Scala: Devnology - Learn A Language Scala

packagenl.devnology.workshop
importjava.util._
classLearnALanguage(today: Calendar) extendsDevnology{
defwelcome(participants: List[Participant]) {
participants.foreach(p => println("welcome" + p))
}
def facilitators() = {
Facilitator("SoemirnoKartosoewito") ::
Facilitator("Jan Willem Tulp") :: Nil
}
}

What’s the plan?
Setup development environment (Eclipse + Scalaplugin)
Form pair-programming pairs
Explanation of basic concepts and syntax
Labs, labs, labs...
... and of course: share knowledge!

Euh...Scala?
Scala runs on JVM and .Net VM
integrates 100% with existing libraries
hybrid language: both functional and OO
statically typed
“everything is an object”
...Immutability, Currying, Tuples, Closures, Higher Order Functions, etc....

Scala in the enterprise

Run Scala as...
Interactive / REPL (Read Eval Print Loop): the scala command starts an interactive shell
As script
Compiled: scalac command compiles Scala code
See: http://www.scala-lang.org/node/166

Variables, Values & Type Inference
varmsg = "welcome to ..." // msg is mutable
msg += " Devnology"
msg = 3 // compiler error

Variables, Values & Type Inference
valmsg = "welcome to ..." // msg is immutable
msg += " Devnology" // compiler error
val n : Int = 3 // explicit type declaration
valname : String = "John"

Functions
def min(x: Int, y: Int) = if (x < y) x else y
// equivalent:
def invert(x: Int) = -x // last statement is return value
def invert(x: Int) : Int = { return –x }
Unit can be considered as java’s void
Watch out!
def invert(x: Int) { // will return Unit, = is absent
-x
}

Every operation is a function call
1 + 2 is the same as1.+(2)
“to” is not a keyword: for (i <- 0 to 10) print(i)
map containsKey ‘a’ is the same as map.containsKey(‘a’)

Lists
valscores = List(1, 2, 3) // immutable
valextraScores: List[Int] = 4 :: 5 :: 6 :: Nil
// allScores = List(1, 2, 3, 4, 5, 6)
// scores = List(1, 2, 3)
// extraScores = List(4, 5, 6)
valallScores = scores ::: extraScores
valevenMoreScores = 7 :: allScores
Nil is synonym for empty list

Foreach
valscores = List(1, 2, 3)
// equivalent
scores.foreach((n: Int) => println(n))
scores.foreach(n => println(n))
scores.foreach(println)

For comprehensions
valscores = List(1, 2, 3)
for (s <- scores)
println(s)
for (s <- scores if s > 1)
println(s)

Arrays
valnames = Array("John", "Jane")
names(0) = "Richard" // Arrays are mutable
val cities = new Array[String](2)
cities(0) = "Amsterdam"
cities(1) = "Rotterdam"
for (i <- 0 to 1)
println(cities(i))

Maps
valtowns = Map("John" -> "Amsterdam", "Jane" -> "Rotterdam") // default immutable Map
var a = towns("John") // returns "Amsterdam"
a = towns get "John”// returns Some(Amsterdam)
a = towns get "John"get // returns "Amsterdam"
var r = towns("Bill") // throws NoSuchElementException
r = towns get "Bill”// returns None
towns.update("John", "Delft") // returns a new Map

Classes & Constructors
class Person(name: String, age: Int) {
if (age < 0) thrownewIllegalArgumentException
defsayHello() { println("Hello, " + name) }
}
require (age >= 0)
defsayHello() { println("Hello, " + name) }
}

def this(name: String) = this(name, 21) // auxiliary
def this(age: Int) = this(“John”, age) // auxiliary
def this() = this(“John”, 21) // auxiliary
}

class Person(name: String, age: Int)
...
val p = new Person("John", 33)
val a = p.age// compiler error
p.name = "Richard" // compiler error
class Person(var name: String, val age: Int)

Companion Objects
// must be declared in same file as Person class
object Person {
defprintName = println("John")
}
valp = Person // Singleton is also an object
Person.printName// similar to static methods in Java/C#

Traits
trait Student {
varage = 10;
def greet() = {
"Hello teacher!"
}
def study(): String // abstract method
}

Extending Traits
class FirstGrader extends Student {
override def greet() = {
"Hello amazing teacher!"
}
override def study(): String = {
“I am studying really hard!"
}
}

Trait Mixin
// compiler error: abstract function study from trait Student is not implemented
valjohn = new Person with Student
traitSimpleStudent {
def greet() = "Hello amazing teacher!"
}
// this is ok
val john = new Person withSimpleStudent
println(john.greet())

Scala Application
object Person
def main(args: Array[String]) { // define a main method
for (arg <- args)
println("Hello " + arg)
}
}
// or extend Application trait
object Person extends Application {
for (name <- List("John", "Jane"))
println("Hello " + name)
}

Pattern Matching
valcolor = if (args.length > 0) args(0) else ""
valfruit match {
case"red"=>"apple"
case"orange" =>"orange"
case"yellow" =>"banana"
case_ => "yuk!"
}

Case Classes
case class Var(name: String) extendsExpr
val v = Var("sum")
adds a Factory method with the name of the class
all parameters implicitly get a val prefix, so they are maintained as fields
“natural” implementation of toString, hashCode and equals
big advantage: support pattern matching

Exceptions
try {
args(0).toFloat
} catch {
case ex: NumberFormatException => println("Oops!")
}

Tuples
valpair = ("John", 28) // Tuple2[String, Int]
println(pair._1)
println(pair._2)
def divProd(x: Int, y:Int) = (x / y, x * y)
valdp = divProd(10, 2)
println(pair._1) // 5
println(pair._2) // 20

LABS!!
Resources:
http://www.scala-lang.org/api
http://scala-tools.org/scaladocs/scala-library/2.7.1/
http://www.codecommit.com/blog/scala/roundup-scala-for-java-refugees
http://blogs.sun.com/sundararajan/entry/scala_for_java_programmers

THANK YOU!

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Scala: Devnology - Learn A Language Scala

by Jan Willem Tulp on Oct 08, 2009

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