This chapter is from the book
This chapter is from the book
This chapter is from the book
4.7 The Option Type
The Option class in the standard library expresses values that might or might not be present. The subclass Some wraps a value. The object None indicates that there is no value.
var friend: Option[String] = Some("Fred")
friend = None // No friend
This is less ambiguous than using an empty string and safer than using null for a missing value.
Option is a generic type. For example, Some("Fred") is an Option[String].
The get method of the Map class returns an Option. If there is no value for a given key, get returns None. Otherwise, it wraps the value inside Some.
val scores = Map("Alice" -> 10, "Bob" -> 7, "Cindy" -> 8)
val alicesScore = scores.get("Alice") // Some(10)
val dansScore = scores.get("Dan") // None
To find out what is inside an Option instance, you can use the isEmpty and get methods:
if alicesScore.isEmpty
then println("No score")
else println(alicesScore.get)
That’s tedious, though. It is better to use the getOrElse method:
println(alicesScore.getOrElse("No score"))
If alicesScore is None, then getOrElse returns "No score".
NOTE
The argument of the getOrElse method is executed lazily. For example, in the call
alicesScore.getOrElse(System.getProperty("DEFAULT_SCORE"))
the call to System.getProperty only happens when alicesScore is empty.
This delayed evaluation is achieved with a “by name” parameter. See Chapter 12 for details.
A more powerful way of working with options is to consider them as collections that have zero or one element. You can visit the element with a for loop:
for score <- alicesScore do println(score)
If alicesScore is None, nothing happens. If it is a Some, then the loop executes once, with score bound to the contents of the option.
You can also use methods such as map, filter, or foreach. For example,
val biggerScore = alicesScore.map(_ + 1) // Some(score + 1) or None val acceptableScore = alicesScore.filter(_ > 5) // Some(score) if score > 5 or None alicesScore.foreach(println) // Prints the score if it exists
TIP
When you create an Option from a value that may be null, simply use Option(value). The result is None if value is null and Some(value) otherwise.