[Haskell-cafe] Fwd: How to make this data type work?

[Vincent Ambo]

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 at yandex.ru>

> Forgot to reply all, as usual.
>
> -------- Пересылаемое сообщение  --------
> 21.06.2013, 12:52, "Miguel Mitrofanov" <miguelimo38 at 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 at 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.
> >  ,
> >  _______________________________________________
> >  Haskell-Cafe mailing list
> >  Haskell-Cafe at haskell.org
> >  http://www.haskell.org/mailman/listinfo/haskell-cafe
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