f (CallMethod "sum" args) = case args of
      List xs -> return $ Number $ sum $ [ x | Number x <- xs ]
      _ -> invalidParams
f (CallMethod "sum" args) = case args of
      List xs -> return $ Number $ sum $ [ x | Number x <- xs ]
      _ -> invalidParams
f (CallMethod "sum" args) = case args of
      List xs -> return $ Number $ sum $ [ x | Number x <- xs ]
      _ -> invalidParams