Hi Frederic, Below is a generic implementation of your class based on your example, that should get you started. Two changes are worth calling out: - I assumed that the binary operation `combine'Shape` is associative. It takes a bit more effort to associate the exact same as `foldl1`. - To avoid duplicating code between `DataSourcePath` and `DataSourceAcq`, I merged them as a single type indexed by a type-level flag. For more information, several tutorials on Haskell generics are findable on search engines. Cheers, Li-yao --- {-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} module G where import GHC.Generics import Data.Kind (Type) -- * Interface data DataSourceShape = DummyDSS Int combine'Shape :: DataSourceShape -> DataSourceShape -> DataSourceShape combine'Shape (DummyDSS x) (DummyDSS y) = DummyDSS (x + y) data DSKind = Path | Acq class DataSource a where data DataSourceT (k :: DSKind) a :: Type ds'Shape :: Monad m => DataSourceT Acq a -> m DataSourceShape withDataSourceP :: String -> DataSourceT Path a -> (DataSourceT Acq a -> m r) -> m r -- | Generic 'ds'Shape' generic'ds'Shape :: (Monad m, Generic (DataSourceT Acq a), GDataSourceAcq (Rep (DataSourceT Acq a))) => DataSourceT Acq a -> m DataSourceShape generic'ds'Shape = g'ds'Shape . from -- | Generic 'withDataSourceP' generic'withDataSourceP :: (Generic (DataSourceT Path a), Generic (DataSourceT Acq a), GDataSourcePath (Rep (DataSourceT Path a)) (Rep (DataSourceT Acq a))) => String -> DataSourceT Path a -> (DataSourceT Acq a -> m r) -> m r generic'withDataSourceP file src gg = g'withDataSourceP file (from src) (gg . to) -- ** Base instance type family DataSourceBase (k :: DSKind) :: Type where DataSourceBase Acq = String DataSourceBase Path = [String] data BaseData instance DataSource BaseData where newtype DataSourceT k BaseData = DataSource'BaseData (DataSourceBase k) ds'Shape _ = pure (DummyDSS 1) withDataSourceP _ _ k = k (DataSource'BaseData "source") -- * Generic example usage data ExampleData instance DataSource ExampleData where data DataSourceT k ExampleData = DataSource'ExampleData (DataSourceT k BaseData) (DataSourceT k BaseData) (DataSourceT k BaseData) (DataSourceT k BaseData) (DataSourceT k BaseData) deriving Generic ds'Shape = generic'ds'Shape withDataSourceP = generic'withDataSourceP -- * Generic implementation class GDataSourceAcq dataAcq where g'ds'Shape :: Monad m => dataAcq x -> m DataSourceShape class GDataSourcePath dataPath dataAcq where g'withDataSourceP :: String -> dataPath x -> (dataAcq x -> m r) -> m r instance GDataSourceAcq f => GDataSourceAcq (M1 i c f) where g'ds'Shape (M1 f) = g'ds'Shape f instance GDataSourcePath f g => GDataSourcePath (M1 i c f) (M1 i c' g) where g'withDataSourceP f (M1 d) gg = g'withDataSourceP f d (gg . M1) instance (GDataSourceAcq f, GDataSourceAcq f') => GDataSourceAcq (f :*: f') where g'ds'Shape (f :*: f') = liftA2 combine'Shape (g'ds'Shape f) (g'ds'Shape f') instance (GDataSourcePath f g, GDataSourcePath f' g') => GDataSourcePath (f :*: f') (g :*: g') where g'withDataSourceP file (f :*: f') gg = g'withDataSourceP file f $ \g -> g'withDataSourceP file f' $ \g' -> gg (g :*: g') instance DataSource a => GDataSourceAcq (K1 i (DataSourceT Acq a)) where g'ds'Shape (K1 acq) = ds'Shape acq instance DataSource a => GDataSourcePath (K1 i (DataSourceT Path a)) (K1 i (DataSourceT Acq a)) where g'withDataSourceP file (K1 acq) gg = withDataSourceP file acq $ \dat -> gg (K1 dat)