Scala Intro

Scala Intro
val you = new Developer[Any] with ScalaKnowledge

Is Scala really hard?

Can I start using it without going mad?

Why zombies?

Scala History

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Scala
2003 AD

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Pizza
GJ
generics in Java
Funnel
Scala

Scala is a pure object-oriented language
Scala is also a functional language
Strongly statically typed
Runs on JVM, compiles to bytecode

Scala – scalable language
def factorial(x: BigInt): BigInt =
if (x == 0) 1 else x * factorial(x - 1)

In contrast with C++,
the more you study Scala, the easier it gets.
Unknown Scala community member

Scala in Production

http://www.scala-lang.org/old/node/1658

Hello …

object HelloZombie extends App {
println("Hello, " + "Zombie!")
}

Another Hello

object HelloZombie {
def main(args: Array[String]) {
println("Hello, " + "Zombie!")
}
}

You do not need CS degree to code in Scala
Application Programmers:
A1 – A3 levels
Library Designers:
L1 – L3 levels

«One can program very productively in Scala on level A1, which one
should be able to pick up in a day or so, coming from Java.
Mastering A2 will doubtlessly increase programmer productivity.
A3 is for expert programmers with more specialized tasks, not
everyone needs to get to that level.»

http://www.scala-lang.org/old/node/8610.html

#1 - Mutability is evil

#2 – Everyhting returns data

#3 – Functions are first-class citizens

Mutability is evil
• Mutable objects are complicated to think about
• Defensive copies

• Need to be synchronized in multithreaded env
• Unobvious side effects

Mutability is evil
val salary = "2000$"
salary = "2500$" // does not compile

var salary = "2000$”
salary = "2500$” // compiles

Everything returns data

val result = if (true) value1 else value2
val result = for (i <- 1 to 10) yield i
def getRandom() = {
Math.random()
}

Functions are first-class citizens

Assign function definitions to variables
val doubled = (x:Int) => x * 2

in Scala a function value is an object!

Scala => pure OOP & full-blown functional language

Sooo many objects around

Baaaad performance, yeah?

http://readwrite.com/2011/06/06/cpp-go-java-scala-performance-benchmark

#1 - Class
#2 - Object = build-in singleton pattern

#3 – Case class
#4 – Abstract class vs Trait
#5 – Multiple inheritance
WAT? Diamond problem solved!

Class
Class is similar to any other languages’ class notion
class MyZombie {
private val index: Int = 0
val name: String = "Zombie0"
}

Class + Constructor

class MyZombie(index: Int, val name: String) {
println("me likes brainz!")
}

Object

Objects are singletons
object LoneZombie {
def growl = println("me sooo sad")
}

Companion Object
class CrawlerZombie(name: String) {
def crawl() = println("crawling...")
}
object CrawlerZombie {
def apply(name: String) = {
println(name + " is here!")
new CrawlerZombie(name)
}
}
val zombie = CrawlerZombie("Mig")
zombie.crawl()

Case Class
Case class = class + “sugar” + compiler hint
case class Zombie(index: Int, name : String)

Case Class Sugar

1. Properties
2. Pattern Matching
3. Companion Object

Abstract class vs Trait

Abstract class defines structure
Trait defines behaviour

Abstract Class
abstract class Undead(name: String) {
def eat() = {
println("Om nom nom")
digest()
}
def digest()
}
class Zombie(name: String) extends Undead(name) {
def digest() = {
println("grab brainz")
}
}

Trait
abstract class Undead(name: String) {
def eat() = {
println("Om nom nom")
digest()
}
def digest()
}
trait Growling{
def growl(msg: String) = println(msg)
def scream(msg: String) = println(msg + " !!!")
}
class Zombie(name: String) extends Undead(name) with Growling {
def digest() = {
growl("grab brainz")
}
}

#1 - Scala basic types are objects
#2 – Scala implement it’s own types, wraps and extends
some Java classes and types

#3 Scala operators are method calls

Value type

Range

Byte

8-bit signed two's complement integer (27 to 27 - 1, inclusive)

Short

16-bit signed two's complement integer
(-215 to 215 - 1, inclusive)

Int

(-231 to 231 - 1, inclusive)

Long

(-263 to 263 - 1, inclusive)

Char

16-bit unsigned Unicode character (0 to
216 - 1, inclusive)

String

a sequence of Chars

Float

32-bit IEEE 754 single-precision float

Double

64-bit IEEE 754 double-precision float

Boolean

true or false

Operators are methods

Operators uses infix notation
val sum = 1 + 2

// Scala invokes (1).+(2)

Null ?

var zombie: Zombie = null
zombie = new Zombie("Alex")

Nothing?

def throwLocalizedException(key: String): Nothing = {
throw LocalizedException(resolveMessage(key))
}

Functions and methods are not the same!

Functions

No side effects
Evaluates only body and parameters
Compiles to FunctionN
Defined mostly in Objects

Functions

object KillerModule {
def kill = (z: Zombie) => println("Killing " + z.name)
}

Methods

Defined in classes
Works with class scope

Methods

class ZombieTortureMachine(z: Zombie) {
def kill() = println("Killing " + z.name)
def starve() = println("Starving " + z.name)
}

Pass function as a parameter
def traverseTree (callback: (Element) => Unit) {
val element = ??? //Getting tree element
callback(element)
}


Return function as a result value
def prepareCalculator(x: Int, y: Int): () => Int = {
println("calculating...")
() => x + y
}

You can omit types in declarations!
The compiler will infer them for you.
Less typing -> less reading -> happier developers 

Type inference with variables

private val index = 0
val name = "Zombie0"
private val index: Int = 0
val name: String = "Zombie0"

Type inference with functions

Scala compiler can not read thoughts 
def func (a: Int, b: String) = { a + b } // compiles
def func1 (a, b) = { a + b } // does not compile

Type inference

def getZombies(severity: Int) = {
if (severity > 10) {
List()
} else {
List(Zombie("Mig"), Zombie("Alex"))
}
} // inferred type = List[Zombie]

Lambda Syntax

val doubler = (n: Int) => n * 2
val yadobler = { n: Int => n * 2 }

Closure is special kind of a function
Closure encloses the local lexical context inside the function body
Closures can be implemented using Anonymous classes in Java
(yuck!)

Simple Closure

val multiplier = 2
val doubler = (n: Int) => n * multiplier

Closure vs Function

Pure function calculates its result solely in terms of its arguments!

Closure can use the outer lexical context for it computations.
One may say that it stores “references” to the outer values.

Sugar : Map Initialization
Java:
Map<String, Integer> mappings = new HashMap<String, Integer>() {{
put("One", 1);
put("Two", 2);
put("Three", 3);
}};

Scala:
val mappings = Map(
"one" -> 1,
"two" -> 2,
"three" -> 3
)

Sugar : Filtering
Java:
List<Integer> numbers = new ArrayList<Integer>(){{
add(1); add(2); add(-55); add(-33); add(122);
}};
List<Integer> negativeNumbers = new ArrayList<Integer>();
for (Integer number : numbers) {
if (number < 0) {
negativeNumbers.add(number);
}
}

Scala:
val numbers = List(1, 2, -55, -33, 122)
val negativeNumbers = numbers.filter(_ < 0)

Sugar : Classification
Java:
List<Integer> numbers = new ArrayList<Integer>(){{
add(1); add(2); add(-55); add(-33); add(122);
}};
List<Integer> negativeNumbers = new ArrayList<Integer>();
List<Integer> positiveNumbers = new ArrayList<Integer>();
for (Integer number : numbers) {
if (number < 0) {
negativeNumbers.add(number);
} else {
positiveNumbers.add(number);
}
}

Scala:
val numbers = List(1, 2, -55, -33, 122)
val (positiveNumbers, negativeNumbers) = numbers.span(_ > 0)

Stuff : Tuples
• Tuples can be viewed as simple immutable collections.
• Tuple can contain up to 22 elements of different types.
• Very useful when you need to return a complex value from
the expression

Stuff : Tuples
val pair = (22, "zombies")
val pair = (22 -> "zombies")
// the type is Tuple2[Int, String]

println(pair._1)
println(pair._2)

Mutable vs Immutable

Prefer immutable collections by default. Period.
Google for details.

Scala uses immutable collections by default
val map = Map("one" -> 1)
//results in scala.collection.immutable.Map[String,Int]

Collections practices
Map, Set and List are mostly used collections in Scala
Use traits’ companion object calls to create
collections when you do not need specific
implementation:
Map("one" -> 1) // good
HashMap("one" -> 1) // not so good
new HashMap("one" -> 1) // won't compile :)

Collections API
example #1
case class User(name: String, password: String)
val users = List("admin:nimda", "user1:asddsa", "root:qwerty")
val mappedUsers = users.map {
user =>
val splitted = user.split(":")
User(splitted(0), splitted(1))
}
// List[User] = List(User(admin,nimda), User(user1,asddsa), User(root,qwerty))

Collections API
example #2
val names = List(
"Alex,Viktor,Eugeny",
"Dmitry,Yegor, Sergey",
"Michael,Sergey")

val splitted = names.flatMap(_.split(",").toList).distinct
// List(Alex, Viktor, Eugeny, Dmitry, Yegor, Michael, Sergey)

Loops & For comprehension
• Scala has a while loop
• Scala has a do-while loop
• They are not expressions (the return type is Unit)

Loops & For comprehension
Because the while loop results in no value, it is often left out of
pure functional languages.
Such languages have expressions, not loops.
Scala includes the while loop nonetheless, because sometimes
an imperative solution can be more readable, especially to
programmers with a predominantly imperative background.

While loop
while (brains.size > 0) {
zombie eat brains
}

Do-While loop
do {
zombie eat brains
} while (brains.size > 0)

Friendly advise

Try use recursion instead of loops!
And don’t feed zombies

For expression
• Looks like a for-loop
• Behaves as a swiss knife 

For + generator

for (zombie <- zombieHorde)
kill(zombie)

For + generator + filter

for (zombie <- zombieHorde if zombie.isMoving)
kill(zombie)

For + generator + filter + yield
This will produce a new collection!

val killedZombies =
for (zombie <- zombieHorde if !zombie.isMoving)
yield zombie

For expression
Can be used instead of map, filter and flatMap combination

Simple matching
val status: String = "R”
val translated = status match {
case "R" => "running”
case "D" => "digesting”
case "E" => "eating brainz”
case _ => "X3"
}

Structure matching
case class User(name: String, lastname: String, age: Int)
val users = List(
User("Alexey", "Migutsky", 25),
User("Alexander", "Albul", 27),
User("John", "Doe", 99)
)
val determined = users.map {
case User("Alexey", _, _) => "L”
case User("Alexander", _, _) => "A”
case _ => "X3"
}.mkString(",")
// L,A,X3

Structure matching
def describe(list: List[Int]) {
list match {
case first :: second :: third :: tail => println("First case")
case head :: tail => println("Second case")
case Nil => println("Empty list")
}
}
describe(List(1,2,3)) // First case
describe(List(1, 2)) // Second case
describe(List()) // Empty list

Exception handling
try {
//something
} catch {
case e: IllegalArgumentException => println("Illegal argument")
case _ => println("Unknown exception")
}

Implicits

The compiler can insert parameters and call conversion methods
automatically based on the types used
This behavior can be achieved using the implicit modifier

#1 - Implicit parameters
#2 – Implicit type conversion
#3 – “Pimp my Library”

Implicit parameters
case class Context(data: List[String])
implicit val context = Context(List("a", "b", "c"))
object SomeService {
def printCtx(implicit ctx: Context) =
println(ctx.data.mkString(","))
}
SomeService.printCtx
// a,b,c

Implicit type conversion
Compiler will use the implicit conversion method automatically!
case class User(first: String, last: String)
def printUserInfo(user: User) {
println("User name is: " + user.first
+ ", last name is: " + user.last)
}
implicit def stringToUser(userString: String): User = {
val split = userString.split(" ")
User(split(0), split(1))
}
printUserInfo("Alexander Albul")

Pimp My Library
case class User(first: String, last: String)
implicit class ExtendedString(str: String) {
def toUser: User = {
val split = str.split(" ")
User(split(0), split(1))
}
}
def printUserInfo(user: User) {
println("User name is: " + user.first +
", last name is: " + user.last)
}
printUserInfo("Alexander Albul".toUser)

Moar Sugar!
For those who survived

Stuff : Object equality
Use == to compare anything

1 == 1.0 // true
List(1,2,3) == List(1,2,3) // true
null == List(1,2,3) // false
List(1,2) == "string" // false

Stuff : Object equality
The equality operator == do the following

1. Check the left side for null
2. If left side is not null, then call equals method
Scala provides a facility for comparing reference equality, as
well, under the name eq. However, eq and its opposite, ne,
only apply to objects that directly map to Java objects.

Stuff : Packages and Imports
• Scala code resides in the Java platform’s global
hierarchy of packages.
• Package definition is at the top of the file.
• The structure is reflected on file system.
package graveyard
class Zombie

• The other way you can place code into packages in Scala is
more like C# namespaces (called packaging)
• Packages can be nested
package graveyard {
package gravestone {
class Zombie
}
}

In Scala, packages and their members can be imported using
import clauses. Imported items can then be accessed by a simple
name like Zombie , as opposed to requiring a qualified name like
graveyard.gravestone.Zombie.
// import Zombie
import graveyard.gravestone.Zombie
// import all undead horde
import graveyard._

• Imports may appear anywhere
• Import may refer to objects (singleton or regular) in addition to packages
• Import let you rename and hide some of the imported members

// please don't do this IRL: aliases must be reasonable!
import graveyard.gravestone.{Zombie => RoseBush}
def hideUndead(): RoseBush = {
import graveyard.gravestone.{Zombie => OldMan}
new OldMan()
}

Stuff : Identifiers
1. CamelCase
2. Alphanumeric for variables – letters + digits
3. Do not use starting $ - it is reserved for compiler variables
4. Do not use _ in names – underscore has other usages

5. Constants has first uppercase letter – math.Pi

Stuff : Method call

Method with 0 parameter can be called without ()
Method with 1 parameter can be called using infix notation
new Test().method
// method call
new Test().function // res1: () => Unit = <function0>
new Test() method1 10 // 13

Stuff : Method call
Use higher-order functions with infix notation, but not with mixed notation

List("abcd","zyxwvu").map (_.toUpperCase).filter (_.length > 5) // bad!

List("abcd","zyxwvu") map (_.toUpperCase) filter (_.length > 5) // good

Stuff : Method calls

Use infix notation only with methods without side effects
(which do not modify internal class state)

List(1,2) mkString "" // ok
new CustomDataset() add "some value" // not ok

Lazy evaluation

An expression that has a value, but that is not
evaluated until it's actually needed (in another words
– until the value is actually read).

Call by-name parameter
Lazy evaluated parameter
def callByValue(x: Int) = {
//parameter get evaluated upon function call
println("x1=" + x)
println("x2=" + x)
}
def callByName(x: => Int) = {
println("x1=" + x) // parameter get evaluated here
println("x2=" + x) // parameter get evaluated again
}

Lazy initialization

When a val is declared with the lazy modifier the righthand side of the value (the definition) will not be
executed until the first time the value is accessed.
val normal = 5 // evaluated immediately
lazy val lzy = 5 // evaluated only upon first read

Stuff : Streams

A stream is like a list except that its elements are
computed lazily. Because of this, a stream can be
infinitely long.

Only those elements requested will be computed.
Otherwise, streams have the same performance
characteristics as lists.

Stuff : Streams

Classical example: Fibonacci sequence
def fibFrom(a: Int, b: Int): Stream[Int] = {
a #:: fibFrom(b, a + b)
}

Tail recursion

Scala compiler can transform a recursive call into a
loop so that it won’t use stack for evaluation!
But it can apply its magic only in case of tail
recursion.

Tail recursion

Tail recursion is simple:
If the last evaluated expression in a function only
makes a recursive call, but does not make
additional computations, than the function is tail
recursive!

Tail recursion : example

def boom(x: Int): Int =
if (x == 0) throw new Exception("boom!")
else boom(x - 1) + 1
@tailrec
def bang(x: Int): Int =
if (x == 0) throw new Exception("bang!")
else bang(x - 1)

Monad
Monad is a name of abstraction.
1.
2.
3.
4.

Chain function calls
“Container” type
Composable with each other
Wrap types, which are stored in the monad, into a
monad type (like zombies – their bites turn
everyone in a zombie)

Monad
You may have been working with monads already!
• Promises are monads!
• jQuery object is a monad!
• Futures (partial monadic API)

Monad
Every monad can:
• Wrap a value into a Monad type
• Unwrap a value from a Monad type
• Work transparently with wrapped value using
Monad interface

Monads like Zombies
Every good zombie can:
• Turn a person into a zombie
• Turn back into a person if cured
• Communicate with other zombies as if those were
people

Monads

Monad is a really tough topic for beginners!
Spend at least a week cranking the theory to create
you own vision of this abstraction!

Option

Scala has a standard type named Option for optional
values.
One can say, that Option is a replacement for Nullobject pattern.
It eliminates the null-checks in the code!
It eliminates Null-Pointer Exceptions!

Option

Option have two forms:
• Some(x)
• None
Scala collections do not return nulls,
they return None!

Option

The most common way to take optional values apart is
through a pattern match.
def show(x: Option[String]) = x match {
case Some(s) => println(s)
case None => println("?")
}

Option
Option is a collection!
Well, it is actually a single-value container 
You can use map, filter, flallen, for-comprehension and
other collection API methods with Option!
http://danielwestheide.com/blog/2012/12/19/the-neophytes-guide-to-scala-part-5-the-option-type.html
http://blog.tmorris.net/scalaoption-cheat-sheet/

Option : Practices
• You should avoid null as much as possible in Scala
• Use Option when you can return something like a “no
value”
• Do not use null-checks in scala, use collection
API/pattern matching over Option

Try

A monad for exception handling.
Its main purpose is to wrap the exception and pass it
around the code (e.g. to another execution context)
Was developed by Twitter team to pass exceptions
across cluster nodes.

Try

Try has two forms:
• Failure[Throwable]
• Success[T]

Try : Example

Try(Integer.parseInt("3")).getOrElse(println("Sorry, I failed"))

Try : Practices
• Use Try rather than a catch block for handling
unexpected failure.
• Use Try when working with Future.
• Exposing Try in a public API has a similiar effect as a
checked exception. Consider using exceptions
instead.

Underscore

Wildcard on steroids!

Underscore usages
Tons of them!
1. “Placeholder syntax”
List(1, 2, 3) map (_ + 2)

2. Partially applied functions
List(1, 2, 3) foreach println _

3. Wildcard pattern
Some(5) match { case Some(_) => println("Yes") }

4. Wildcard imports
import java.util._
http://stackoverflow.com/questions/8000903/what-are-all-the-uses-of-an-underscore-in-scala

Thanks!

val you = new Developer[Any] with ScalaKnowledge

Resources
• Scala Twitter School [link]
• Effective Scala [link]
• Scala for Java Programmers [link]
• The Neophyte’s Guide to Scala [link] [book]
• Functional Programming Principles in Scala @ Coursera [link]
• Programming in Scala [book]
• ScalaUA skype chat [link]

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