Path Dependent Types Example.md

case class NetworkSwitch(name: String) { thisSwitch =>
  case class Port(nr: Int) {
    val switch = thisSwitch
  }
  def switchPortOn(port: Port): Unit = { println(s"Switching on $port") }
}
 
def switchOffAnyPort(port: NetworkSwitch#Port): Unit =
  println(s"switching off $port on ${port.switch}")

Using this:

scala> val switch1 = new NetworkSwitch("sw1")
scala> val switch2 = new NetworkSwitch("sw2")
scala> val port1 = switch1.Port(12)
scala> val port2 = switch2.Port(25)
 
scala> switchOffAnyPort(port1)
switching off Port(12) on NetworkSwitch(sw1)
 
scala> switchOffAnyPort(port2)
switching off Port(25) on NetworkSwitch(sw2)

scala> switch1.switchPortOn(port1)   // OK: switch-on a port on a switch to which the port "belongs"
Switching on Port(12)

scala> switch1.switchPortOn(port2)   // Not OK: mix-up port& switch - demonstrates the usefulness of Path Dependent Types
                            ^
       error: type mismatch;
        found   : switch2.Port
        required: switch1.Port