Foo.hs

data ApplyKey b a where 
  F :: ApplyKey (a -> b)
  A :: ApplyKey a
  
  
instance Reflex t => Apply (Event t) where
  evf <.> evx = fmapMaybe fromDMap e' where 
    e' merge (DMap.fromList [F :=> evf, A :=> evx])
    fromDMap m = case (DMap.lookup F m, DMap.lookup A m) of
      (Just (Identity f), Just (Identity a)) -> Just (f a)
      _ -> Nothing