![Syntax
def max(ints: List[Int]): Int = {
def max(x:Int, y:Int) : Int = if(x>y) x else y
ints.fold(Int.MinValue)(max(_,_))
}
val maxVar = max(List(1,2,3,4))
vals and vars
name: Type[typeParam]
def method(param1:pType, param2:pType): mType = { … }
array(2) instead of array[2]
not mandatory semicolon(;)
operators
Alternative method call “object method arg”
Type inference
Named parameters
Integer max(List<Integer> ints){
Integer max = Integer.MIN_VALUE;
for(Integer intEl : ints)
if(intEl > max)
max = intEl;
return max;
}](https://image.slidesharecdn.com/meetscala-151030073456-lva1-app6892/85/Meet-scala-5-320.jpg)
![Assertions and Unit Testing
Static assert() and non-static ensuring()
JUnit, TestNG, ScalaTest, specs
import org.specs._
import Element.elem
object ElementSpecification extends Specification {
"A UniformElement" should {
"have a width equal to the passed value" in {
elem('x', 2, 3).width must be_==(2)
}
"have a height equal to the passed value" in {
elem('x', 2, 3).height must be_==(3)
}
"throw an IAE if passed a negative width" in {
elem('x', -2, 3) must throwA[IllegalArgumentException]
}
}
}](https://image.slidesharecdn.com/meetscala-151030073456-lva1-app6892/85/Meet-scala-10-320.jpg)
![Type Parameterization(Generics)
Types and type parameters are guessed if possible(target typing)
No raw types
Generic types have by default nonvariant (or, “rigid”) subtyping
Lower and upper bounds
scala> val q = Queue(1, 2, 3)
q: Queue[Int] = Queue(1, 2, 3)
def doesNotCompile(q: Queue) {}
//variance
trait Q[T] {…} // nonvariant, Q[String] is not instance of Q[AnyRef]
trait Q[+T] {…} // covariant, Q[String] is instance of Q[AnyRef]
trait Q[-T] {…} // contravariant, Q[AnyRef] is instance of Q[String]
class Queue[+T] (private val leading: List[T], private val trailing: List[T] ) {
def enqueue[U >: T](x: U) = new Queue[U](leading, x :: trailing) // ...
}
def orderedMergeSort[T <: Ordered[T]](xs: List[T]): List[T] = {…}](https://image.slidesharecdn.com/meetscala-151030073456-lva1-app6892/85/Meet-scala-12-320.jpg)
![Traits
Interfaces on steroids
Multiple inheritance
Linear order
Order of mixins is important!
abstract class IntQueue {
def get(): Int
def put(x: Int)
}
class BasicIntQueue extends IntQueue {
private val buf = new ArrayBuffer[Int]
def get() = buf.remove(0)
def put(x: Int) { buf += x }
}
trait Doubling extends IntQueue {
abstract override def put(x: Int) { super.put(2 * x) }
}
trait Incrementing extends IntQueue {
abstract override def put(x: Int) { super.put(x + 1) }
}
trait Filtering extends IntQueue {
abstract override def put(x: Int) {
if (x >= 0) super.put(x)
}
}
val queue = (new BasicIntQueue with Incrementing with Filtering)
• If the behavior will not be reused, then make it a concrete class
• If it might be reused in multiple, unrelated classes, make it a trait.
• If you want to inherit from it in Java code, use an abstract class.
• If you plan to distribute it in compiled form … its more complicated
• If efficiency is very important, lean towards using a class.
• If you still do not know, start with a trait.](https://image.slidesharecdn.com/meetscala-151030073456-lva1-app6892/85/Meet-scala-17-320.jpg)
![Case Classes, Pattern Matching and Extractors
abstract class Expr
case class Var(name: String) extends Expr
case class Number(num: Double) extends Expr
case class UnOp(operator: String, arg: Expr) extends Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr
A bit like enums
Automatically added factory method,
toString, hashCode, equals, copy
Deep matches
Sealed classes
Subclasses in the same file
Matching coverage check
Extractors – allows you to treat class like a case class
def simplifyAll(expr: Expr): Expr = expr match {
case UnOp("", UnOp("", e)) => simplifyAll(e) // ‘’is its own inverse
case BinOp("+", e, Number(0)) => simplifyAll(e) // ‘0’ is a neutral element for ‘+’
case BinOp("*", e, Number(1)) => simplifyAll(e) // ‘1’ is a neutral element for ‘*’
case UnOp(op, e) => UnOp(op, simplifyAll(e))
case BinOp(op, l, r) => BinOp(op, simplifyAll(l), simplifyAll(r))
case _ => expr
}
def describe(x: Any) = x match {
case 5 => "five"
case true => "truth"
case "hello" => "hi!"
case Nil => "the empty list"
case _ => "something else"
}
object EMail {
// The injection method (optional)
def apply(user: String, domain: String) = user +"@"+ domain
// The extraction method (mandatory)
def unapply(str: String): Option[(String, String)] = {
val parts = str split "@"
if (parts.length == 2) Some(parts(0), parts(1)) else None
}
}](https://image.slidesharecdn.com/meetscala-151030073456-lva1-app6892/85/Meet-scala-18-320.jpg)
![Actors and Concurrency
Anti „synchronized” way
Actor – thread + mailbox + actions
Good style:
Actors should not block
Communicate only via messages
Prefer immutable messages
Akka
Typesafe actors comming
object NameResolver extends Actor {
import java.net.{InetAddress, UnknownHostException}
def act() {
react {
case (name: String, actor: Actor) =>
actor ! getIp(name)
act()
case "EXIT" =>
println("Name resolver exiting.")
// quit
case msg =>
println("Unhandled message: "+ msg)
act()
}
}
def getIp(name: String): Option[InetAddress] = {
try {
Some(InetAddress.getByName(name))
} catch {
case _:UnknownHostException => None
}
}
}
scala> NameResolver.start()
res0: scala.actors.Actor = NameResolver$@90d6c5
scala> NameResolver ! ("www.scalalang.org", self)
scala> self.receiveWithin(0) { case x => x }
res2: Any = Some(www.scalalang.org/128.178.154.102)
scala> NameResolver ! ("wwwwww.scalalang.org", self)
scala> self.receiveWithin(0) { case x => x }
res4: Any = None](https://image.slidesharecdn.com/meetscala-151030073456-lva1-app6892/85/Meet-scala-21-320.jpg)
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Meet scala
- 1. Meet scala Wojciech Pituła For AMG.net
- 2. Agenda What is it? Old features done better New features Why scala?
- 3. What is it? JVM language Object-functional Scripting Java compatible CONCISE
- 4. Old features done better Syntax Classes and Objects Basic Types and Operations Built-in Control Structures(if, while, switch) Packages and Imports Assertions and Unit Testing Collections Type Parameterization(Generics)
- 5. Syntax def max(ints: List[Int]): Int = { def max(x:Int, y:Int) : Int = if(x>y) x else y ints.fold(Int.MinValue)(max(_,_)) } val maxVar = max(List(1,2,3,4)) vals and vars name: Type[typeParam] def method(param1:pType, param2:pType): mType = { … } array(2) instead of array[2] not mandatory semicolon(;) operators Alternative method call “object method arg” Type inference Named parameters Integer max(List<Integer> ints){ Integer max = Integer.MIN_VALUE; for(Integer intEl : ints) if(intEl > max) max = intEl; return max; }
- 6. Classes and Objects Singleton companion objects Default constructor Mandatory override modifier One namespace for fields and methods class Rational(n: Int, d: Int) { require(d != 0) private val g = gcd(n.abs, d.abs) val numer = n / g val denom = d / g def this(n: Int) = this(n, 1) def + (that: Rational): Rational = new Rational( numer * that.denom + that.numer * denom, denom * that.denom ) def * (that: Rational): Rational = new Rational(numer * that.numer, denom * that.denom) override def toString = numer +"/"+ denom private def gcd(a: Int, b: Int): Int = if (b == 0) a else gcd(b, a % b) } Evaluated at construction time Default public modifier No get/set convention Complicated access modifiers you don’t want to know about
- 7. Basic Types and Operations All the Java literals (”””str”””) raw, multiline string(escapes doesn’t work) Operators are methods, (1).+(2) Object equality checked by ==, !=, not by .equals() Reference equality checked by eq() method foo(bar) is equal to foo.apply(bar) Rich primitive types by implicit conversion Any, AnyRef, Nothing
- 8. Built-in Control Structures(if, while, switch) They return values! Complicated for expression Producing collections with for and yield Catch with pattern matching match – switch on steroids No break and continue println(if (!args.isEmpty) args(0) else "default.txt") def grep(pattern: String) = for ( file <- filesHere if file.getName.endsWith(".scala"); line <- fileLines(file) if line.trim.matches(pattern) ) yield (file +": "+ line.trim) try { val f = new FileReader("input.txt") // Use and close file } catch { case ex: FileNotFoundException => // Handle missing file case ex: IOException => // Handle other I/O error } def describe(x: Any) = x match { case 5 => "five" case true => "truth" case "hello" => "hi!" case Nil => "the empty list" case _ => "something else" }
- 9. Packages and Imports Java-like or C#-like packages Imports can be declared anywhere And are much more powerful package bobsrockets { package navigation { // In package bobsrockets.navigation class Navigator package tests { // In package bobsrockets.navigation.tests class NavigatorSuite } } } // easy access to Fruit import bobsdelights.Fruit // easy access to all members of bobsdelights import bobsdelights._ // easy access to all members of Fruits import bobsdelights.Fruits._ //import only Apple and Orange import Fruits.{Apple, Orange} //import all Fruits but Orange and rename Apple to McIntosh import Fruits.{Apple => McIntosh, Orange =>_, _}
- 10. Assertions and Unit Testing Static assert() and non-static ensuring() JUnit, TestNG, ScalaTest, specs import org.specs._ import Element.elem object ElementSpecification extends Specification { "A UniformElement" should { "have a width equal to the passed value" in { elem('x', 2, 3).width must be_==(2) } "have a height equal to the passed value" in { elem('x', 2, 3).height must be_==(3) } "throw an IAE if passed a negative width" in { elem('x', -2, 3) must throwA[IllegalArgumentException] } } }
- 11. Collections Defaults are immutable! Tuples Sequences(Arrays, Lists, ArrayBuffers, ListBuffers) map, flatMap, filter, collect, fold, … Set, Map, MutableSet, MutableMap val tuple = (1, “hello”, 3.1) val (integer, str, dbl) = tuple Array(1.0, 2.0, 3.0) List(1, 2, 3, 4) Set(‘a’, ‘b’, ‘c’) Map("hi" -> 2, "there" -> 5)
- 12. Type Parameterization(Generics) Types and type parameters are guessed if possible(target typing) No raw types Generic types have by default nonvariant (or, “rigid”) subtyping Lower and upper bounds scala> val q = Queue(1, 2, 3) q: Queue[Int] = Queue(1, 2, 3) def doesNotCompile(q: Queue) {} //variance trait Q[T] {…} // nonvariant, Q[String] is not instance of Q[AnyRef] trait Q[+T] {…} // covariant, Q[String] is instance of Q[AnyRef] trait Q[-T] {…} // contravariant, Q[AnyRef] is instance of Q[String] class Queue[+T] (private val leading: List[T], private val trailing: List[T] ) { def enqueue[U >: T](x: U) = new Queue[U](leading, x :: trailing) // ... } def orderedMergeSort[T <: Ordered[T]](xs: List[T]): List[T] = {…}
- 13. New features Scripting Functional/immutable objects Functions and Closures Traits Case Classes, Pattern Matching and Extractors Implicit Conversions and Parameters Working with XML Actors and Concurrency
- 14. Scripting Scala console Script files Good debug/testing tool #!/bin/sh exec scala "$0" "$@" !# // Say hello to the first argument println("Hello, "+ args(0) +"!") $ scala Welcome to Scala version 2.8.1. Type in expressions to have them evaluated. Type :help for more information. scala> 1 + 2 res0: Int = 3 scala> res0 * 3 res1: Int = 9
- 15. Functional/immutable objects Matter of convention No mutable state(vals of immutable types only) “Mutating” methods return new object Mutable alternatives(builders) + Easier to reason about + Pass freely around the system + Thread safe + Safe hash keys - COPY
- 16. Functions and Closures Methods Nested/local functions Function literals and values(first-class and higher-class) Placeholder syntax Closures Repeated parameters Named arguments Default values Tail recursion – no stack overflow and overhead scala> var increase = (x: Int) => x + 1 increase: (Int) => Int = <function1> scala> increase(10) res0: Int = 11 val someNumbers = List(11, 10, 5, 0, 5, 10) someNumbers.foreach((x: Int) => println(x)) someNumbers.filter(x => x > 0) someNumbers.map(_ * 2) val f = (_: Int) + (_: Int) ; f(5, 10) def sum(a: Int, b: Int, c: Int) = a + b + c val a = sum _ //partially applied function // also correct: val a = sum val b = sum(1, _: Int, 3) def makeIncreaser(more: Int) = (x: Int) => x + more //closure val inc1 = makeIncreaser(1) // inc1(10) = 11 val inc9999 = makeIncreaser(9999) //inc9999(-9) = 9990 def approximate(guess: Double): Double = if (isGoodEnough(guess)) guess else approximate(improve(guess)) // last call in body is recursive def div(x :Int, y :Int=1) { x*y } div( y=2 , x=3) div(3) scala> var increase = (x: Int) => x + 1 increase: (Int) => Int = <function1> scala> increase(10) res0: Int = 11 scala> var increase = (x: Int) => x + 1 increase: (Int) => Int = <function1> scala> increase(10) res0: Int = 11 scala> var increase = (x: Int) => x + 1 increase: (Int) => Int = <function1> scala> increase(10) res0: Int = 11 scala> var increase = (x: Int) => x + 1 increase: (Int) => Int = <function1> scala> increase(10) res0: Int = 11
- 17. Traits Interfaces on steroids Multiple inheritance Linear order Order of mixins is important! abstract class IntQueue { def get(): Int def put(x: Int) } class BasicIntQueue extends IntQueue { private val buf = new ArrayBuffer[Int] def get() = buf.remove(0) def put(x: Int) { buf += x } } trait Doubling extends IntQueue { abstract override def put(x: Int) { super.put(2 * x) } } trait Incrementing extends IntQueue { abstract override def put(x: Int) { super.put(x + 1) } } trait Filtering extends IntQueue { abstract override def put(x: Int) { if (x >= 0) super.put(x) } } val queue = (new BasicIntQueue with Incrementing with Filtering) • If the behavior will not be reused, then make it a concrete class • If it might be reused in multiple, unrelated classes, make it a trait. • If you want to inherit from it in Java code, use an abstract class. • If you plan to distribute it in compiled form … its more complicated • If efficiency is very important, lean towards using a class. • If you still do not know, start with a trait.
- 18. Case Classes, Pattern Matching and Extractors abstract class Expr case class Var(name: String) extends Expr case class Number(num: Double) extends Expr case class UnOp(operator: String, arg: Expr) extends Expr case class BinOp(operator: String, left: Expr, right: Expr) extends Expr A bit like enums Automatically added factory method, toString, hashCode, equals, copy Deep matches Sealed classes Subclasses in the same file Matching coverage check Extractors – allows you to treat class like a case class def simplifyAll(expr: Expr): Expr = expr match { case UnOp("", UnOp("", e)) => simplifyAll(e) // ‘’is its own inverse case BinOp("+", e, Number(0)) => simplifyAll(e) // ‘0’ is a neutral element for ‘+’ case BinOp("*", e, Number(1)) => simplifyAll(e) // ‘1’ is a neutral element for ‘*’ case UnOp(op, e) => UnOp(op, simplifyAll(e)) case BinOp(op, l, r) => BinOp(op, simplifyAll(l), simplifyAll(r)) case _ => expr } def describe(x: Any) = x match { case 5 => "five" case true => "truth" case "hello" => "hi!" case Nil => "the empty list" case _ => "something else" } object EMail { // The injection method (optional) def apply(user: String, domain: String) = user +"@"+ domain // The extraction method (mandatory) def unapply(str: String): Option[(String, String)] = { val parts = str split "@" if (parts.length == 2) Some(parts(0), parts(1)) else None } }
- 19. Implicit Conversions and Parameters Implicit conversion Only definitions marked implicit are available An inserted implicit conversion must be in scope as a single identifier, or be associated with the source or target type of the conversion. One-at-a-time Rule: Only one implicit is tried Whenever code type checks as it is written, no implicits are attempted. Implicit parameters implicit def function2ActionListener(f: ActionEvent => Unit) = new ActionListener { def actionPerformed(event: ActionEvent) = f(event) } class PreferredPrompt(val preference: String) object Greeter { def greet(name: String)(implicit prompt: PreferredPrompt) { println("Welcome, "+ name +". The system is ready.") println(prompt.preference) } } object JoesPrefs { implicit val prompt = new PreferredPrompt("Yes, master> ") } import JoesPrefs._ Greeter.greet("Joe")
- 20. Working with XML XML literals Compiled Xpath-like syntax Pattern matching scala> val yearMade = 1955 scala> <a> { if (yearMade < 2000) <old>{yearMade+”r.”}</old> else xml.NodeSeq.Empty } </a> scala> <a><b><c>hello</c></b></a> "b" res10: scala.xml.NodeSeq = <b><c>hello</c></b> scala> <a><b><c d="world">hello</c></b></a> "c" "@d" res12: scala.xml.NodeSeq = world abstract class CCTherm { ... def toXML = <cctherm> <description>{description}</description> <yearMade>{yearMade}</yearMade> <dateObtained>{dateObtained}</dateObtained> <bookPrice>{bookPrice}</bookPrice> <purchasePrice>{purchasePrice}</purchasePrice> <condition>{condition}</condition> </cctherm> } abstract class CCTherm { ... def fromXML(node: scala.xml.Node): CCTherm = new CCTherm { val description = (node "description").text val yearMade = (node "yearMade").text.toInt val dateObtained = (node "dateObtained").text val bookPrice = (node "bookPrice").text.toInt val purchasePrice = (node "purchasePrice").text.toInt val condition = (node "condition").text.toInt }}
- 21. Actors and Concurrency Anti „synchronized” way Actor – thread + mailbox + actions Good style: Actors should not block Communicate only via messages Prefer immutable messages Akka Typesafe actors comming object NameResolver extends Actor { import java.net.{InetAddress, UnknownHostException} def act() { react { case (name: String, actor: Actor) => actor ! getIp(name) act() case "EXIT" => println("Name resolver exiting.") // quit case msg => println("Unhandled message: "+ msg) act() } } def getIp(name: String): Option[InetAddress] = { try { Some(InetAddress.getByName(name)) } catch { case _:UnknownHostException => None } } } scala> NameResolver.start() res0: scala.actors.Actor = NameResolver$@90d6c5 scala> NameResolver ! ("www.scalalang.org", self) scala> self.receiveWithin(0) { case x => x } res2: Any = Some(www.scalalang.org/128.178.154.102) scala> NameResolver ! ("wwwwww.scalalang.org", self) scala> self.receiveWithin(0) { case x => x } res4: Any = None
- 22. Why scala? Conciseness Immutability Modernity Convenience
- 23. References Programming in Scala Free at http://www.artima.com/pins1ed/ Scala for Java Programmers Tutorial http://docs.scala-lang.org/tutorials/scala-for-java-programmers.html