Scala for Jedi

SCALA ДЛЯ ДЖЕДАЕВ
Владимир Парфиненко
vladimir.parﬁnenko@gmail.com
@cypok

OPTION
val set = Set(42, null, "Yoda")
val found = set find { _ == null }

if (found != null) {
// huh, we've found it or not?
println(s"Found $found!")
} else {
println("Not found.")
}

OPTION
val set = Set(42, null, "Yoda")
val found = set find { _ == null }

found match {
case Some(elem) => println(s"Found $elem!")
case None => println("Not found.")
}

olation
2 .10

string interp

TYPES ON STEROIDS
sealed abstract class Option[+A] {
def get: A
}

final case class Some[+A](x: A) extends Option[A] {
def get = x
}

case object None extends Option[Nothing] {
def get = throw new NoSuchElementException("None.get")
}

TYPE COVARIANCE
not im 2 .10
pleme
class Cell[T](x: T) { nte d
def get: T = ???
def set(x: T) { ??? }
}

val c1 = new Cell[String]("Anakin")

val c2: Cell[Any] = c1 // type mismatch

TYPE COVARIANCE

class Cell[+T](x: T) {
def get: T = ???
}


val c2: Cell[Any] = c1
val jedi = c2.get

TYPE COVARIANCE

class Cell[+T](x: T) {
def get: T = ???
def set(x: T) { ??? } // compilation error
}


val c2: Cell[Any] = c1
c2.set(37)

TYPE COVARIANCE

class Cell[-T](x: T) {
def get: T = ??? // compilation error
def set(x: T) { ??? }
}

val c1 = new Cell[Any](37)

val c2: Cell[String] = c1
val jedi = c2.get

TYPE COVARIANCE

class Cell[-T](x: T) {
def set(x: T) { ??? }
}

val c1 = new Cell[Any]('Anakin)

val c2: Cell[String] = c1
c2.set("Darth Vader")

LOWER & UPPER BOUNDS
class List[+A] {
def ++[B >: A](that: List[B]): List[B] = ???
}

val strs = List("Leia", "Organa")
val syms = List('Anakin, 'Skywalker)
val all = strs ++ syms // List[java.io.Serializable]

def max[A <: Ordered[_]](xs: List[A]): A = ???
val accounts: List[Money]
max(accounts)

CONTEXT BOUNDS
trait Jedi { def force: Double }

val jediOrder: Set[Jedi]
val grandMaster = jediOrder.max
// No implicit Ordering defined for Jedi.

implicit val jediOrdering = new Ordering[Jedi] {
def compare(j1: Jedi, j2: Jedi) = j1.force.compare(j2.force)
}

val grandMaster = jediOrder.max

trait TraversableOnce[+A] {
def max[B >: A](implicit cmp: Ordering[B]): A = ???
def max[A : Ordering]: A = ???
}

TYPES HIERARCHY
Any

AnyVal AnyRef/Object

ScalaObject

Value classes Primitives Java classes
2.10 Scala classes

Null

Nothing

PARALLEL COLLECTIONS
val nums = (1 to 10).par
nums foreach { x => print(x + " ") }
// 1 2 8 9 10 7 5 6 3 4

nums reduce { _ - _ }
// -15

nums reduce { _ - _ }
// 5

nums reduce { _ + _ }
// 55

val word = Seq("G", "r", "i", "e", "v", "o", "u", "s")
word.par reduce { _ ++ _ }
// Grievous

2.10
FUTURES

• Хранилище для значения, которое будет получено в
будущем

• Получениезначения может быть выполнено асинхронно и
не блокировать программу

• Значение может быть не получено вовсе

FUTURES
val f: Future[List[String]] = future {
session.getRecentPosts
}

f onSuccess {
case posts =>
for (post <- posts) println(post)
}

f onFailure {
case t =>
println("An error has occured: " + t.getMessage)
}

FOR

val files: Seq[File]

for (file <- files) {
println(file.getName)
}

FOR

def fileLines(f: File): Seq[String] = ???

for (file <- files) {
if (!file.getName.startsWith(".")) {
for (line <- fileLines(file)) {
if (line.nonEmpty) {
println(file + ": " + line)
}
}
}
}

FOR
for {
file <- files
if !file.getName.startsWith(".")
line <- fileLines(file)
if line.nonEmpty
} println(file + ": " + line)

files withFilter { !_.getName.startsWith(".") } foreach { file =>
fileLines withFilter { _.nonEmpty } foreach { line =>
println(file + ": " + line)
}
}

FOR
val lines = for {
file <- files
if !file.getName.startsWith(".")
line <- fileLines(file)
if line.nonEmpty
} yield (file + ": " + line)

files withFilter { !_.getName.startsWith(".") } flatMap { file =>
fileLines(file) withFilter { _.nonEmpty } map { line =>
file + ": " + line
}
}

FUTURES

val usdQuote = future { connection.getCurrentValue(USD) }
val chfQuote = future { connection.getCurrentValue(CHF) }

val purchase = for {
usd <- usdQuote
chf <- chfQuote
if isProfitable(usd, chf)
} yield connection.buy(amount, chf)

purchase onSuccess {
case _ => println(s"Purchased $amount CHF")
}

FUTURES
val rateQuote = future {
connection.getCurrentValue(USD)
}

val purchase = rateQuote map { quote =>
if (isProfitable(quote)) connection.buy(amount, quote)
else throw new Exception("not profitable")
}

purchase onSuccess {
case _ => println(s"Purchased $amount USD")
}

filter, fallbackTo, recoverWith, andThen, …

PROMISES
val p = promise[T]
val f = p.future

val producer = future {
val r = produceSomething()
p success r
continueDoingSomethingUnrelated()
}

val consumer = future {
startDoingSomething()
f onSuccess {
case r => doSomethingWithResult()
}
}

PROMISES
def first[T](f: Future[T], g: Future[T]): Future[T] = {
val p = promise[T]

f onSuccess {
case x => p.tryComplete(x)
}

g onSuccess {
case x => p.tryComplete(x)
}

p.future
}

2.10
ACTORS & AKKA

• Акторы – это приватные данные и описание поведения

• Общение между акторами только через сообщения с
неизменяемыми данными

• Акторы могут создавать других акторов и управлять ими в
случае исключительных ситуаций

ACTORS
class MyActor extends Actor {
def receive = {
case "test" => log("received test")
case _ => log("received unknown message")
}
}

object Main extends App {
val system = ActorSystem("MySystem")
val myActor = system.actorOf(Props[MyActor], name = "myactor")
myActor ! "test"
}

ACTORS
class Squarer extends Actor {
def receive = {
case x: Int => sender ! (x * x)
case x: Double => sender ! (x * x)
}
}

class Mathematician extends Actor {
val squarer = context.actorOf(Props[Squarer], name = "squarer")
def receive = {
case x: Int =>
(squarer ? x) onComplete {
case Success(x) => log(x)
case Failure => fail()
}
}
}

{
on[ A] =
ti
n)) : Op
ea trait
=> Bool Provi
, p : (A def s der {
rec Lis t[A] }
mth:
Int
@ tail A]( xs:
f ind[ p)
def tch
{
ne (ta il,
x s ma l = > No => find class
e Ni tail lse LazyO
c as :: hea d) e lazy ne ex
tends
head ) S ome( val s Provi
case ead) ??? mth = der {
p(h {
if ( }
} }

}
assert(assertion: Boolean, message: => Any): Unit

assert(true, { ??? })
assert(false, { ??? })

?
my t = ??
2.10 age : In ?
pack o o { v al x t = ??
implicit
class Ro cla ss F e[this] y : In = ???
ckString vat val nt
def rock
() = ??? (str: St
ring) { pri e[ Foo] al z: I
} pr ivat [my] v
e
"foo".ro pr ivat
ck()
}

val xml = <and><much>More!</much></and>

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