import Data.Constraint (Dict(..), withDict)
import qualified Data.ByteString as BS

data DBEntry = DBEntry { siteVariant :: SiteVariant
                       , html :: BS.ByteString
                       }

data SiteVariant = A | B

data SiteA = SiteA

data SiteB = SiteB

-- | all sites to be transformed into General
data General = General

class IntoGeneral a where
  fromHTML :: Dict (IntoGeneral a) -> BS.ByteString -> General

instance IntoGeneral SiteA where
  fromHTML _ = undefined

-- | Depending on `siteVariant` in `DBEntry`, `fromHTML` is calibrated with Proxy
fromDBEntry :: Dict (IntoGeneral a) -> DBEntry -> General
fromDBEntry dict (DBEntry siteVariant html) = case siteVariant of
                                                A -> withDict dict (@SiteA) $ fromHTML html
                                                B -> withDict dict SiteB $ fromHTML html

-- Question: how to make fromDBEntry work?