Examples for binding patterns to variables

Author: gkepka

_overviews/scala3-book/control-structures.md

@@ -810,6 +810,53 @@ speak(Person("Bam Bam"))   // "Bam Bam says, Bam bam!"
 {% endtab %}
 {% endtabs %}
 
+#### Binding matched patterns to variables
+
+You can bind the matched pattern to a variable to use type-specific behavior.
+
+{% tabs pattern-binding class=tabs-scala-version %}
+{% tab 'Scala 2' for=pattern-binding %}
+```scala
+trait Animal {
+  val name: String
+}
+case class Cat(name: String) extends Animal {
+  def meow: String = "Meow"
+}
+case class Dog(name: String) extends Animal {
+  def bark: String = "Bark"
+}
+
+def speak(animal: Animal) = animal match {
+  case c @ Cat(name) if name == "Felix" => println(s"$name says, ${c.meow}!")
+  case d @ Dog(name) if name == "Rex" => println(s"$name says, ${d.bark}!")
+  case _ => println("I don't know you!")
+}
+
+speak(Cat("Felix")) // "Felix says, Meow!"
+speak(Dog("Rex"))   // "Rex says, Bark!"
+```
+{% endtab %}
+{% tab 'Scala 3' for=pattern-binding %}
+```scala
+trait Animal:
+  val name: String
+case class Cat(name: String) extends Animal:
+  def meow: String = "Meow"
+case class Dog(name: String) extends Animal:
+  def bark: String = "Bark"
+
+def speak(animal: Animal) = animal match
+  case c @ Cat(name) if name == "Felix" => println(s"$name says, ${c.meow}!")
+  case d @ Dog(name) if name == "Rex" => println(s"$name says, ${d.bark}!")
+  case _ => println("I don't know you!")
+
+speak(Cat("Felix")) // "Felix says, Meow!"
+speak(Dog("Rex"))   // "Rex says, Bark!"
+```
+{% endtab %}
+{% endtabs %}
+
 ### Using a `match` expression as the body of a method
 
 Because `match` expressions return a value, they can be used as the body of a method.

_tour/pattern-matching.md

@@ -236,6 +236,43 @@ def goIdle(device: Device): String = device match
 
 `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.
+
+{% tabs pattern-matching-variable-binding class=tabs-scala-version %}
+{% tab 'Scala 2' for=pattern-matching-variable-binding %}
+```scala mdoc
+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(model) => s"$model: ${p.screenOff}" 
+  case c @ Computer(model) => s"$model: ${c.screenSaverOn}"
+}
+```
+{% endtab %}
+{% tab 'Scala 3' for=pattern-matching-variable-binding %}
+```scala
+sealed trait Device
+case class Phone(model: String) extends Device:
+  def screenOff = "Turning screen off"
+
+case class Computer(model: String) extends Device:
+  def screenSaverOn = "Turning screen saver on..."
+
+
+def goIdle(device: Device): String = device match
+  case p @ Phone(model) => s"$model: ${p.screenOff}"
+  case c @ Computer(model) => s"$model: ${c.screenSaverOn}"
+```
+{% endtab %}
+{% endtabs %}
+
 ## Sealed types
 
 You may have noticed that in the examples above the base types are qualified