Is there a reason why you can't use an explicit type variable?
{-# LANGUAGE OverloadedStrings, ExistentialQuantification #-}
import Data.Aeson
import Control.Applicative
import Control.Monad (mzero)
data ActionData j
= (FromJSON j, ToJSON j) => AD j j
instance ToJSON (ActionData j) where
toJSON (AD o n) = object [ "oldData" .= o
, "newData" .= n ]
instance (ToJSON j, FromJSON j) => FromJSON (ActionData j) where
parseJSON (Object v) = AD
<$> v .: "oldData"
<*> v .: "newData"
parseJSON _ = mzero
2013/6/21 Miguel Mitrofanov <miguelim...@yandex.ru>
> Forgot to reply all, as usual.
>
> -------- Пересылаемое сообщение --------
> 21.06.2013, 12:52, "Miguel Mitrofanov" <miguelim...@yandex.ru>:
>
> Actually, this is not the real error you should care about. Try removing
> FromJSON instance completely, and you'll get a lot more. And these are
> fundamental: you have to decide what "j" to use when serializing. Haskell
> won't automagically substitute some suitable type for you.
>
> So, that's a classic mismatch: for serializing (ToJSON) you need your "j"
> type to be known to the AD value (meaning: it should be quantified
> existentially), but for deserializing you need it to be any type
> (quantified universally).
>
> All in all, AD seems to be the wrong type.
>
> 21.06.2013, 12:18, "Magicloud Magiclouds" <magicloud.magiclo...@gmail.com
> >:
>
> > data ActionData = AD { oldData :: (FromJSON j, ToJSON j) => j
> > , newData :: (FromJSON j, ToJSON j) => j}
> > instance ToJSON ActionData where
> > toJSON (AD o n) = object [ "oldData" .= o
> > , "newData" .= n ]
> > instance FromJSON ActionData where
> > parseJSON (Object v) = AD
> > <$> v .: "oldData"
> > <*> v .: "newData"
> > parseJSON _ = mzero
> >
> > I got when compile:
> > No instance for (FromJSON (forall j. (FromJSON j, ToJSON j) => j))
> > arising from a use of `.:'
> > Possible fix:
> > add an instance declaration for
> > (FromJSON (forall j. (FromJSON j, ToJSON j) => j))
> > In the second argument of `(<$>)', namely `v .: "oldData"'
> > In the first argument of `(<*>)', namely `AD <$> v .: "oldData"'
> > In the expression: AD <$> v .: "oldData" <*> v .: "newData"
> >
> > --
> > 竹密岂妨流水过
> > 山高哪阻野云飞
> >
> > And for G+, please use magiclouds#gmail.com.
> > ,
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