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

Steffen Schuldenzucker sschuldenzucker at uni-bonn.de
Mon Oct 3 17:18:28 CEST 2011

On 10/03/2011 10:42 PM, Magicloud Magiclouds wrote:
> 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?

If 'users' and 'medias' are actually of a general type (like "for all a 
with ToJson a, users describes a value of type a"), use Jesse's 
suggestion. Otherwise ("there is an a with ToJson a such that users 
describes a value of type a"), you might want to use existentials:

{-# LANGUAGE ExistentialQuantification #-}
data SomeToJson = forall a. (ToJson a) => SomeToJson a

instance ToJson SomeToJson where
     toJson (SomeToJson x) = toJson x  -- I guess your class looks like 

And then:
r <- post site token "user.addMedia" $ Map.fromList
     [("users", SomeToJson users), ("medias", SomeToJson medias)]

As a last remark, I needed this pattern exactly once, namely for dealing 
with rank 2 types in rendering functions using takusen. I can conclude 
that requiring it is often an indicator for a major design flaw in your 
program. In this case:

Why not:

-- assuming that there is an
-- instance ToJson Json where toJson = id
r <- post site token "user.addMedia" $ Map.fromList
    [("users", toJson users), ("medias", toJson medias)]


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