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"

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