Stepping Up: A Brief Intro to Scala

Derek Chen-Becker, October 10th, 2009

A little background Scala began around 2003, 2.0 in 2006,

currently at 2.7.5 (stable) Created by Martin Odersky (EPFL), of GJ,

and later Javac v5 fame Compiles to 100% Java Bytecode

Generally works flawlessly with Java Leverages JIT: performance ±5% of Java

Excellent community (mailing lists, IRC) In use at Twitter, Seimens, Xerox, and others

Not quite déjà vu...Scala combines features from many different languages to form a unique hybrid:

Scala is pure OO. Everything is an object, even primitives

Scala has first-class function objects. Much of the library leverages function arguments

Scala has a comprehensive type system and a flexible inheritance mechanism

Method names have less restrictions. This allows for operator overloading and nice DSLs

Immutability is strongly supported and encouraged. This is important for multi-core

Statics and Singletons

Scala makes a distinction between instance and static methods and fields by using a separate “object” entity to represent the latter

Objects are VM singletons Objects can be used to create custom

factories and pattern matching extractors (more in a moment)

public class Bike { private String brand; private int gear;

public Bike(String brand, int gear) { this.brand = brand; this.gear = gear; }

public int getGear() { return gear; }

public void setGear(int gear) { this.gear = gear; }

public String getBrand() { return brand; } public String toString() { return String.format("This is a %s in gear %d", brand, gear); }}

Scala is Succinct

class Bike (val brand : String, var gear : Int) { override def toString = "This is a %s in gear %d" .format(brand, gear)}

Scala has Case Classes

case class Book(title : String, author : String)

val classic = Book("A Tale of Two Cities", "Dickens")

val Book(title,author) = classic

Case classes are defined basically like normal classes

By default are immutable (good!) Constructor parameters are automatically vals Automatically get a companion object with

apply/unapply, as well as overrides for equals, hashCode and toString on the class itself

Scala has Case Classes, part 2

class Vehicle(val description : String, val topSpeed : Long)

case class Airplane(description : String, topSpeed : Long, var fuel : Double) extends Vehicle(description,speed) { def takeOff {...}}

Can inherit from other classes (although inheriting from other case classes is being deprecated)

Can have their own methods just like normal classes

Scala is Functional

Functions in Scala are first-class objects, and can be passed as arguments, assigned to variables, etc.

val printLog =

{ msg : String => // or ⇒ println(System.currentTimeMillis + ":" + msg) }

List("one", "two", "three").foreach(printLog)

Scala is Functional, Part 2

Functions (and Scala's type system) allow you to do things like define new control structures in a type-safe way:

// Structural typing allows any type with a “close” methoddef using[A <: { def close() : Unit }, B] (resource : A ) (f : A => B) : B = try { f(resource) } finally { resource.close } val firstLine = using(new BufferedReader(new FileReader("/etc/hosts"))) { reader => reader.readLine }

Scala has Closures

// Note that this example is not thread-safedef logAndAdd(name : String) = { var counter = 0

val logFunc = { msg : String => println(name + "-" + counter + ":" + msg) counter += 1 }

logFunc}

Closures capture variables in the scope of their definition. These are ideal for callbacks, among other things.

Scala is Higher Level

case class User(name : String, age : Int)val people = List(User("Fred", 35), User("Barney", 32),...)

val sorted = people.sort(_.age < _.age).take(5)

// The same as :val sorted = people.sort({(a,b) => a.age < b.age}).take(5) val under30 = sorted.takeWhile(_.age < 30)

val youngest5 = sorted.take(5)

val oldest5 = sorted.takeRight(5).reverse

In particular, collections support complex operations directly:

Scala has For-Comprehensions

val prefixes = List("/tmp", "/work", System.getProperty("user.home"))

val suffixes = List("txt", "xml", "html") def locateFiles(name : String) : List[String] = for (prefix <- prefixes; suffix <- suffixes; filename <- Some("%s/%s.%s".format(prefix, name, suffix)) if (new File(filename)).exists) yield filename

For-comprehensions provide an elegant construct for tying multiple sources and criteria together to extract information:

Scala does Pattern Matching

def matchIt (in : Any) = in match { case 1 | 14 | "test" => {...} case x : Int => {...} case x : Double if x > 42.0 => {...} case Vehicle(description, _) => {...} case <head>{ stuff }</head> => {...} case _ => {...}}

Like the “switch” statement on steroids Incredibly powerful for expressing

conditional logic Cases are tested in order

Scala does Custom Matching My favorite Scala example (more in the

handout)// Regexps in Scala define a custom unapply to match on groupsval colonSplit = """(\w+)\:(\w+)""".r

// It's easy to define your own as wellobject IntVal { def unapply(in : String) = try { Some(in.toInt) } catch { case _ => None }}

def parts (in : String) = in match {// Matching allows nested use of extractors (here we want Ints) case colonSplit(IntVal(number), value) if number == 14 => {...} case colonSplit(IntVal(number), value) => {...} case colonSplit(key, value) => {...} case _ => {...}}

Scala has Traits At their most basic, traits are 1-to-1 with Java

interfaces Support concrete implementations of methods Support both concrete and abstract

implementations of vals and vars

trait Named { val version = "1.0.0" val name : String def printName { println(name) }}

Scala Enforces Trait Composition

Traits can restrict which classes and traits they can be mixed into

Traits can also define what “this” means

trait CreepyPerson extends Named { self : Person =>

override def printName { println("Creepy! " + name + ", size = " + this._size) }}

class A extends CreepyPerson {} // Error!

Scala Mixes In Traits Classes can have multiple traits mixed in,

but can still only extend one superclass Disambiguation of method calls is done via

a process called “Linearization”. Basically, right-to-left

trait Messageable { def msg (msg : String) }

trait Emailable extends Messageable {...}

trait SMSable extends Messageable {...}

class iPhone extends SMSable with Emailable {...}

Scala has Native XML

XML literals are part of Scala's syntax Xpath-like expressions and pattern

matching supported for extractionval simpleFragment = <entries> <entry><name>Fred</name></entry> <entry><name>Barney</name></entry> </entries>

(simpleFragment \\ "name").toList.map(_.text)

simpleFragment.child.foreach({ case <entry><name>{name}</name></entry> => println(name); case _ => })

Scala has Native XML, Part 2

Values can be embedded in XML literals via braces.

Interpretation of values is via toString, although scala.xml.{PCData,Unparsed} can do special handling

// <name>Fred&amp;Barney</name>val embedFragment = <name>{ "Fred&Barney" }</name> val noEscapes = // <name>Betty&Wilma</name> <name>{ Unparsed("Betty&Wilma") }</name>

Conclusion

Scala lets you write less boilerplate and concentrate on the logic

Interoperates very well with the existing Java ecosystem

IDE support is still not where it needs to be, but it's functional

Odersky, et. al, are committed to long-term growth and stability

Further Reading

http://www.scala-lang.org/ http://scala.sygneca.com/ Books:

Programming in Scala by Odersky, Venners & Spoon

Beginning Scala by David Pollak Programming Scala by Wampler & Payne (online) Programming Scala by Venkat Subramaniam Steps in Scala by Loverdos & Syropolous The Definitive Guide to Lift (shameless plug)