2. Week 02: Scala Primer

This lecture is an introduction to Scala programming language. It is intentionally brief, as all the YSC4231 attendies are assumed to have familiarity with either OCaml or Java (or both). Therefore, this tutorial provides a quick overview of Scala’s object-oriented and functional features

The accompanying code for this lecture is available in the branch 01-intro (under the folder src/main/scala/intro) of the course repository.

If unsure about some of the feature, you are encouraged to take a look at one of multiple Scala tutorials available online. The following are the recommended ones:

2.1. Using IntelliJ IDEA

We will be using IntelliJ IDEA as an Integrated Development Environment (IDE) for programming in Scala. It will allow us to manage large multi-file projects with multiple executables and automated tests. The following link provides instructions on getting IntelliJ and creating a simple Scala project in it:

You can also open the course repository from IntelliJ via File -> Open dialogue.

IntelliJ IDEA provides a convenient way to build projects, navigate to files and definitions, and refactor them consistently. You are encouraged to this tutorial to master the following aspects of using IntelliJ for managing Scala projects:

  • Creating new files

  • Using code completion

  • Building and rebuilding the project

  • Navigating to definitions and files (see the menu Navigate)

  • Creating runtim configurations

Use this key binding reference for your convenience. Alternatively, just remember how to invoke the Find Action dialogue (Ctrl + Shift + A or Command + Shit + A, depending on the system) and type in your query to get a hint.

The following simple executable is implemented in the file SquareOf5.scala (use IntelliJ navigation to locate it!):

object SquareOf5 extends App {
  def square(x: Int) = x * x
  val s = square(5)
  println(s"Result $s")
}

To run it in the IDE, right-click on SquareOf5 in the editor (or on the file in the navigator tree pane on the left of the editor) and choose Run ‘SquareOf5’. Alternatively, you can click on the green triangle next to the object definition. The result of executing the program will appear in the execution pane at the bottom of the screen.

An alternative way to define an executable object is via the main method. For instance, it can be done as follows:

object SquareOf6 {
  def main(args: Array[String]): Unit = {
    val s = SquareOf5.square(6)
    println(s"Result $s")
  }
}

Here, the executable object SquareOf6 uses the function square defined within the object SquareOf5.

2.2. Using SBT

Scala Build Tool is a build system used to manage dependencies and compile Scala projects, similar to OCaml’s dune and opam. The tutorial on obtaining and running sbt is available:

You can try executing sbt test from the root of the primer project. This will compile it and automatically run all the tests in the project.

Finally, to run a Scala console similar to OCaml utop, start sbt and then type console in it. You can now import the code from the packages and objects of your project, as well as create new class instances.

2.3. Object-Oriented Elements of Scala

Scala inherits many object-oriented features from languages such as C++ and Java. That said, it’s object-oriented model can still be understand in terms of OCaml abstractions. Definitions of functions and data types in Scala can be placed in either of the module-like environments:

Objects are similar to OCaml’s first-order modules. They may contain data type definitions, functions and mutable references. Each object is unique, hence all values in it are global. Examples of objects are shown above. Objects are the “final” units of any Scala project: all executable code is run starting from some object’s main method (or is an object is defined as extends App) as demonstrated above.

Classes are “factories” of objects. Similarly, they may contain definitions of functions (called methods in Scala), as well as data types. However, each class can be used to create multiple instances, each having its own state (including mutable variables, called fields). For instance, the following class features one mutable field greeting and three methods:

class Printer(val initial: String) {
  private var greeting = initial

  def printMessage(): Unit = println(greeting + "!")

  def printNumber(x: Int): Unit = {
    println("Number: " + x)
  }

  def setGreeting(s: String) = {
    greeting = s
  }
}

It takes a value of type String, which can be then used within the body of the class. For instance, it is used to initialise a mutable field greeting. It can be later changed via a method setGreeting by passing a new String value. The class can be then instantiated and used as follows:

object UsePrinter {
  def main(args: Array[String]): Unit = {
    val printer = new Printer("Hello")
    printer.printMessage()
    printer.printNumber(42)
  }
}

Traits in Scala are used as “templates” containing some data types, fields, and methods, which can be used to “mix-in” to classes and objects that are create later. Traits cannot be instantiated or executed: they should be considered as “dictionaries” of definitions to be used by classes and objects. The following example shows a trait, a class that extends it, and an object that uses the resulting class:

trait Logging {
  // Signature of a method to be defined later
  def log(s: String): Unit

  def warn(s: String) = log("WARN: " + s)

  def error(s: String) = log("ERROR: " + s)
}


// A class that mixes in Logging's functionality
class PrintLogging extends Logging {
  def log(s: String) = println(s)
}

object UseLogging extends App {
  val logger = new PrintLogging
  logger.warn("Hmm...")
}

One class and object can extend multiple traits. In addition, an
object and a class can extend precisely one class.

More details on object-oriented model of Scala can be found in the following tutorials:

Classes, objects and traits in Scala are located under a source root in a hierarchy of folders that correspond to packages. Packages are declared at the beginning of each file and should mimic the folder hierarchy, for instance:

package runners.squares

is a package ascribed to SquareOf5 and SquareOf6 objects that are located in a folder runners/squares under the source root of a project.

Classes and traits in scala can be polymorphic, i.e., taking type parameters. For instance, the following class corresponds to OCaml’s polymorphic pair data type with immutable fields first and second or arbitrary types P and Q:

class Pair[P, Q](val first: P, val second: Q)

object UsePair extends App {
  val p = new Pair("abc", 42)
  println(p.first)
  println(p.second)
}

2.4. Functional Elements of Scala

Scala allows to define classes in a way that they could be used in a pattern matching, similar to OCaml’s data types. For this, they need to be defined using the keyword case. The typical pattern is to define a trait that corresponds to the “abstract data type” and a number of case classes that extend it. For instance, this is how the familiar option type is implemented in Scala (ignore the + in front of the type parameter for now):

trait Option[+T]

case class Some[T](elem: T) extends Option[T]

case object None extends Option[Nothing]

Notice that None is defined as an object, since it doesn’t take any parameters, and hence should only exist as a single instance. This is how case classes are used in pattern matching:

object PrintOption {

  def printOption(o: Option[String]): Unit = {
    o match {
      case Some(e) =>
        println("Some: " + e)
      case None =>
        println("Nothing!")
    }
  }

  def main(args: Array[String]): Unit = {
    val s = Some("Hello")
    printOption(s)
  }
}

Scala has support for first-class functions-as-values. The following code snippet declares a lambda-expressions for multiplying a number by two:

val twice: Int => Int = (x: Int) => x * 2

Notice that it is ascribed the type Int => Int. However, the compiler can infer this type automatically cased on the argument type of x, hence we can shorten the definition and write in one of the two following ways:

val twice = (x: Int) => x * 2

or:

val twice: Int => Int = x => x * 2

First-class functions allow manipulating blocks of code as if they were first-class values. For instance, the following example uses lazy (by-name) parameter to declare a run-twice method, which runs the specified block of code body twice:

def runTwice(body: =>Unit): Unit = {
  body
  body
}

// This will print Hello twice.
runTwice ({
  println("Hello!")
})

Scala provides for-comprehensions as a convenient way to travers and transform collections. For instance, the following loop prints all elements of a list ls:

val ls = List(1, 2, 3, 4)
for (e <- ls) {
  println(e)
}

The standard functions on collections, such as map, foldLeft, filter etc. are all available as methods of the corresponding data types. For instance, the following code prints the list that is obtained by multiplying all elements of ls by 3:

println(ls.map(x => x * 3))

Notice that the parameter type of x of the function passed to map is inferred from the type of map itself and does not have to be declared explicitly.

Scala features both immutable (purely functional) and mutable collections. All collections mix-in the functionality from the Seq[T] trait of generic collections of elements of type T. Concrete collections can be created and manipulated as follows:

val messages : Seq[String] = Seq("Hello", "World", "!")
val messageMap : Map[Int, String] = Map(1 -> "Hello", 2 -> "World", 3 -> "!")

// Convert all elements of `messages` to strings (which they already are),
// concatenate with the " " as a separator and print the result:
println(messages.mkString(" "))

//Create a new map by adding a key-value pair (4 -> "Yay") to messageMap
println(messageMap + (4 -> "Yay"))

We recommend to use IntelliJ auto-completion to discover available methods for different data types. The following tutorial provides a great overview of Scala collections:

2.5. Imperative elements of Scala

Scala allows one to use arrays in the same way one uses them in OCaml and Java. The code below shows some examples of creating using arrays:

object ArrayExample {

  var index = 0
  val nextIndex = () => {
    val tmp = index
    index = index + 1
    tmp
  }

  def main(args: Array[String]): Unit = {

    // Create an array of booleans of size 5
    val arr1 = new Array[Boolean](5)

    // Print all elements of the array
    for (i <- 0 to arr1.length - 1) {
      println(arr1(i))
    }

    // Create an array of size 5 filled by repeating the computation
    // passed as a second parameter
    val arr2 = Array.fill(5)({ nextIndex() * nextIndex() })

    // Print all elements of this array
    for (i <- arr2.indices) {
      println(arr2(i))
    }
  }
}

The following reference provides a brief guide on using arrays in conjunction with other Scala collections:

2.6. Working with Threads in Scala

Threads in Scala can be created as instances of a class that extends Thread, overriding its method run. Consider the following object that features a mutable list field and three methods to manipulate with it:

object ConcurrentManipulation {

  private var workingList: List[Int] = Nil

The first method is used to add an element to a list in a mutually-exclusive fashion:

def addToList(i: Int) = {
  this.synchronized {
    workingList = i :: workingList
  }
}

The two other methods are removing elements from the list. The first one does it without synchronisation, and the second one enforces mutual exclusion:

def removeFromListWithoutSync = {
  // Notice how synchornisation is missing
  // This can cause troubles: which?
  workingList match {
    case Nil => None
    case ::(head, tl) =>
      // Wait for some time
      Thread.sleep(10)
      workingList = tl
      Some(head)
  }
}

def removeFromList = this.synchronized{
  workingList match {
    case Nil => None
    case ::(head, tl) =>
      // Wait for some time
      Thread.sleep(10)
      workingList = tl
      Some(head)
  }
}

Notice that in removeFromListWithoutSync, to make things worse, we added a delay of the form Thread.sleep(10) that makes a thread executing this method to stop for 10 milliseconds.

We can now create two threads for adding and removing elements of workingList:

class Adder(start: Int) extends Thread {
  override def run() = {
    // Get my thread id
    val id = Thread.currentThread().getId
    val end = start + 9
    for (i <- start to end) {
      addToList(i)
      println(s"Thread $id: Added $i")
    }
  }
}

class Remover extends Thread {
  override def run() = {
    // Get my thread id
    val id = Thread.currentThread().getId
    var result: Option[Int] = None
    do {
      result = removeFromList
      result match {
        case Some(value) =>
          println(s"Thread $id: Removed $value")
        case None => // Do nothing
      }
    } while (result.isDefined) // Loop while the list is not depleted
  }
}

Time to put this all together. The following method first creates two threads to add elements to the list and then starts them, so they could run concurrently (via the start() method). It then waits for them to finish their execution (via the join() method) and starts two more threads to remove the elements:

def main(args: Array[String]): Unit = {
  // Create two threads without executing them
  val adder1 = new Adder(1)
  val adder2 = new Adder(11)

  // Start two threads in parallel with this one
  adder1.start()
  adder2.start()

  // Wait in this thread while those two will finish
  adder1.join()
  adder2.join()

  println()

  // Make two new threads
  val remover1 = new Remover
  val remover2 = new Remover

  // Start two threads in parallel with this one
  remover1.start()
  remover2.start()

  // Wait in this thread while those two will finish
  remover1.join()
  remover2.join()
}

Let us run an application to see the result of its concurrent execution, which should be similar to the following:

Thread 11: Added 11
Thread 10: Added 1
Thread 11: Added 12
Thread 10: Added 2
Thread 11: Added 13
Thread 11: Added 14
Thread 11: Added 15
Thread 11: Added 16
Thread 10: Added 3
Thread 11: Added 17
Thread 10: Added 4
Thread 11: Added 18
Thread 11: Added 19
Thread 11: Added 20
Thread 10: Added 5
Thread 10: Added 6
Thread 10: Added 7
Thread 10: Added 8
Thread 10: Added 9
Thread 10: Added 10

Thread 12: Removed 10
Thread 13: Removed 9
Thread 12: Removed 8
Thread 12: Removed 7
Thread 13: Removed 6
Thread 13: Removed 20
Thread 12: Removed 19
Thread 13: Removed 5
Thread 12: Removed 18
Thread 13: Removed 4
Thread 12: Removed 17
Thread 13: Removed 16
Thread 12: Removed 15
Thread 13: Removed 14
Thread 12: Removed 3
Thread 13: Removed 13
Thread 12: Removed 2
Thread 13: Removed 12
Thread 13: Removed 1
Thread 12: Removed 11

As we can see the threads are fetching the elements in the interleaved order, but the set of removed elements is the same as the set of added ones.

One, let’s try to use removeFromListWithoutSync instead of removeFromList in the implementation of the Remover thread. Can you explain the result? Can you fix removeFromListWithoutSync by adding the necessary synchronisation to it to avoid this problem?

As the last not, please, pay attention to the difference between run and start methods of thread instances. The first one can only be used to to define the things the thread must do (you must not call it), while the second is used to begin the execution of a thread concurrently with the current one.

2.7. Writing Automated Tests in Scala

Scala provides many frameworks for implementing automated tests. We will use ScalaTest. The tests located under the test source root of the project and adhering to ScalaTest’s conventions will be automatically executed by SBT and IntelliJ.

Feel free to copy the following template for your tests from the Scala primer project:

import org.scalatest.{FunSpec, Matchers}

// The test needs to extend those two traits in order to
// get access to the functions used below
class BasicTest extends FunSpec with Matchers {

  // Describe a set of tests for some class of cases
  describe("A simple test") {

    // Describe an individual test
    it("should always succeed") {
      // Write your code here
      // Use assert statements to make the test pass or fail
      assert(2 * 2 == 4)
    }
  }

  // Another set of tests
  describe("A square function") {
    it ("should work correctly") {
      // Import all from object SquareOf5
      import primer.runners.squares.SquareOf5._
      assert(square(10) == 100)
    }
  }
}

You can run an individual test suite (a class) in IntelliJ by right-clicking on its name and choosing Run .... The same can be done and a finer level of granularity by clicking on the individual describe-component of a test suite. Finally, you can run all tests in the project by right-clicking on the test source folder in the project view on the left of the screen and choosing Run 'ScalaTest in scala...'.