class EventHandler(EventClass)
  macro make_callback(event_class)
    alias Callback = Callback, {{ event_class }} -> Nil
  end
  make_callback(EventClass)

  def initialize
    @s = Hash(EventClass, Array(Callback)).new
  end

  def add(
       event,
       &block : Callback, EventClass -> Nil
     ) : Callback, EventClass -> Nil
    @s[event.class] ||= Array(Callback).new
    @s[event.class] << block
    block
  end
end

class Event; end
class EventA < Event; end
class EventB < Event; end

h = EventHandler.new(Event)
a1 = h.add(EventA.new) { |cb, event| puts :EventA }
a2 = h.add(EventA.new) { |cb, event| puts :EventA2 }
b1 = h.add(EventB.new) { |cb, event| puts :EventB }
h.remove(a1)