th-example.hs

data Hi =
  Hi1 Int
  | Hi2 String
  | Hi3 [Int]
  deriving (Eq, Show)

-- generate this:
class AsHi a where
  _Hi ::
    a
    -> Hi
  _Hi1 ::
    a
    -> Maybe Int
  _Hi1 a =
    case _Hi a of
      Hi1 n ->
        Just n
      _ ->
        Nothing
  _Hi2 ::
    a
    -> Maybe String
  _Hi2 a =
    case _Hi a of
      Hi2 s ->
        Just s
      _ ->
        Nothing
  _Hi3 ::
    a
    -> Maybe [Int]
  _Hi3 a =
    case _Hi a of
      Hi3 n ->
        Just n
      _ ->
        Nothing

instance AsHi Hi where
  _Hi =
    id