planeengine; public Plane(WingType type) {... public Engine getengine() { return planeengine; public void fly() {...

13 Traits Functional Features Java Example with Interfaces III public class FlyingCar extends Vehicle implements IPlane, ICar { public FlyingCar(WingType wingtype, TyreType tyretype) {... public Engine getengine() {... public void fly() {... public void drive() {... implement methods multiple times complicated hierarchy creating Car without being a Vehicle possible

14 Traits Functional Features Now in with Traits I abstract class Vehicle { def engine:engine trait Car extends Vehicle { private var carengine:engine =... override def engine = carengine def drive() = {... trait Plane extends Vehicle { private var planeengine:engine =... override def engine = planeengine def fly() = {...

15 Traits Functional Features Now in with Traits II class FlyingCar(wingType:WingType, tyretype:tyretype) extends Plane with Car { // engine from Car class FlyingCar(wingType:WingType, tyretype:tyretype) extends Car with Plane { // engine from Plane

16 Functional Features Traits & in the Real World Traits multiple withs possible override also possible trait cannot have any "class" parameters in the Real World Webframework Lift Twitter