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

[Magicloud Magiclouds]

Hi there,
  I am using the "toJson = id" way, which doesn't seem to cause other problem.
  Thank you.

On Mon, Oct 3, 2011 at 11:18 PM, Steffen Schuldenzucker
<sschuldenzucker at uni-bonn.de> wrote:
> 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 this?
>
> 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)]
>
> Cheers!
>



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