How is this a type mismatch? - scala

Question

val eventListeners = new HashMap[Class[Event], ArrayBuffer[Event => Unit]]
def addEventListener[A <: Event](f: A => Unit)(implicit mf: ClassManifest[A]): A => Unit = {
eventListeners.getOrElseUpdate(mf.erasure.asInstanceOf[Class[Event]], ArrayBuffer[Event => Unit]()) += f
f
}
Is throwing:
error: type mismatch;
found : (A) => Unit
required: (this.Event) => Unit
eventListeners.getOrElseUpdate(mf.erasure.asInstanceOf[Class[Event]], ArrayBuffer[Event => Unit]()) += f
Why is it saying that it found (A) => Unit? The value of f is a function that is (Event) => Unit. Isn't A just a type parameter, not the signature?
Example call:
addEventListener { e:FooEvent => .... }

Answer 1

The class Function1 is contra-variant on its parameter. Ie, its type is Function1[-T, +R].
That means a function of Any => Unit is a subtype of Event => Unit, but for A a subtype of Event, A => Unit is a _super_type of Event => Unit.
Ergo, the problem. If you change the type parameter to A >: Event, it should work.

Answer 2

You are promising your ArrayBuffer that you will give it a function that can take any Event and turn it into a Unit (presumably doing something interesting along the way).
But you are giving it a function that can only take As, which may not encompass all Events. That is clearly not what you've promised, so the compiler complains.
You need to figure out what ought to happen in that case, and write code accordingly. For example, you could create a new function g that does nothing in case it receives an Event that, according to your class manifest, is not an A, and otherwise applies f. Or you could require all listeners to take events of all sorts, and be responsible themselves for throwing away the events that they don't want to bother with.
Edit: just to make things clearer with an example,
abstract class Fruit { def tasty: String }
class Banana extends Fruit { def tasty = "Yum!" }
abstract class SeededFruit extends Fruit {
def seedCount: Int
def tasty = "Mmm, mmm."
}
class Peach extends SeededFruit { def seedCount = 1 }
class Apple extends SeededFruit { def seedCount = 5 }
val tellAboutSeeds = (sf: SeededFruit) => println("There are "+sf.seedCount+"seeds")
val fruitTeller = new collection.mutable.ArrayBuffer[Fruit=>Unit]
fruitTeller += tellAboutSeeds // If this worked...
fruitTeller(0)(new Banana) // ...we'd be in trouble!