Pattern matching is a mechanism for checking a value against a pattern. A successful match can also deconstruct a value into its constituent parts. It is a more powerful version of the switch statement in Java and it can likewise be used in place of a series of if/else statements.
Syntax
A match expression has a value, the match keyword, and at least one case clause.
import scala.util.Random
val x: Int = Random.nextInt(10)
x match {
case 0 => "zero"
case 1 => "one"
case 2 => "two"
case _ => "other"
}
import scala.util.Random
val x: Int = Random.nextInt(10)
x match
case 0 => "zero"
case 1 => "one"
case 2 => "two"
case _ => "other"
The val x above is a random integer between 0 and 9. x becomes the left operand of the match operator and on the right is an expression with four cases. The last case _ is a “catch all” case for any other possible Int values. Cases are also called alternatives.
Match expressions have a value.
def matchTest(x: Int): String = x match {
case 1 => "one"
case 2 => "two"
case _ => "other"
}
matchTest(3) // returns other
matchTest(1) // returns one
def matchTest(x: Int): String = x match
case 1 => "one"
case 2 => "two"
case _ => "other"
matchTest(3) // returns other
matchTest(1) // returns one
This match expression has a type String because all of the cases return String. Therefore, the function matchTest returns a String.
Matching on case classes
Case classes are especially useful for pattern matching.
sealed trait Notification
case class Email(sender: String, title: String, body: String) extends Notification
case class SMS(caller: String, message: String) extends Notification
case class VoiceRecording(contactName: String, link: String) extends Notification
Notification is a sealed trait which has three concrete Notification types implemented with case classes Email, SMS, and VoiceRecording. Now we can do pattern matching on these case classes:
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"
case VoiceRecording(name, link) =>
s"You received a Voice Recording from $name! Click the link to hear it: $link"
}
}
val someSms = SMS("12345", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
println(showNotification(someSms)) // prints You got an SMS from 12345! Message: Are you there?
println(showNotification(someVoiceRecording)) // prints You received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
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"
case VoiceRecording(name, link) =>
s"You received a Voice Recording from $name! Click the link to hear it: $link"
val someSms = SMS("12345", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
println(showNotification(someSms)) // prints You got an SMS from 12345! Message: Are you there?
println(showNotification(someVoiceRecording)) // prints You received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
The function showNotification takes as a parameter the abstract type Notification and matches on the type of Notification (i.e. it figures out whether it’s an Email, SMS, or VoiceRecording). In the case Email(sender, title, _) the fields sender and title are used in the return value but the body field is ignored with _.
Pattern guards
Pattern guards are boolean expressions which are used to make cases more specific. Just add if <boolean expression> after the pattern.
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
}
}
val importantPeopleInfo = Seq("867-5309", "[email protected]")
val someSms = SMS("123-4567", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
val importantEmail = Email("[email protected]", "Drinks tonight?", "I'm free after 5!")
val importantSms = SMS("867-5309", "I'm here! Where are you?")
println(showImportantNotification(someSms, importantPeopleInfo)) // prints You got an SMS from 123-4567! Message: Are you there?
println(showImportantNotification(someVoiceRecording, importantPeopleInfo)) // prints You received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
println(showImportantNotification(importantEmail, importantPeopleInfo)) // prints You got an email from special someone!
println(showImportantNotification(importantSms, importantPeopleInfo)) // prints You got an SMS from special someone!
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
val importantPeopleInfo = Seq("867-5309", "[email protected]")
val someSms = SMS("123-4567", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
val importantEmail = Email("[email protected]", "Drinks tonight?", "I'm free after 5!")
val importantSms = SMS("867-5309", "I'm here! Where are you?")
println(showImportantNotification(someSms, importantPeopleInfo)) // prints You got an SMS from 123-4567! Message: Are you there?
println(showImportantNotification(someVoiceRecording, importantPeopleInfo)) // prints You received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
println(showImportantNotification(importantEmail, importantPeopleInfo)) // prints You got an email from special someone!
println(showImportantNotification(importantSms, importantPeopleInfo)) // prints You got an SMS from special someone!
In the case Email(sender, _, _) if importantPeopleInfo.contains(sender), the pattern is matched only if the sender is in the list of important people.
Matching on type only
You can match on the type like so:
sealed trait Device
case class Phone(model: String) extends Device {
def screenOff = "Turning screen off"
}
case class Computer(model: String) extends Device {
def screenSaverOn = "Turning screen saver on..."
}
def goIdle(device: Device): String = device match {
case p: Phone => p.screenOff
case c: Computer => c.screenSaverOn
}
sealed trait Device
case class Phone(model: String) extends Device:
def screenOff = "Turning screen off"
case class Computer(model: String) extends Device:
def screenSaverOn = "Turning screen saver on..."
def goIdle(device: Device): String = device match
case p: Phone => p.screenOff
case c: Computer => c.screenSaverOn
def goIdle has a different behavior depending on the type of Device. This is useful when the case needs to call a method on the pattern. It is a convention to use the first letter of the type as the case identifier (p and c in this case).
Binding matched patterns to variables
You can use variable binding to get type-dependent behavior while simultaneously extracting fields from the matched pattern.
def goIdleWithModel(device: Device): String = device match {
case p @ Phone(model) => s"$model: ${p.screenOff}"
case c @ Computer(model) => s"$model: ${c.screenSaverOn}"
}
def goIdleWithModel(device: Device): String = device match
case p @ Phone(model) => s"$model: ${p.screenOff}"
case c @ Computer(model) => s"$model: ${c.screenSaverOn}"
Sealed types
You may have noticed that in the examples above the base types are qualified
with the keyword sealed. This provides extra safety because the compiler
checks that the cases of a match expression are exhaustive when the base
type is sealed.
For instance, in the method showNotification defined above, if we forget
one case, say, VoiceRecording, the compiler emits a warning:
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"
}
}
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"
This definition produces the following warning:
match may not be exhaustive.
It would fail on pattern case: VoiceRecording(_, _)
The compiler even provides examples of input that would fail!
On the flip side, exhaustivity checking requires you to define all the subtypes
of the base type in the same file as the base type (otherwise, the compiler
would not know what are all the possible cases). For instance, if you try
to define a new type of Notification outside of the file that defines
the sealed trait Notification, it will produce a compilation error:
case class Telepathy(message: String) extends Notification
^
Cannot extend sealed trait Notification in a different source file
Notes
Scala’s pattern matching statement is most useful for matching on algebraic types expressed via case classes.
Scala also allows the definition of patterns independently of case classes, using unapply methods in extractor objects.
More resources
- More details on match expressions in the Scala Book