Scala tips

Scala Option and Some

The absence of a value during rurtime should be somehow handled otherwise the will be a runtime error. The absent value is a null value. If we have a value of type D that may be absent (for example in method, function, class constructor), Scala uses an instance (object) of the (abstract) classOption[D] as its container. The class Option has two sub classes Some and None. Therefore, an instance of Option is an instance of either Some or None. For a variable x of type Option[D], the factory (of Option[D]) creates Some(x) if x != null or None if x == null .

The following are examples of situations to use Option:

1- Creating a type-Option val:

val optionalIntValue: Option[Int] = Some(1)
val optionalValue = Some("abc") // Scala infers the type
// or
// val optionalIntValue: Option[Int] = None  // type is needed

2- When a function may or may not return a value; we want to give it the option to return a value or not:

def myFunc: Option[String] = { ... }

3- When a parameter of a function can be present or absent, i.e being optional to be given:

def myFunc(param1: Option[D]): ...

case class MyClass(param1: Option[D], param2: String)

Keeping in mind that Option is just a type as a container for a value that can be absent, there are several ways to use an Option-type variable. Examples:

1- The most general way is to use an Option with a match. We can define the output of a method as an Option and handle the output. For example, a function that reads user input. User may just press Enter/Cancel and input nothing.

def readUserInput(): Option[processedUserInputType] = { ... }
readUserInput() match {
      case Some(outputOftheFunction) => // Do something with outputOftheFunction
      case None =>    // Do something when there is no output
    } 

This example is to show how to make a parameter of a function optional for the user. If not provided, it will be considered as a predefined value None.

def show(x: Option[String], y: Option[Int] = None) = {
  x match {
    case Some(s) => println(s)
    case None => println ("?")
  }
  y match {
    case Some(number) => println( s"the number is ${number}")
    case None => println("no number")
  }
}

val myInput1 = Some("hello bear")
show(myInput1) //  hello bear    no number
show(None) // ?       no number
show(None, y=Some(123)) // ?       the number is 123

2- Extracting the value of an Option using getOrElse method. A default value for the None case is mandatory.

val someValue1 =  Some(123)
val someValue2 : Option[Int] = None

someValue1.getOrElse(100) // 123
someValue2.getOrElse(100) // 100