1. Multiparadigm Programming in Scala
H. Conrad Cunningham
James C. Church
Computer and Information Science
University of Mississippi

2. What is Multiparadigm Programming?
Definition:
A multiparadigm programming language provides “a framework in which programmers can work in a variety of styles, freely intermixing constructs from different paradigms.” [Tim Budd]
Programming paradigms:
imperative versus declarative (e.g., functional, logic)
other dimensions – object-oriented, component-oriented, concurrency-oriented, language-oriented
4 April 2009

3. Why Learn Multiparadigm Programming?
Tim Budd:
“Research results from the psychology of programming indicate that expertise in programming is far more strongly related to the number of different programming styles understood by an individual than it is the number of years’ experience in programming.”
The “goal of multiparadigm computing is to provide ... a number of different problem-solving styles” so that a programmer can “select a solution technique that best matches the characteristics of the problem to be solved”.
4 April 2009

4. Why Teach Multiparadigm Programming?
Contemporary imperative and object-oriented languages increasingly have functional programming features, e.g.,
higher order functions (closures)
list comprehensions
New explicitly multiparadigm (object-oriented/functional) languages are appearing, e.g.,
Scala on the Java platform (and .Net in future)
F# on the .Net platform
4 April 2009

5. Scala
Programming language developed by Martin Odersky’s team at EPFL in Switzerland
Executes on the Java platform
Integrates with Java
Has growing usage (e.g., twitter.com)
Multiparadigm language
Object-oriented (with generics and mixins)
Functional (similar to Haskell and SML)
Extensible (method calls as operators, currying, closures, by-name parameters)
Actor-based concurrency-oriented programming
Language-oriented programming
Statically typed with Hindley-Milner type inference
4 April 2009

6. Scala References Website http://www.scala-lang.org
Martin Odersky. Scala Tutorial for Java Programmers.
Martin Odersky. Scala By Example.
Martin Odersky, Lex Spoon, and Bill Venners. Programming in Scala: A Comprehensive Step-By-Step Guide, Artima, Inc., 2009.
Books from Apress and Pragmatic Bookshelf in May, O’Reilly in August, Cambridge late 2009
4 April 2009

7. Defining Hello World object HelloWorld { // Mississippi version
def main(args: Array[String]){
println("Hey world!") }
Singleton object named HelloWorld (also replaces static methods and variables)
Method main defined (procedure)
Parameter args of type Array[String]
Array is generic class with type parameter
4 April 2009

8. Interpreting Hello World
> scala
This is a Scala shell.
Type in expressions to have them evaluated.
Type :help for more information.
scala> object HelloWorld {
| def main(args: Array[String]) {
| println("Hey world!")
| } |}
defined module HelloWorld
scala> HelloWorld.main(null)
Hey world!
unnamed0: Unit = ()
scala>:q
4 April 2009

9. Compiling & Executing Hello World
> scalac HelloWorld.scala
> scala HelloWorld
Hey world!
4 April 2009

10. Numbers are Objects Consider expression 1 + 2 * 3 / x
Operators are method calls (like Smalltalk)
Operator symbols are identifiers
Expression above is same as (1).+(((2).*(3))./(x))
4 April 2009

11. Functions are Objects object Timer {
def oncePerSecond(callback:() => Unit){
while (true) {
callback(); Thread sleep 1000
} // 1-arg method sleep used as operator }
def welcome() {
println("Welcome to CCSC:MS!")
def main(args: Array[String]) {
oncePerSecond(welcome)
4 April 2009

12. Timer Execution scala> :l Timer.scala Loading Timer.scala...
defined module Timer
scala> Timer.main(null)
Welcome to CCSC:MS! …
4 April 2009

13. Anonymous Functions object Timer {
def oncePerSecond(callback:() => Unit){
while (true) {
callback(); Thread sleep 1000 }
def main(args: Array[String]) {
oncePerSecond(
() => println("Welcome to CCSC:MS!") )
4 April 2009

14. Classes class Complex(real: Double, imag: Double){ def re = real
def im = imag }
Class primary constructor combined with class body
Parameters of class private constants within class
Parameterless methods re and im
Return types of re and im inferred from expression (cannot be inferred for recursive functions)
Thus more concise syntax
4 April 2009

15. Method Overriding // Complex.scala
class Complex(real: Double, imag: Double) {
def re = real
def im = imag
override def toString =
re + (if (im < 0.0) "" else "+") +
im + "i“ }
Classes extend class AnyRef by default
Methods must explicitly override parent method
if expressions
4 April 2009

16. Using Classes and Objects
scala> :load Complex.scala
Loading Complex.scala...
defined class Complex
scala> val x = new Complex(1,-3)
x: Complex = i
scala> x.toString
res0: java.lang.String = i
4 April 2009

17. Case Classes abstract class Tree // Expression Trees
case class Sum(l: Tree, r: Tree)
extends Tree
case class Var(n: String) extends Tree
case class Const(v: int) extends Tree
Algebraic data types as in functional languages
Keyword new not needed to create instances (objects)
Getters defined automatically for constructor parameters
equals method defined on structure of instances
Pattern matching can be used to decompose
4 April 2009

18. Pattern Matching object Expressions { type Environ = String => Int
def eval(t: Tree, env: Environ): Int = t match {
case Sum(l,r) => eval(l,env) + eval(r,env)
case Var(n) => env(n)
case Const(v) => v }
def derive(t: Tree, v: String): Tree = t match {
case Sum(l,r) => Sum(derive(l,v),
derive(r,v))
case Var(n) if (v == n) => Const(1)
case _ => Const(0)
4 April 2009

19. Test Expression Trees def main(args: Array[String]) { val exp: Tree =
Sum(Sum(Var("x"),Var("x")),
Sum(Const(7),Var("y")))
val env: Environ =
{ case "x" => 5 case "y" => 7 }
println("Expression: " + exp)
println("Evaluation with x=5, y=7: " +
eval(exp,env))
println("Derivative relative to x:\n " +
derive(exp, "x"))
println("Derivative relative to y:\n " +
derive(exp, "y")) }
4 April 2009

20. Execute Expression Trees
scala> :load Expressions.scala
Loading Expressions.scala... …
scala> Expressions.main(null)
Expression: Sum(Sum(Var(x),Var(x)),Sum(Const(7),Var(y)))
Evaluation with x=5, y=7: 24
Derivative relative to x:
Sum(Sum(Const(1),Const(1)),Sum(Const(0),Const(0)))
Derivative relative to y:
Sum(Sum(Const(0),Const(0)),Sum(Const(0),Const(1)))
4 April 2009

21. Defs, Vals, and Vars Three types of identifier definitions
def defines functions with parameters; RHS expression evaluated each time called
val defines unchanging values; RHS expression evaluated immediately to initialize
var defines storage location whose values can be changed by assignment statements; RHS expression evaluated immediately to initialize
4 April 2009

22. Traits trait Ord { // Order comparison operators
def < (that: Any): Boolean // abstract
def <=(that: Any): Boolean =
(this < that) || (this == that)
def > (that: Any): Boolean =
!(this <= that)
def >=(that: Any): Boolean =
!(this < that) }
Like Java interfaces except can have concrete methods
Can be “mixed-in” to class
Note < abstract; others defined with < and equals
4 April 2009

23. Date Class with Mixin Trait Ord
class Date(y: Int, m: Int, d: Int)
extends Ord {
def year = y
def month = m
def day = d
override def toString(): String =
year + "-" + month + "-" + day
// need definition of < and equals }
Can only extend only one class or trait
May mix-in additional classes using keyword with
4 April 2009

24. Date Class Equals Method
override def equals(that: Any): Boolean =
that.isInstanceOf[Date] && {
val o = that.asInstanceOf[Date]
o.day == day && o.month == month &&
o.year == year }
isInstanceOf[T] checks whether object is an instance of the given type T
asInstanceOf[T] casts static type to T if compatible with dynamic type of object
Value of last statement of function is returned
4 April 2009

25. Date Class < Method def <(that: Any): Boolean = {
if (!that.isInstanceOf[Date])
error("Cannot compare " + that +
" and a Date")
val o = that.asInstanceOf[Date]
(year < o.year) ||
(year == o.year &&
(month < o.month ||
(month == o.month && day < o.day))) }
4 April 2009

26. DateTest object DateTest { def main(args: Array[String]) {
val x = new Date(1,1,2000)
val y = new Date(12,31,2001)
println("x = " + x)
println("y = " + y)
println("x < y: " + (x<y))
println("x > y: " + (x>y)) }
4 April 2009

27. DateTest Output > scala DateTest x = 1-1-2000 y = 12-31-2001
x < y: true
x > y: false
4 April 2009

28. Scala Functions Are first-class values – i.e., functions are objects
Can be higher-order – take functions as arguments or return them as result
Can be anonymous
May be curried – take arguments one at a time, allowing partial application
Are often passed in a closure – with references to free variables they maninpulate
Provide ability to build powerful libraries of higher-order functions
4 April 2009

29. Curried Functions scala> def add(x: Int, y: Int) = x + y
add: (Int,Int)Int
scala> add(1,3)
res0: Int = 4
scala> def addc(x: Int)(y: Int) = x + y
addc: (Int)(Int)Int
scala> addc(1)(3)
res1: Int = 4
4 April 2009

30. Partial Application scala> def addc(x: Int)(y: Int) = x + y
addc: (Int)(Int)Int
scala> val z = addc(1) _
z: (Int) => Int = <function>
scala> z(3)
res2: Int = 4
4 April 2009

31. Closures scala> val inc = 10 inc: Int = 10
scala> def incre(x: Int) = x + inc
incre: (Int)Int
scala> def app(y: Int, g: (Int=>Int)) = g(y)
app: (Int,(Int) => Int)Int
scala> app(13,incre)
res0: Int = 23
4 April 2009

32. List Processing // Not actual Scala API code
abstract class List[+A]{ …
def map[B](f: (A)=> B): List[B] =
this match {
case Nil => this
case x::xs => f(x)::xs.map(f) } …
case object Nil extends List[Nothing]
case final class ::[B]
(private hd: B, val tl: List[B])
extends List[B]
4 April 2009

33. Using List Map scala> val xs = List(3,4,5)
xs: List[Int] = List(3, 4, 5)
scala> val triples = xs.map(x => 3*x)
triples: List[Int] = List(9, 12, 15)
4 April 2009

34. More List Processing // Not actual Scala API code
abstract class List[+A]{ …
def filter(p: (A) => Boolean): List[A] =
this match {
case Nil => this
case x::xs =>
if (p(x)) x::xs.filter(p)
else xs.filter(p) } …
4 April 2009

35. Using List Filter scala> val xs = List(3,4,5,6)
xs: List[Int] = List(3, 4, 5, 6)
scala> val evens = xs.filter(x => x%2==0)
evens: List[Int] = List(4, 6)
4 April 2009

36. Other Higher Order List Methods
flatMap
foldLeft, foldRight
reduceLeft, reduceRight
takeWhile, dropWhile
span, break
foreach
4 April 2009

37. For Comprehensions scala> for(i <- 1 to 30;
| j <- List(2,3,5,7);
| if i % j == 0) yield (i,j)
res0: Seq.Projection[(Int, Int)] = RangeG((2,2), (3,3), (4,2), (5,5), (6,2), (6,3), (7,7), (8,2), (9,3), (10,2), (10,5), (12,2), (12,3), (14,2), (14,7), (15,3), (15,5), (16,2), (18,2), (18,3), (20,2), (20,5), (21,3), (21,7), (22,2), (24,2), (24,3), (25,5), (26,2), (27,3), (28,2), (28,7), (30,2), (30,3), (30,5))
4 April 2009

38. Actors in Scala
4 April 2009

39. Motivation Concurrency is hard!
Real World is parallel and distributed.
Erlang's notion of a process:
Concurrent processes should pass messages to other processes rather than share memory.
Erlang's processes are part of the language.
Scala's actors are part of the library.
4 April 2009

40. Actors Actors act independent of other actors. Actors have mailboxes.
Actors communicate by sending messages to other actors.
Actors will check their mailbox and react to their messages.
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41. Message in a Bottle Any object can be sent to an Actor
case object myMessageObject
...
myActor ! myMessageObject
4 April 2009

42. Please Mr. Postman How urgent is it?
react: I need it now!
receiveWithin: I need it soon!
receive: I'll wait.
All three methods will perform pattern matching on the objects received.
4 April 2009

43. Overacting import scala.actors._ object SillyActor extends Actor {
def act() { // Defines how our actor acts
for (i <- 1 to 5) {
println(“I'm acting!”)‏
Thread.sleep(1000)‏ }
...
SillyActor.start() // Begins acting
4 April 2009

44. Vegetable Launcher case object Tomato case object Lettuce
object VegetableLauncher extends Actor {
def act() {
for (i <- 1 to 5) {
VegetableCatcher ! Tomato // Send it!
Thread.sleep(1000)‏
VegetableCatcher ! Lettuce // Send it! }
4 April 2009

45. Vegetable Catcher object VegetableCatcher extends Actor { def act() {
loop {
react { // Non-blocking call
// Pattern Matching
case Lettuce =>
println(“I caught a lettuce!”)‏
case Tomato =>
println(“I caught a tomato!”)‏ }
4 April 2009

46. Lights, Camera, ... VegtableLauncher.start()‏ VegtableCatcher.start()‏
SillyActor.start()‏
I'm acting!
I caught a tomato!
I caught a lettuce!
4 April 2009

47. Dining Philosophers Five philosophers compete for limited resources.
Deadlocks are possible.
Solution implemented with a waiter.
11 actors total:
5 philosophers
5 chopsticks (A mutex actor)‏
1 waiter (A singleton object actor)‏
4 April 2009

48. Philosophers must still wait for chopsticks after sitting.
The waiter only allows four to sit. After that, names are on the wait list.
Philosophers must still wait for chopsticks after sitting.
4 April 2009

49. Six Messages import scala.actors._ import scala.actors.Actor._
import java.util.Random
// Philosopher and Waiter communication
case object NeedToEat
case object HaveASeat
case object DoneEating
// Philosopher and Chopstick communication
case object NeedChopstick
case object HeresAChopstick
case object GiveBackChopstick
4 April 2009

50. Chopstick “Mutex” class Chopstick extends Actor { def act() { loop {
react {
case NeedChopstick =>
sender ! HeresAChopstick
receive {
case GiveBackChopstick => 1 }
4 April 2009