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 <miguelimo38@yandex.ru>

Forgot to reply all, as usual.

-------- Пересылаемое сообщение --------
21.06.2013, 12:52, "Miguel Mitrofanov" <miguelimo38@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.magiclouds@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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