changing particle draw condition

Now particles are drawn when moving the mouse whilst pressing the left
mouse button

examples/example03.hs

@@ -104,12 +104,15 @@ update sec = do
   ev <- liftIO $ SDL.pollEvents
   mapM_ (\e ->
     case SDL.eventPayload e of
-      SDL.MouseButtonEvent dat -> do
-          let (SDL.P (SDL.V2 x y)) = SDL.mouseButtonEventPos dat
+      SDL.MouseMotionEvent dat ->
+        if SDL.ButtonLeft `elem` SDL.mouseMotionEventState dat
+        then do
+          let (SDL.P (SDL.V2 x y)) = SDL.mouseMotionEventPos dat
           vx <- liftIO $ randomRIO (-20, 20)
           vy <- liftIO $ randomRIO (-20, 20)
           life <- liftIO $ randomRIO (1, 5)
-          -- traceM $ "velocity is: " ++ show vx ++ " " ++ show vy
+          traceM $ "position is: " ++ show x ++ " " ++ show y
+          traceM $ "velocity is: " ++ show vx ++ " " ++ show vy
           putAffection $ ud
             { coordinates = Just (fromIntegral x, fromIntegral y)
             , partsys     = (partsys ud)
@@ -122,6 +125,8 @@ update sec = do
                     }) : (psParts $ partsys ud)
                 }
             }
+        else
+          return ()
       SDL.WindowClosedEvent _ -> do
         traceM "seeya!"
         put $ ad