[Haskell-cafe] Problem on using class as type.

Hi, I have a function: post :: (ToJson p, FromJson q) => String -> String -> String -> Map.Map String p -> IO q Now I'd like to call it like: r <- post site token "user.addMedia" (Map.fromList [ ("users", users :: ToJson) , ("medias", medias :: ToJson) ]) So I got the problem. If I used things like "users :: ToJson", then class used as a type error occurred. But if I did not use them, since users and medias were actually different types, then fromList failed, required the type of medias the same with users. How to resolve the conflict? -- 竹密岂妨流水过 山高哪阻野云飞