Hello, I defined a typeclass like this with a data familly class HasConfig a where data BinocularsConfig a :: * new :: (MonadIO m, MonadLogger m, MonadThrow m) => Maybe FilePath -> m (BinocularsConfig a) getConfig :: Maybe FilePath -> IO (Either String (BinocularsConfig a)) combineWithCmdLineArgs :: BinocularsConfig a -> Maybe ConfigRange -> Maybe (Path Abs Dir) -> BinocularsConfig a update :: (MonadIO m, MonadLogger m, MonadThrow m) => FilePath -> m (BinocularsConfig a) the instance is for now instance HasConfig PreConfig where data BinocularsConfig PreConfig = BinocularsPreConfig { _binocularsPreConfigProjectionType :: ProjectionType } deriving (Eq, Show) [...] Since I use Ini to serialize/un-serialize the configuration, I need lens for this type My question is how can I generate the like easily makeLenses ''(BinocularsConfig PreConfig) does not work thanks for considering Frederic