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Scala tutorial

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Introduction

Scala is both an object-oriented and functional programming language. It is a statically-typed language, but uses type inference so the user is not required to annotate code with redundant type information.

Online compiler: Scala Fiddle

Basics

Values vs. Variables

  • val is a constant
  • var is a variable
val x = 2;
x = 3; // does not compile
var x = 2;
x = 3; // compiles

Declaring type of variable

Types of values can be inferred, but you can also explicitly state the type:

val x: Int = 2

Functions

(x: Int) => x + 1                   // anonymous (no-name) function
val getTheAnswer = () => 42         // named function (0 parameters)
val addOne = (x: Int) => x + 1      // named function (1 parameter)
val add = (x: Int, y: Int) => x + y // named function (2 parameters)

Blocks & Returning values

A block evaluates to the last line in the block.

println({
  val x = 1 + 1
  x + 1
}) // 3

For methods, the return statement is not needed, since the value of the last line is returned.

Methods

0 parameters

def name: String = System.getProperty("user.name")

println("Hello, " + name + "!")

1 parameter list

def add(x: Int, y: Int): Int = x + y

println(add(1, 2)) // 3

2 parameter lists

def addThenMultiply(x: Int, y: Int)(multiplier: Int): Int = (x + y) * multiplier

println(addThenMultiply(1, 2)(3)) // 9

Classes

Unit is like void in Java

class Greeter(prefix: String, suffix: String) {
  def greet(name: String): Unit =
    println(prefix + name + suffix)
}
val greeter = new Greeter("Hello, ", "!")
greeter.greet("Scala developer") // Hello, Scala developer!

Case Classes

case classes are immutable and compared by value

case class Point(x: Int, y: Int)

The new keyword is not necessary when creating a Case Class

val point = Point(1, 2)
val anotherPoint = Point(1, 2)
if (point == anotherPoint) {
  println("same point") // this will print
}

Objects

The object keyword is used to create a singleton. This feature replaces the static keyword in Java.

object IdFactory {
  private var counter = 0
  def create(): Int = {
    counter += 1
    counter
  }
}
val newId: Int = IdFactory.create()
println(newId) // 1
val newerId: Int = IdFactory.create()
println(newerId) // 2

Traits

Traits are like interfaces in Java

trait Greeter {
  def greet(name: String): Unit
}

Traits can have default implementations:

trait Greeter {
  def greet(name: String): Unit =
    println("Hello, " + name + "!")
}
  • Extend traits using extends keyword
  • Override an implementationusing override keyword.
class DefaultGreeter extends Greeter

class CustomizableGreeter(prefix: String, postfix: String) extends Greeter {
  override def greet(name: String): Unit = {
    println(prefix + name + postfix)
  }
}

val greeter = new DefaultGreeter()
greeter.greet("Scala developer") // Hello, Scala developer!

val customGreeter = new CustomizableGreeter("How are you, ", "?")
customGreeter.greet("Scala developer") // How are you, Scala developer?

Main method

Syntax for Scala's main method:

object Main {
  def main(args: Array[String]): Unit =
    println("Hello, Scala developer!")
}

Unified Types

Type Hierarchy

Classes

Private Members and Getter/Setter Syntax

Setters use special syntax: _=

class Point {
  private var _x = 0
  private val bound = 100

  // Getter
  def x = _x

  // Setter
  def x_= (newValue: Int): Unit = {
    if (newValue < bound) _x = newValue else printWarning
  }

  private def printWarning = println("WARNING: Out of bounds")
}

val point1 = new Point
point1.x = 99
point1.y = 101 // prints the warning

Alternative explanation on getters/setters

Tuples

Accessing the elements

val ingredient = ("Sugar" , 25)

println(ingredient._1) // Sugar
println(ingredient._2) // 25

Pattern matching on tuples

Grabbing values out of the tuple:

val (name, quantity) = ingredient
println(name) // Sugar
println(quantity) // 25

Pattern matching using foreach and case:

val planets = List(("Mercury", 57.9), ("Venus", 108.2), ("Earth", 149.6))

planets.foreach{
  case ("Earth", distance) =>
    println(s"Our planet is $distance million kilometers from the sun")
  case _ =>
}

Pattern matchin in for loops:

val numPairs = List((2, 5), (3, -7))
for ((a, b) <- numPairs) {
  println(a * b)
}

Tuples and case classes

Variables in tuples don't have names. If you want them to have names, use a "case class" instead.

Example: case class Planet(name: String, distance: Double)

Class composition with Mixins

This is how you can extend a subclass, and also have a "trait":

abstract class A {
  val message: String
}
class B extends A {
  val message = "I'm an instance of class B"
}
trait C extends A {
  def loudMessage = message.toUpperCase()
}
class D extends B with C

val d = new D
println(d.message)  // I'm an instance of class B
println(d.loudMessage)  // I'M AN INSTANCE OF CLASS B

Higher Order Functions

Higher order functions are functions that taking another function as a parameter.

val salaries = Seq(2, 7, 4)
val newSalaries = salaries.map(x => x * 2) // List(4, 14, 8)

You an alternatively write the 2nd line as:

val newSalaries = salaries.map(_ * 2)

Nested Methods

You can nest methods in Scala.

Multiple Parameter Lists (Currying)

Main use cases:

  1. Passing a value to the 1st parameter list helps the 2nd parameter list infer the type
  2. Partial application

Here is an example of "partial application" that lets us define B as List[Int]:

def foldLeft[B](z: B)(op: (B, A) => B): B
val numbers = List(1, 2, 3, 4, 5)
val numberFunc = numbers.foldLeft(List[Int]()) _

val squares = numberFunc((xs, x) => xs :+ x*x)
print(squares) // List(1, 4, 9, 16, 25)

val cubes = numberFunc((xs, x) => xs :+ x*x*x)
print(cubes)  // List(1, 8, 27, 64, 125)

Pattern matching

"Switch statements" from Java are called "match expressions" in Scala.

Matching on case classes

abstract class Notification

case class Email(sender: String, title: String, body: String) extends Notification

case class SMS(caller: String, message: String) extends Notification
def showNotification(notification: Notification): String = {
  notification match {
    case Email(sender, title, _) =>
      s"You got an email from $sender with title: $title"
    case SMS(number, message) =>
      s"You got an SMS from $number! Message: $message"
  }
}

val someSms = SMS("12345", "Are you there?")
println(showNotification(someSms))  // prints You got an SMS from 12345! Message: Are you there?

Pattern guards

Pattern guards are boolean expressions that make cases more specific:

def showImportantNotification(notification: Notification, importantPeopleInfo: Seq[String]): String = {
  notification match {
    case Email(sender, _, _) if importantPeopleInfo.contains(sender) =>
      "You got an email from special someone!"
    case SMS(number, _) if importantPeopleInfo.contains(number) =>
      "You got an SMS from special someone!"
    case other =>
      showNotification(other) // nothing special, delegate to our original showNotification function
  }
}

Other functionality

  • matching on type - You can also match on "type" of the object
  • sealed - Traits and classes can be marked sealed which means all subtypes must be declared in the same file. This assures all subtypes are known. This is useful for pattern matching because we don't need a "catch all" case.

Singleton Objects

Companion objects

Scala uses companion objects instead of Java's static keyword.

An object with the same name as a class is called a companion object. Conversely, the class is the object’s companion class. A companion class or object can access the private members of its companion.

Use a companion object for methods and values which are not specific to instances of the companion class.

import scala.math._

case class Circle(radius: Double) {
  import Circle._
  def area: Double = calculateArea(radius)
}

object Circle {
  private def calculateArea(radius: Double): Double = Pi * pow(radius, 2.0)
}

val circle1 = Circle(5.0)

circle1.area

Regular Expression Patterns

Any string can be converted to a regular expression using the .r method.

import scala.util.matching.Regex

val numberPattern: Regex = "[0-9]".r

numberPattern.findFirstMatchIn("awesomepassword") match {
  case Some(_) => println("Password OK")
  case None => println("Password must contain a number")
}

For Comprehensions

for each loop example:

case class User(name: String, age: Int)

val userBase = List(User("Travis", 28),
  User("Kelly", 33),
  User("Jennifer", 44),
  User("Dennis", 23))

val twentySomethings = for (user <- userBase if (user.age >=20 && user.age < 30))
  yield user.name  // i.e. add this to a list

twentySomethings.foreach(name => println(name))  // prints Travis Dennis

for each loop example with 2 iterators:

def foo(n: Int, v: Int) =
   for (i <- 0 until n;
        j <- 0 until n if i + j == v)
   println(s"($i, $j)")

foo(10, 10) // prints (1, 9) (2, 8) (3, 7) (4, 6) (5, 5) (6, 4) (7, 3) (8, 2) (9, 1)

Generic Classes

A generic Stack:

class Stack[A] {
  private var elements: List[A] = Nil
  def push(x: A) { elements = x :: elements }
  def peek: A = elements.head
  def pop(): A = {
    val currentTop = peek
    elements = elements.tail
    currentTop
  }
}
val stack = new Stack[Int]
stack.push(1)
stack.push(2)
println(stack.pop)  // prints 2
println(stack.pop)  // prints 1

Variances

class Foo[+A] // Covariant class
class Bar[-A] // Contravariant class
class Baz[A]  // Invariant class

Covariance

For some class List[+A], making A covariant implies that for two types A and B where A is a subtype of B, then List[A] is a subtype of List[B]

  • Given
    • A is Animal
    • B is Cat
    • Animal is a subtype of Cat
  • Then
    • List[Animal] is a subtype of List[Cat]

Scala's List class is sealed abstract class List[+A], where the type parameter A is covariant

Contravariance

Opposite of Covariance.

Let's say we literally had a printer that prints Animals. Then it should be able to print Cats as well. Covariance can help us model this scenario.

  • Given
    • A is Animal
    • B is Cat
    • Animal is a subtype of Cat
    • abstract class Printer[-A]
    • AnimalPrinter extends Printer[Animal]
    • CatPrinter extends Printer[Cat]
    • def printMyCat(printer: Printer[Cat])
  • Then
    • Printer[Animal] can be passed into printMyCat

Invariance

Generic classes in Scala are invariant by default. This means that they are neither covariant nor contravariant.

Example: If we make a custom Container class, then Container[Cat] is not a Container[Animal]. The reverse is not true either.

Upper Type Bounds

An upper type bound T <: A declares that type variable T refers to a subtype of type A

  • Given
    • Cat extends Pet
    • Lion extends Animal
    • class PetContainer[P <: Pet](p: P) { def pet: P = p }
  • Then
    • new PetContainer[Cat](new Cat) compiles
    • new PetContainer[Lion](new Lion) fails to compile

Common Pitfall: "Variances" and "Upper Type Bounds" are 2 different concepts. Notice you cannot replace P <: Pet with +P since then new PetContainer[Lion](new Lion) would incorrectly succeed.

Lower Type Bounds

Opposite of Upper Type Bounds.

Common pitfall: "functions are contravariant in their parameter types and covariant in their result types". When making a type covariant by using +B, we will run into a problem when using B as a "parameter type" to a function. This is solved by using U >: B instead. See Lower Type Bounds for a full example.

Inner Classes

Scala allows classes to have other classes as members.

Compound Types

If you have 2 traits: Cloneable and Resetable, then the syntax for a function to take an object with those 2 traits is:

def cloneAndReset(obj: Cloneable with Resetable): Cloneable = {
  //...
}

If it was 3 or more traits, the syntax would be Trait1 with Trait2 with Trait3

Self-type

Unlikely I'll use this often. Self-types are used when 1 trait depends on another trait, but doesn't "extend" it. See Self-type for an example.

Implicit Parameters

This is when we mark a variable implicit in it's declaration, and implicit where it's used in a function, and the compiler will try to match the 2 together.

  • Given:
    • value: implicit val stringMonoid: Monoid[String]
    • value: implicit val intMonoid: Monoid[Int]
    • method definition: def sum[A](xs: List[A])(implicit m: Monoid[A]): A
  • Then
    • m will be matched with intMonoid if we call sum(List(1, 2, 3))

See Implicit Parameters for the full example.

Polymorphic Methods

Omitting the Type

The compiler knows businessName is a String:

val businessName = "Montreux Jazz Café"

The compiler knows an Int will be returned:

def squareOf(x: Int) = x * x

For recursive functions, the compiler can't know the return type. The following code will fail:

def fac(n: Int) = if (n == 0) 1 else n * fac(n - 1)

Compiler can also infer the types:

case class MyPair[A, B](x: A, y: B)
val p = MyPair(1, "scala") // type: MyPair[Int, String]

def id[T](x: T) = x
val q = id(1)              // type: Int

Operators

It's easy to "overload" an operator:

case class Vec(x: Double, y: Double) {
  def +(that: Vec) = Vec(this.x + that.x, this.y + that.y)
}

val vector1 = Vec(1.0, 1.0)
val vector2 = Vec(2.0, 2.0)

val vector3 = vector1 + vector2
vector3.x  // 3.0
vector3.y  // 3.0

By-name Parameters

By-name parameters are only evaluated when used. They are in contrast to by-value parameters. To make a parameter called by-name, prepend => to its type:

def calculate(input: => Int) = input * 37

By-name parameters have the advantage that they are not evaluated if they aren’t used in the function body. On the other hand, by-value parameters have the advantage that they are evaluated only once.

This ability to delay evaluation of a parameter until it is used can help performance if the parameter is computationally intensive to evaluate.

Default Parameter Values

class Point(val x: Double = 0, val y: Double = 0)

val point0 = new Point(1);    // point (1, 0)
val point1 = new Point(y = 3) // point (0, 3)

For point1, we use y=3 (a named argument) since "if the caller omits an argument, any following arguments must be named."

Packages and Imports

Imports

import users._  // import everything from the users package
import users.User  // import the class User
import users.{User, UserPreferences}  // Only imports selected members
import users.{UserPreferences => UPrefs}  // import and rename for convenience

More notes

Class constructor: private, public, read-only, mutable variables:

class Ok[T](statusCode: Int, result: T) // private fields, but present on the constructor

class Ok[T](val statusCode: Int, val result: T) // public, read-only fields

class Ok[T](var statusCode: Int, var result: T) // public, mutable fields

On a case class, "when you use the case keyword, you do not need to use val to make a field public and read-only":

case class Ok[T](statusCode: Int, result: T)

Null, null, Nil, Nothing, None, and Unit in Scala

  • Null – it's a Trait.
  • null – it's an instance of Null - similar to Java null.
  • Nil – represents an empty List of anything of zero length.
  • Nothing - it's a Trait. Its a subtype of everything, but not superclass of anything. There are no instances of Nothing.
  • None – used with Option which has exactly 2 subclasses: Some and None. None is used to represent a sensible return value to avoid null pointer exceptions.
  • Unit – type used in method that doesn’t return a value.

References

Notes are summarized from Tour of Scala. All sections other than "Implicit Conversions" were well written.

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