CheatsheetPattern maching
Scala uses the keywords match and case as well as the => notation to achieve pattern matching. A pattern matching block is evaluated as any other block, hence it have a return value and type.
NB: the _ case is used as the default case.
scala
def writtenInt(x: Int): String =
x match
case 1 => "one"
case 2 => "two"
case 3 => "three"
case _ => "other"
println(writtenInt(1)) // prints "one"
println(writtenInt(3)) // prints "three"
println(writtenInt(4)) // prints "other"
Matching on case classes
Case classes are especially useful for pattern matching as their class members can be extracted directly.
scala
sealed abstract class Message
case class Email(sender: String, title: String, body: String) extends Message
case class SMS(caller: String, message: String) extends Message
def printMessage(m: Message): Unit =
m match
case Email(sender, title, body) =>
println(s"From: <$sender>")
println(s"Subject: $title")
println(body)
case SMS(caller, message) =>
println(s"$caller: $message")
printMessage(Email("alice@gmail.com", "News from Australia", "Hi Bob, [...], see you soon!"))
printMessage(SMS("Bob", "Hi Alice, thanks for the mail!"))
Outputs:
From: <alice@gmail.com>
Subject: <News from Australia>
Hi Bob, [...], see you soon!
Bob: Hi Alice, thanks for the mail!
Matching on type
Scala lets you matching on type only:
NB: the _ used in the example bellow is not the same as seen previously. It only means that we do not care about the instance matched.
scala
def sendNotification(m: Message): Unit =
m match
case e: Email => println("You received a mail")
case _: SMS => println("You received a SMS")
Pattern guards
Pattern guards are boolean expressions which are used to make cases more specific. Just add if <boolean expression> after the pattern.
scala
def writtenInt(x: Int)(allowOdd: Boolean): String =
x match
case 1 if allowOdd => "one"
case 2 => "two"
case 3 if allowOdd => "three"
case _ => "other"
println(writtenInt(1)(true)) // prints "one"
println(writtenInt(3)(false)) // prints "other"
println(writtenInt(4)(false)) // prints "other"