pattern-matching.md

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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 _ => "many"
}

The val x above is a random integer between 0 and 10. 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 number greater than 2. Cases are also called alternatives.

Match expressions have a value.

def matchTest(x: Int): String = x match {
  case 1 => "one"
  case 2 => "two"
  case _ => "many"
}
matchTest(3)  // many
matchTest(1)  // 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.

abstract class 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 an abstract super class 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(email, title, _) =>
      s"You got an email from $email 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))  // 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(email, title, _) the fields email and title are used in the return value but the body field is ignored with _.

Pattern guards

Pattern guards are simply 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(email, _, _) if importantPeopleInfo.contains(email) =>
      "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("867-5309", "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))
println(showImportantNotification(someVoiceRecording, importantPeopleInfo))
println(showImportantNotification(importantEmail, importantPeopleInfo))
println(showImportantNotification(importantSms, importantPeopleInfo))

In the case Email(email, _, _) if importantPeopleInfo.contains(email), the pattern is matched only if the email is in the list of important people.

Matching on type only

You can match on the type like so:

abstract class 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) = 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).

Sealed classes

Traits and classes can be marked sealed which means all subtypes must be declared in the same file. This assures that all subtypes are known.

sealed abstract class Furniture
case class Couch() extends Furniture
case class Chair() extends Furniture

def findPlaceToSit(piece: Furniture): String = piece match {
  case a: Couch => "Lie on the couch"
  case b: Chair => "Sit on the chair"
}

This is useful for pattern matching because we don't need a "catch all" case.

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.