The Scala programming language has been gaining momentum recently as an alternative (and some might say successor) to Java on the JVM. This talk will start with an introduction to basic Scala syntax and concepts, then delve into some of Scala's more interesting and unique features. At the end we'll show a brief example of how Scala is used by the Lift web framework to simplify dynamic web apps.

1. Stepping Up: A Brief Intro to Scala
Derek Chen-Becker, October 10th, 2009

2. 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

3. 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

4. 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)

5. Scala is Succinct
public class Bike {
private String brand;
private int gear;
public Bike(String brand, class Bike (val brand : String,
int gear) { var gear : Int) {
this.brand = brand; override def toString =
this.gear = gear; "This is a %s in gear %d"
} .format(brand, 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);
}
}

6. Scala has Case Classes

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
case class Book(title : String, author : String)
val classic =
Book("A Tale of Two Cities", "Dickens")
val Book(title,author) = classic

7. Scala has Case Classes, part 2

Can inherit from other classes (although inheriting
from other case classes is being deprecated)

Can have their own methods just like normal
classes
class Vehicle(val description : String,
val topSpeed : Long)
case class Airplane(description : String,
topSpeed : Long,
var fuel : Double)
extends Vehicle(description,speed) {
def takeOff {...}
}

8. 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)

9. 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” method
def 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
}

10. Scala has Closures
Closures capture variables in the scope of their
definition. These are ideal for callbacks, among
other things.
// Note that this example is not thread-safe
def logAndAdd(name : String) = {
var counter = 0
val logFunc = { msg : String =>
println(name + "-" + counter + ":" + msg)
counter += 1
}
logFunc
}

11. Scala is Higher Level
In particular, collections support complex
operations directly:
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

12. Scala has For-Comprehensions
For-comprehensions provide an elegant
construct for tying multiple sources and
criteria together to extract information:
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

13. Scala does Pattern Matching

Like the “switch” statement on steroids
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Incredibly powerful for expressing
conditional logic

Cases are tested in order
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 _ => {...}
}

14. Scala does Custom Matching

My favorite Scala example (more in the
handout)
// Regexps in Scala define a custom unapply to match on groups
val colonSplit = """(w+):(w+)""".r
// It's easy to define your own as well
object 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 _ => {...}
}

15. 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) }
}

16. 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!

17. 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 {...}

18. Scala has Native XML

XML literals are part of Scala's syntax

Xpath-like expressions and pattern
matching supported for extraction
val 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 _ =>
})

19. 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>

20. 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

21. Further Reading

http://www.scala-lang.org/

http://scala.sygneca.com/

Books:

Programming in Scala by Odersky, Venners &
Spoon
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Beginning Scala by David Pollak

Programming Scala by Wampler & Payne (online)

Programming Scala by Venkat Subramaniam
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Steps in Scala by Loverdos & Syropolous
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The Definitive Guide to Lift (shameless plug)