Multiparadigm Programming in Scala

1. Multiparadigm Programming in Scala Adapted from presentation by H. C. Cunningham and J. C. Church University of Mississipi

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, etc. ScalaMulti

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 yearsof 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”. ScalaMulti

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 ScalaMulti

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, Foursquare, and Linkedin) 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 ScalaMulti

6. Why Scala? (Coming from Java/C++) • Runs on the JVM • Can use any Java code in Scala • Almost as fast as Java (within 10%) • Much shorter code • Odersky reports 50% reduction in most code over Java • Local type inference • Fewer errors • No Null Pointer problems • More flexibility • As many public classes per source file as you want • Operator overloading

7. Scala References • Websitehttp://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, 2nd Edition, Artima, Inc., 2010. • Books on Scala: http://www.scala-lang.org/node/959 ScalaMulti

8. Scala object system • Class-based • Single inheritance • Can define singleton objects easily (no need for static which is not really OO) • Traits, compound types, and views allow for more flexibility

9. Basic Scala • Use var to declare variables: var x = 3; x += 4; • Use val to declare values (final vars) val y = 3; y += 4; // error • Notice no types, but it is statically typed var x = 3; x = “hello world”; // error • Type annotations: var x : Int = 3;

10. Functional Scala • Defining lambdas – nameless functions val f = x :Int => x + 42; f : mapping :int->int • Closures! A way to haul around state var y = 3; val g = {x : Int => y += 1; x+y; } • Maps (and a cool way to do some functions) List(1,2,3).map(_+10).foreach(println) • Filtering (and ranges!) 1 to 100 filter (_ % 7 == 3) foreach (println) • (Feels a bit like UNIX pipes?)

11. Defining Hello World object HelloWorld { 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 ScalaMulti

12. 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 ScalaMulti

13. Compiling & Executing Hello World > scalacHelloWorld.scala > scalaHelloWorld Hey world! ScalaMulti

14. 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)) ScalaMulti

15. Functions are Objects object Timer { defoncePerSecond(callback:() => Unit){ while (true) { callback(); Thread sleep 1000 } // 1-arg method sleep used as operator } def welcome() { println("Welcome to CS3180!") } def main(args: Array[String]) { oncePerSecond(welcome) } } ScalaMulti

16. Timer Execution scala> :l Timer.scala Loading Timer.scala... defined module Timer scala> Timer.main(null) Welcome to CS3180! Welcome to CS3180! Welcome to CS3180! … ScalaMulti

17. Anonymous Functions object Timer { defoncePerSecond(callback:() => Unit){ while (true) { callback(); Thread sleep 1000 } } def main(args: Array[String]) { oncePerSecond( () => println("Welcome to CS3180!") ) } } ScalaMulti

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

19. 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 ScalaMulti

20. Using Classes and Objects scala> :load Complex.scala Loading Complex.scala... defined class Complex scala> val x = new Complex(1,-3) x: Complex = 1.0-3.0i scala> x.toString res0: java.lang.String = 1.0-3.0i ScalaMulti

21. 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 • Cf. Algebraic data types as in functional languages • Keyword new not needed to create instances (objects) • Getters defined automatically for constructor parameters • Pattern matching can be used to decompose • equals method defined on structure of instances ScalaMulti

22. 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) } ScalaMulti

23. 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")) } } ScalaMulti

24. 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))) ScalaMulti

25. 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 ScalaMulti

26. 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 that < is abstract; others defined with < and equals ScalaMulti

27. 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 deftoString(): String = year + "-" + month + "-" + day // … need definition of < and equals } • Can only extend one class or trait • May mix-in additional classes using keyword with ScalaMulti

28. 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 ScalaMulti

29. 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))) } ScalaMulti

30. 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)) } } ScalaMulti

31. DateTest Output > scalaDateTest x = 1-1-2000 y = 12-31-2001 x < y: true x > y: false ScalaMulti

32. 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 ScalaMulti

33. 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 ScalaMulti

34. Partial Application scala> defaddc(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 ScalaMulti

35. Closures scala> val inc = 10 inc: Int = 10 scala> defincre(x: Int) = x + inc incre: (Int)Int scala> defapp(y: Int, g: (Int=>Int)) = g(y) app: (Int,(Int) => Int)Int scala> app(13,incre) res0: Int = 23 ScalaMulti

36. Using List Map scala> valxs = 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) scala> val evens = xs.filter(x => x%2==0) evens: List[Int] = List(4) ScalaMulti

37. Other Higher Order List Methods • flatMap • foldLeft, foldRight • reduceLeft, reduceRight • takeWhile, dropWhile • span, break • foreach ScalaMulti

38. For Comprehensions scala> for(i <- 1 to 30; | j <- List(2,3,5,7); | if i % j == 0) yield (i,j) • res0: scala.collection.immutable.IndexedSeq[(Int, Int)] = Vector((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)) ScalaMulti

39. Scala class hierarchy

40. Actors in Scala ScalaMulti

41. 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. ScalaMulti

42. 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. ScalaMulti

43. Message in a Bottle • Any object can be sent to an Actor case object myMessageObject ... myActor ! myMessageObject ScalaMulti

44. 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. ScalaMulti

45. 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 ScalaMulti

46. 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! Thread.sleep(1000) } } } ScalaMulti

47. 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!") } } } } ScalaMulti

48. Lights, Camera, ... VegetableLauncher.start() VegetableCatcher.start() SillyActor.start() I'm acting! I caught a tomato! I'm acting! I caught a lettuce! I'm acting! I caught a tomato! I'm acting! I caught a lettuce! I'm acting! I caught a tomato! I caught a lettuce! I caught a tomato! I caught a lettuce! I caught a tomato! I caught a lettuce! ScalaMulti