Types.hs

data RedditJsonReply a where
    GoodJsonReply :: FromJSON a => a -> RedditJsonReply a
    BadJsonReply  :: FromJSON a => [Text] -> RedditJsonReply a

errors :: RedditJsonReply a -> [Text]
errors (BadJsonReply e) = e
errors _                = error "Used 'errors' on GoodJsonReply."

reply :: RedditJsonReply a -> a
reply (GoodJsonReply a) = a
reply _                 = error "Used 'reply' on BadJsonReply."

instance FromJSON a => FromJSON (RedditJsonReply a) where
    parseJSON (Object v) = do
        e <- v .: "json" >>= (.: "errors")
        case e of
            []  -> v .: "json" >>= (.: "data") >>= parseJSON
            e'  -> return $ BadJsonReply e'

    parseJSON _ = mzero