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

Magicloud Magiclouds magicloud.magiclouds at gmail.com
Mon Oct 3 15:42:59 CEST 2011

  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?

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