ToJSON a => parseJSON val :: Parser a

data Query' f r where
  AccountInfo'   :: f A.Account -> Query' f A.Account
  AccountByName' :: Text -> Query' f A.Account
  QId'           :: (ToJSON a, FromJSON a) => a ->  Query' f a
  QAnd'          :: (ToJSON a, FromJSON a, ToJSON b, FromJSON b) => Query' f a -> Query' f b -> Query' f (a,b)

data QueryBox' where
  Qbox :: (ToJSON r, FromJSON r) => Query' Keyed r -> QueryBox'

defineToJSONQuery ''Query' ''Keyed

instance forall a b c. (ToJSON a, FromJSON a, ToJSON b, FromJSON b, ToJSON c, FromJSON c) => FromJSON QueryBox' where
  parseJSON (Object o) = do
    conKey <- o .: "tag"
    val    <- o .: "contents"
    case (conKey :: String) of
      "AccountInfo'"   -> Qbox . AccountInfo' <$> parseJSON val
      "AccountByName'" -> Qbox . AccountByName' <$> parseJSON val
      "QId'" -> Qbox . QId' <$> (parseJSON val :: Parser a)
      "QAnd'" -> do
        vs <- parseJSON val
        (Qbox q) <- (parseJSON $ vs !! 0) 
        (Qbox q') <- (parseJSON $ vs !! 1) 
        return $ Qbox $ QAnd' (q :: Query' Keyed b) (q' :: Query' Keyed c)
      anystring -> fail "Not implemented yet."


Error:
/home/thomasd/Documents/github/bd/beautilytics/lib/Beautilytics/TransH/Query.hs:212:22:
        Could not deduce (ToJSON a) arising from a use of ‘parseJSON’
        from the context (ToJSON a1,
                          FromJSON a1,
                          ToJSON b1,
                          FromJSON b1,
                          ToJSON c1,
                          FromJSON c1)
          bound by the instance declaration
          at lib/Beautilytics/TransH/Query.hs:202:10-111
        The type variable ‘a’ is ambiguous
        Note: there are several potential instances:
          instance ToJSON AdGoal -- Defined in ‘Beautilytics.AdTypes’
          instance ToJSON AdInterest -- Defined in ‘Beautilytics.AdTypes’
          instance ToJSON AdPlacement -- Defined in ‘Beautilytics.AdTypes’
          ...plus 417 others
        In the expression: parseJSON
        In a stmt of a 'do' block: (Qbox q) <- (parseJSON $ vs !! 0)
        In the expression:
          do { vs <- parseJSON val;
               (Qbox q) <- (parseJSON $ vs !! 0);
               (Qbox q') <- (parseJSON $ vs !! 1);
               return
               $ Qbox $ QAnd' (q :: Query' Keyed b) (q' :: Query' Keyed c) }
    
    /home/thomasd/Documents/github/bd/beautilytics/lib/Beautilytics/TransH/Query.hs:214:32:
        Couldn't match type ‘r’ with ‘b1’
          ‘r’ is a rigid type variable bound by
              a pattern with constructor
                Qbox :: forall r.
                        (ToJSON r, FromJSON r) =>
                        Query' Keyed r -> QueryBox',
              in a pattern binding in
                   'do' block
              at lib/Beautilytics/TransH/Query.hs:212:10
          ‘b1’ is a rigid type variable bound by
               the instance declaration at lib/Beautilytics/TransH/Query.hs:202:10
        Expected type: Query' Keyed b1
          Actual type: Query' Keyed r
        Relevant bindings include
          q :: Query' Keyed r
            (bound at lib/Beautilytics/TransH/Query.hs:212:15)
        In the first argument of ‘QAnd'’, namely ‘(q :: Query' Keyed b)’
        In the second argument of ‘($)’, namely
          ‘QAnd' (q :: Query' Keyed b) (q' :: Query' Keyed c)’
    
    /home/thomasd/Documents/github/bd/beautilytics/lib/Beautilytics/TransH/Query.hs:214:54:
        Couldn't match type ‘r1’ with ‘c1’
          ‘r1’ is a rigid type variable bound by
               a pattern with constructor
                 Qbox :: forall r.
                         (ToJSON r, FromJSON r) =>
                         Query' Keyed r -> QueryBox',
               in a pattern binding in
                    'do' block
               at lib/Beautilytics/TransH/Query.hs:213:10
          ‘c1’ is a rigid type variable bound by
               the instance declaration at lib/Beautilytics/TransH/Query.hs:202:10
        Expected type: Query' Keyed c1
          Actual type: Query' Keyed r1
        Relevant bindings include
          q' :: Query' Keyed r1
            (bound at lib/Beautilytics/TransH/Query.hs:213:15)
        In the second argument of ‘QAnd'’, namely ‘(q' :: Query' Keyed c)’
        In the second argument of ‘($)’, namely
          ‘QAnd' (q :: Query' Keyed b) (q' :: Query' Keyed c)’