Example of contravariance
- by Misha
I am thinking of the following example to illustrate why contravariance is useful.
Let's consider a GUI framework with Widgets, Events, and Event Listeners.
abstract class Event;
class KeyEvent extends Event
class MouseEvent extends Event
trait EventListener[-Event] { def listen(e:Event) }
Let Widgets define the following methods:
def addKeyEventListener(listener:EventListener[KeyEvent])
def addMouseEventListener(listener:EventListener[MouseEvent])
These methods accept only "specific" event listeners, which is fine. However I would like to define also "kitchen-sink" listeners, which listen to all events, and pass such listeners to the "add listener" methods above.
For instance, I would like to define LogEventListener to log all incoming events
class LogEventListener extends EventListener[Event] {
def listen(e:Event) { log(event) }
}
Since the trait EventListener is contravariant in Event we can pass LogEventListener to all those "add listener" methods without losing their type safety.
Does it make sense ?
