Introduction to programming in scala

Introduction to Programming in Scala May 4th, 2017

Hungai Amuhinda @hungai hungaikev hungaikev.in hungaikevin@gmail.com

Agenda ❖ Introduction ❖ Scala Concepts ❖ OOP ❖ FP ❖ Collections

Scala: Scala is a Java like programming language which unifies object - oriented and functional programming.

Scala Concepts ● Scala is an object oriented language in pure form: every value is an object and every operator is a method. ● “ + ”, “ - ”, “ * ”, “ / ” are all methods SCAVANNAH 2017

Scala Concepts ● Everything is an expression ● Expression is an instruction to execute something that will return a value. ● An expression evaluates to a result or results in a value SCAVANNAH 2017

Scala Class Hierarchy scala.Any scala.AnyVal scala.AnyRef scala.Int scala.Byte scala.Boolean scala.Char scala.Stringscala.List scala.Seq scala.Map

Scala Data Types ● Boolean ● Byte ● Short ● Int ● Long SCAVANNAH 2017 ● Float ● Double ● Char ● String

Scala Concepts(Advanced Type System) ● Static ● Inferred (Type Inference) ● Structural ● Strong SCAVANNAH 2017

Program with Scala (REPL) Read Evaluate Print Loop $ scala Demo 03 SCAVANNAH 2017

Program with Scala (REPL) Read Evaluate Print Loop $ scala -Dscala.color Demo 04 SCAVANNAH 2017

Program with Scala (Worksheet) Work with worksheets ● IntelliJ ● Scala IDE or Eclipse with Scala Plugin ● Ensime Demo 04 SCAVANNAH 2017

Program with Scala (Main) object Demo1 { def main (args: Array[String]): Unit = { println(“Hello Everyone”) println(“Welcome to today's event”) } } Demo 01 SCAVANNAH 2017

Program with Scala (App) object Demo2 extends App { val todaysEvent = “Scavannah” lazy val fun = (0 to 4).map(x => “fun”).mkString(“ ”) println(“Hello world”) println(“Welcome to ” + todaysEvent + “! n”) } Demo 02 SCAVANNAH 2017

Scala Concepts SCAVANNAH 2017 ❖ var, val ❖ If expressions ❖ def ❖ Block expressions ❖ While - Loops ❖ For - Loops ❖ Nested Function ❖ Recursion vs Tail recursion ❖ Pattern Matching

Scala Concepts ❖ var - variable Something that is able or likely to change or be changed (Mutable) Demo 05 SCAVANNAH 2017 ❖ val - value A value is an expression that cannot be changed any further (Wiki) (Immutable) It's similar to final in Java

Expression with Semicolon? val x = 5566 val y = 87 val java = “Java” ; val scala = “scala” Demo 06 SCAVANNAH 2017 If you have multiple expressions in one line, you will need semicolons(;). Otherwise you don't need it.

If Expressions ❖ If has return value (expression) ❖ Scala has no ternary operator (? : ) val value = 0 val negative = Demo 07 SCAVANNAH 2017 If (value < 0 ) true else false Everything is an expression

def def max (x: Int, y: Int): Int = { If (x > y) x else y } Demo 08 SCAVANNAH 2017 Function body in Curly braces Equals sign Result type of function Parameters Function name “def” starts a function definition

def Demo 09 SCAVANNAH 2017 def max (x: Int, y: Int): Int = { If (x > y) return x else return y }

def Demo 010 SCAVANNAH 2017 def max (x: Int, y: Int) = { If (x > y) x else y } No Result type

def Demo 011 SCAVANNAH 2017 def max (x: Int, y: Int) = If (x > y) x else y No Curly brackets

Summary of def ❖ You don't need a return statement - Last expression of a block will be the return value ❖ You don't need return type in method or function definition. - Scalac will infer your return type in most cases. Its a good habit to have a return type, especially if your API has a public interface ❖ You don't need curly brackets - If you have multiple lines of code, using curly brackets ({ }) is a good habit SCAVANNAH 2017

Block expressions (Curly brackets) val n = 5 val factorial = { var result = 1 for (i <- 1 to n ) result = result * i result } Demo 012 SCAVANNAH Last expression (result) in block will be the return value, then it will be assign result to “factorial”

While Loop var n = 10 var sum = 0 while (n > 0) { sum = sum + 1 n = n - 1 } Demo 013 SCAVANNAH

For - Loop Demo 014 SCAVANNAH var sum = 0 for (i <- 1 to 10) { sum += 1 } println(sum)

Nested Function def min (x: Int, y: Int): Int = { def max (x: Int, y: Int) = { if (x > y) x else y } if (x >= max(x,y)) y else x } Demo 015 SCAVANNAH

Recursion vs Tail - recursion def factorial(n : Int): BigInt = { If (n == 0) 1 else n * factorial(n - 1) } factorial (15) factorial (150) factorial(15000) // java.lang.StackOverflowError Demo 016 SCAVANNAH

Recursion vs Tail - recursion def factorial(n : Int): BigInt = { def helpFunction(acc : Int, n: Int) : BigInt = { If (n == 0) acc else helpFunction(acc * n , n -1 ) } helpFunction(1,n) } factorial(15000) Demo 017 SCAVANNAH “Tail - Recursion”

Recursion vs Tail - recursion import scala.annotation.tailrec def factorial(n : Int): BigInt = { @tailrec def helpFunction(acc : Int, n: Int) : BigInt = { If (n == 0) acc else helpFunction(acc * n , n -1 ) } helpFunction(1,n) } factorial(15000) Demo 018 SCAVANNAH “You have to add a return type, when the function is recursive” “Add annotation is a good habit. Compiler can check whether or not it can be optimised. ”

Pattern Matching ❖ Pattern matching is a feature that is very common in functional languages ❖ It matches a value against an expression ❖ Each pattern points to an expression ❖ It's similar to “switch case” but its more general. There are some differences: - No fall through - Each condition needs to return a value(Everything in scala is an expression) - It can match anything SCAVANNAH 2017

Pattern Matching def matchString(x : String): String = { x match { case “Dog” => x case “Cat” => “Not a cat person” case _ => “Neither Dog or Cat” } matchString(“Dog”) matchString(“Human”) Demo 019 SCAVANNAH

Scala (Object Oriented) SCAVANNAH 2017 ❖ Classes ❖ Extends , with, override ❖ Abstract classes ❖ Objects, Companion objects ❖ Traits ❖ Case classes

Classes class Employee(id : Int, val name: String, address: String, var salary: Long ) val employee = new Employee(1,”Hungai”,”Kakamega”,40L) Demo 020 SCAVANNAH Primary Constructor val in constructor will give you a getter var in constructor will give you a getter and setter

Extends, with, override. ❖ Scala is single inheritance like Java. ❖ Scala - extends = Java - extends ❖ Scala - with = Java - implements ❖ Scala - override = Java - @Override SCAVANNAH Single inheritance enables a derived class to inherit properties and behaviour from a single parent class

Abstract classes. ❖ Abstract classes are just like normal classes but can have abstract methods and abstract fields ❖ In scala, you don't need the keyword abstract for methods and fields in an abstract class SCAVANNAH

Abstract classes. abstract class Animal (val name: String) { val footNumber: Int val fly : Boolean def speaks : Unit } class Dog(name: String) extends Animal (name) { override val footNumber : Int = 4 override val fly = false override def speak : Unit = println(“Spark”) } class Bird(name : String) extends Animal(name) { override val footNumber : Int = 2 override val fly = true override def speaks: Unit = println(“chatter”) } Demo 021 SCAVANNAH Subclasses should be in the same file.

Objects. ❖ A singleton object is declared using the object keyword. ❖ When a singleton object shares the same name with a class it's referred to as a Companion object. ❖ A companion object and its classes can access each others private methods or fields SCAVANNAH

Singleton Object. object MathUtil { def doubleHalfUp(precision: Int, origin: Double): Double { val tmp = Math.pow(10,precision) Math.round(origin * tmp)/ tmp } } Demo 022 SCAVANNAH

Case Classes. Case classes are just regular classes that are : ➔ Immutable by default ➔ Decomposable through pattern matching ➔ Compared by structural equality instead of by reference. When you declare a case class the scala compiler does the following for you: ➔ Creates a class and its companion object ➔ Implements the ‘apply’ method that you can use a factory. This lets you create instances of the class without the ‘new’ keyword SCAVANNAH

Case classes. abstract class Notification case class Email(sourceEmail: String, title: String, body: String) extends Notification. case class SMS (sourceNumber: String, message: String) extends Notification. case class VoiceRecording(contactName: String, link: String) extends Notification. val emailNotification = Email(“h@hungaikev.in”,”Scavannah”,”Todays lesson”) println(emailNotification) Demo 023 SCAVANNAH

The Lambda Calculus. Demo 012 SCAVANNAH Alonzo Church 1930 ❖ Theoretical foundation ❖ Pure functions - no state ❖ Not a programming language

Lambda Calculus. ƛx . x + 1 SCAVANNAH ExpressionsVariable Function Application

Lambda Calculus. ƛx . x + 1 // Scala Translation { x : Int => x + 1 } Demo 024 SCAVANNAH

Scala (Functional) SCAVANNAH 2017 ❖ Functional Concepts ❖ First class functions ❖ Anonymous functions ❖ Higher order functions

Functional Concepts. ❖ Immutability (Referential Transparency - Expressions always evaluates to the same result in any context) - No side effects (Modifies state, Not predictable) ❖ Functions as values - Functions as objects ❖ Higher order functions - Input: Takes one or more functions as parameters, Output: Returns a function as result Demo 012 SCAVANNAH

Anonymous functions (Lambdas). Demo 012 SCAVANNAH

Anonymous functions. ((x : Int ) => x * x) (0 until 10).map ((value: Int) => value * value) (0 until 10).map (value => value * value ) (0 until 10).map (value => value + 1) (0 until 10).map (_ + 1) Demo 025 SCAVANNAH

High-order Functions. def calculateTax(rate: BigDecimal => BigDecimal, salary: BigDecimal) : BigDecimal = { rate(salary) } val kenyaTax = (salary: BigDecimal) => { if (salary > 413201) salary * 0.396 else salary * 0.3 } val tzTax: BigDecimal => BigDecimal = _ * 0.25 calculateTax(kenyaTax,413201) calculateTax(tzTax,100) Demo 026 SCAVANNAH

High-order Functions. def calculateTax(rate: BigDecimal => BigDecimal, salary: BigDecimal) : BigDecimal = { rate(salary) } def kenyaTax (salary: BigDecimal) = { calculateTax(x => if (salary > 413201) salary * 0.396 else salary * 0.3, salary ) } def tzTax(salary: BigDecimal ) = calculateTax( _ * 0.25, salary) kenyaTax(413202) tzTax(100) Demo 027 SCAVANNAH

High-order Functions. def calculateTax(rate: BigDecimal => BigDecimal) : (BigDecimal ) => BigDecimal = { rate } def kenyaTax = calculateTax(x => if (x > 413201) x * 0.396 else x * 0.3 ) def tzTax = calculateTax( x => x * 0.25) kenyaTax(413202) tzTax(100) calculateTax(kenyaTax)(413202) calculateTax(tzTax)(100) Demo 028 SCAVANNAH

High-order Functions - Curry. def calculateTax(rate: BigDecimal => BigDecimal) (salary : BigDecimal ) : BigDecimal = { rate (salary) } def kenyaTax(salary : BigDecimal) = calculateTax(x => if (x > 413201) x * 0.396 else x * 0.3 )(salary) def tzTax(salary : BigDecimal) = calculateTax( x => x * 0.25)(salary) kenyaTax(413202) tzTax(100) Demo 029 SCAVANNAH

Collections ❖ SCAVANNAH ❖ Concept of Collections ❖ Hierarchy of Collections - Immutable - Mutable ❖ Immutable List

Collections. ❖ Scala supports mutable collections and immutable collections. ❖ A mutable collection can be updated or extended in place. This means you can change, add or remove elements as a side effect. ❖ Immutable collections never change. You have still operations that stimulate additions, removals, updates by creating a new collection and leave the old unchanged. SCAVANNAH

Collections Hierarchy SCAVANNAH

Hierarchy of Immutable Collections SCAVANNAH

Hierarchy of mutable Collections SCAVANNAH

Immutable List // Construct a List val list1 = List(1,2,3) val list2 = 1 :: 2 :: 3 :: 4 :: 5 :: Nil val list3 = List(“apples”, “oranges”,”bananas”) val list4 = List(5,5,6,6) ::: List(8,7) Demo030 SCAVANNAH Pronounced “cons”

Immutable List (API) val list = List(4,3,0,1,2) ➔ list.head ➔ list.tail ➔ list.length ➔ list.max ➔ list.min ➔ list.sum ➔ list.contains(2) ➔ list.drop ➔ list.reverse Demo031 SCAVANNAH ➔ list.sortWith(_ > _) ➔ list.map(x => x * 2) ➔ list.map(_ * 2) ➔ list.reduce((x,y) => x + y) ➔ list.filter(_ % 2 == 0) ➔ list.groupBy(x => x % 2 == 0)

Summary of Scala. ❖ Keep it simple ❖ Don’t pack too much in one expression ❖ Find meaningful names ❖ Prefer functional ❖ Careful with mutable objects SCAVANNAH

Further Reading $ References. ❖ Scala Official Docs ❖ Cousera - Martin Odesky ❖ Creative Scala ❖ Scala School by Twitter ❖ Scala 101 by Lightbend SCAVANNAH

Introduction to programming in scala

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