gistfile1.txt

{-# LANGUAGE OverloadedStrings #-}
data GUIEvent = KeyDownEvent { key_down__ctrl :: !Bool
                             , key_down__shift :: !Bool
                             , key_down__code :: !Int }
              | MouseEvent { mouse__x :: !Int
                           , mouse__y ::!Int }
                  deriving (Show)

instance FromJSON GUIEvent where
  parseJSON = withObject "GUIObject" parseGUIEvent
  parseGUIEvent v = parseKeyDownEvent v <|> parseMouseEvent
  parseKeyDownEvent = do
    tag <- v .: "tag"
    guard (tag == String "KeyDownEvent")
    KeyDownEvent <$> v .: "ctrlKey"
                 <*> v .: "shiftKey"
                 <*> v .: "keyCode"
  parseMouseEvent = do
    tag <- v .: "tag"
    guard (tag == String "MouseEvent")
    MouseEvent <$> v .: "x"
               <*> v .: "y"