Scala Technion – Institute of Technology Software Design (236700) Based on slides by: Sagie Davidovich, Assaf Israel Author: Gal Lalouche - Technion 2015 © 1

What is Scala? Scala is a static object-functional language  It supports fully object-oriented code  Classes, methods, inheritance, visibility modifiers,…  It is actually more object-oriented than Java, due to lack of primitives, no static members  It supports fully functional code  Lambdas, Closure, Currying, pattern matching, lazy evaluation, tail recursion,…  Most importantly, it allows the developer to mix and match the two  Currying on methods, inheriting functions, …  Compiles to Java bytecode  So it’s easy to call Java from Scala and vice-versa Author: Gal Lalouche - Technion 2015 © 2

A note on the “new” features in Java 8 All the new features in Java 8 have been in Scala for years  Higher order functions  optional values  Lambda Expressions  The syntactic sugar of Functional Interfaces can be implemented using implicit  default methods on interfaces are actually weak traits  Streams and easy parallelization Author: Gal Lalouche - Technion 2015 © 3

Java: Scala: Java vs Scala code Author: Gal Lalouche - Technion 2015 © 4 class Person { private final String name; private final int age; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } public boolean canDrink() { return age >= 21; } public String getFirstName() { return name.split(" ")[0]; } } class Person (val name: String, val age: Int) { def canDrink = age >= 21 val getFirstName: String = name split " " apply 0 }

Java: Scala: Case classes Author: Gal Lalouche - Technion 2015 © 5 case class Person(id: Long) // includes getters/hash/equals/toString public class Person { long public String toString() { return "Person [id=" + id + "]"; public int hashCode() { return result = 31 + (id ^ (id >>> 32)); public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Person other = (Person) obj; return id != other.id; }

Java vs. Scala, at a glance Java 8  Has primitives  No operator overloading  Statements and expressions  Has static methods and members  Some type inference  Library collections are mutable  default methods  Basic switch functionality  Basic foreach functionality  No extension methods  Has checked exceptions  Type Erasure (JVM property)  Little to no syntactic sugar Scala  Everything is an object  No concept of "operators", just methods  Everything is an expression  Uses a companion object/Singleton  Stronger type inference  Collections are immutable by default  traits  Powerful pattern matching (a la ML)  Powerful for comprehensions  Can define implicit coercions  Doesn’t have checked exceptions  Can get around type erasure  A lot (too much?) of syntactic sugar 6

Everything is an object Author: Gal Lalouche - Technion 2015 © 7  Scala has no primitive types  Int is the type of integers and Double is the type of Double Precision  All classes extends Any ( this is called a unified type system )  Nothing extends everything (this is called a bottom type)  is actually syntactic sugar for 1.+(2)  Scala allows you to forgoe the. and parenthesis under certain conditions  This also means that operator overloading is supported implicitly  Simply define a method named + or *, or / or pretty much anything…  For example, XML parsing: val forecast = weather \ "channel" \\ "item" \ "forecast"

Everything is an expression 8  In Scala, all statements are actually expressions  Although some may be of type Unit ( Scala’s void) val x: Int = if (flag) 10 else 20 // no need for ?: val compound = { // doSomeCalculation … $ // last argument is considered the return value } val y: Seq[Int] = for (i <- 0 to 10) yield i // list comprehensions var z = 0 val unit: Unit = (z = 10) val file = try { readFile() } catch { case _: Exception => readOtherFile() }

Object companion Author: Gal Lalouche - Technion 2015 © 9  Scala has no static members / methods  Instead, it uses the Singleton pattern and the object keyword Wait, isn’t Singleton evil?  Well, usually…  But objects are more powerful than simple static methods, since they can also be passed around, extend classes, etc.  Java  Scala Author: Gal Lalouche - Technion 2015 © class Person { String name; static boolean isLegal(String name) { //… } class Person(val name: String) { } object Person { def isLegal(name: String) = //… }

Type inference 10 Scala has strong type inference (though not as strong as Haskell/ML)  For variables  For methods  For Lambdas  We can also state the explicit return type to have it enforced by the compiler var x = 3 // discouraged in Scala val str = "Hello world!" // str’s reference cannot be changed def getString = { return "Hello world!" } List("1","2","3") map (i => i.toInt()) def getString: String = 4 // won’t compile val getInt: Int = "foobar" // won’t compile

Immutability 11 So you want to write immutable code in Java…  We can use the final ( val in Scala) modifier, but that only applies to the reference itself  What about the reference d object? It can be mutable!  If we want our classes to be truly immutable we have to work hard  We can use immutable objects, but most of the standard java classes and data structures are mutable  This means we have to resort to defensive copying, which is verbose, slow and error-prone  In Scala, all data structures are immutable (“persistent”) by default  This also gives us free generic covariance (i.e., List[Int] extends List[Any])

Traits 12 Scala replaces interfaces with traits  Traits are behavioural units, designed for software reuse  Can be composed to create new traits  An alternative to multiple inheritance (e.g., C++) and mixins (e.g., Ruby) Difference between Traits, Mixins and Multiple Inheritance  Composed traits are flat, mixins are linear and multiple inheritance is a directed acyclic graph  Traits only have a default constructor  Trait conflicts are solved explicitly by the programmer

Traits – Conflict resolution 13 Trait conflicts are resolved explicitly  Conflicts can only arise when there are two implementations with the same signature  Since there are no constructors, and all conflicts are resolved explicitly, the model is much simple than multiple inheritance trait Hodor { def speak = "Hodor!" } trait Stark { def speak = "Winter is coming" } class StarkHodor extends Hodor with Stark { override def speak = "Hodor is coming!" // we can also use super[Hodor].speak }

Traits vs. default Methods 14 Traits can be viewed as more powerful default methods  default methods cannot implement non-default methods  Traits can have state interface Bar { void bar(); } interface Foo { default void bar() {…} } class FooBar implements Foo, Bar { // the method bar is considered in conflict and needs to resolved } trait Counter { var x = 0 }

Traits vs. default Methods (cont.)s 15 Trait composition can be used to provide required implementation trait Bar { def bar // this is a required method } trait Foo { def bar = {…} // this is a provide } class FooBar extends Foo with Bar { // doesn’t need to implement bar }

Traits vs. default Methods (cont.) 16  Traits can be composed in runtime (this actually borrows from mixins, i.e., conflicts are resolved linearly )  Traits can access the implementing class instance.  A trait can require more than just methods from its implementing class; it can also impose lower bounds  This is also used for dependency injection (the “cake pattern”) trait Dancer { def dance = … } class Cat { … } val dancingCat = new Cat with Dancer dancingCat.dance

Pattern matching Author: Gal Lalouche - Technion 2015 © 17 Scala has a powerful pattern matching mechanism  Simple switch  Type matching Author: Gal Lalouche - Technion 2015 © val y = 10 val x = y match { case 3 => … case 4 => … case _ => … //default } val x = y match { case s: String => … case i: Int => … case d: Double => … case tuple: (Int, String) => … } // no match will throw an exception

Pattern matching – sealed classes Author: Gal Lalouche - Technion 2015 © 18 Sealed classes can only be extended in the same source file  This generates a compile error for none-exhaustive search Author: Gal Lalouche - Technion 2015 © sealed abstract class LogMessage case class StringMessage(message:String) extends LogMessage case class ExceptionMessage(exception:Throwable) extends LogMessage case class BothMessage(message:String, exception:Throwable) extends LogMessage class Logger { def debug(l:LogMessage) = log(10,l) def info(l:LogMessage) = log(5,l) def error(l:LogMessage) = log(1,l) def log(level:Int, l:LogMessage): Unit = l match { case StringMessage(msg) => println(msg) case ExceptionMessage(exception:Error) => exception.printStackTrace }

Pattern matching (cont.) Author: Gal Lalouche - Technion 2015 © 19  Functional progamming  Regular expressions  Much more… Author: Gal Lalouche - Technion 2015 © def contains[T](list: List[T], x: T): Boolean { case Nil => false case e :: xs => if (e == x) true else contains(xs, x) } val Name = new Regex("""(\w+)\s+(\w+)""") "Gal Lalouche" match { case Name(firstName, lastName) => println(firstName + " " + lastName) case _ => println("No match") }

Thing we don’t have time to cover Author: Gal Lalouche - Technion 2015 © 20 We only scratched the surface  Implicit methods  Implicit parameters  For comprehensions  Lazy evaluations  Views (similar to stream, but lazier )  Actors (expressive distributed model)  Much, much more… Author: Gal Lalouche - Technion 2015 ©

If you want to know more… Author: Gal Lalouche - Technion 2015 © 21  Suggested Reading:  Programming in Scala (The de-facto guide) Programming in Scala  Scala in Depth Scala in Depth  See more here:  Coursera:  Functional Programming Principles in Scala (by the language creator) Functional Programming Principles in Scala  Principles of Reactive Programming (advanced course, by the language creator and several API designers) Principles of Reactive Programming Author: Gal Lalouche - Technion 2015 ©

So is Scala perfect? Author: Gal Lalouche - Technion 2015 © 22 No language is perfect  Scala takes the “everything but the kitchen sink” approach  This gives us great power  But W ith Great Power, Comes Great Responsibility! Author: Gal Lalouche - Technion 2015 ©

So is Scala perfect? (cont.) Author: Gal Lalouche - Technion 2015 © 23  All java code looks pretty much the same  There no macros, no extension methods, little to no syntactic sugar (especially before Java 8)  You always know where the method comes from  With the exception of static imports  This makes Java very verbose …  …but also very simple to read Author: Gal Lalouche - Technion 2015 ©

“Too much syntactic sugar causes cancer of the semi-colons” Author: Gal Lalouche - Technion 2015 © 24  Scala has much built into the language (i.e., not part of a library)  Java‘s map :  Scala’s map : Author: Gal Lalouche - Technion 2015 © // sum numbers 1 to 100 var sum = 0; for (i <- to 100) sum += i (for (i <- 1 to 100) yield i) sum 1.to(100).reduce((x: Int, y: Int) => x.+(y)) 1.to(100).reduce((x, y) => x + y) 1 to 100 reduce (_ + _) Stream map(Function mapper) map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That

“Too much syntactic sugar causes cancer of the semi-colons” (cont.) Author: Gal Lalouche - Technion 2015 © 25 It is very easy to create DSLs in Scala  Scalatest (a unit-testing framework)  The above code compiles  But what’s going on here? is should a method? is be ? what is equal or in ? Author: Gal Lalouche - Technion 2015 © "A List" should "pop values in last-in-first-out order" in { val $ = new MutableList[Int] $ add 1 $ add 2 $ at 0 should be equal 1 $ at 1 should be equal 2 }

“Too much syntactic sugar causes cancer of the semi-colons” (cont.) Author: Gal Lalouche - Technion 2015 © 26 You can define new control structures You can create Python-like syntax Author: Gal Lalouche - Technion 2015 © def unless(b: Boolean)(f: => Unit) { if (!b) f } //… unless(x < 10) { println("x >= 10") } implicit def richBoolean(b: Boolean) = new { def or(other: Boolean) = b || other def and(other: Boolean) = b && other } if ((b or c) and d) { // … }

“Too much syntactic sugar causes cancer of the semi-colons” (cont.) Author: Gal Lalouche - Technion 2015 © 27 ProcessBuilder API Author: Gal Lalouche - Technion 2015 ©

Summary Author: Gal Lalouche - Technion 2015 © 28  Scala combines functional and object oriented approaches  It is lightweight and expressive, supports DSL with ease, and fully interoperable with Java  However, when a language let’s you do everything, developers also have to know everything  Especially when working in teams  Syntactic sugar is cool when working alone, but can quickly get out of hand Author: Gal Lalouche - Technion 2015 ©