Hello - I have a typeclass like this .. class (Monad m) => DB m where query :: Text -> m (Maybe Account) ... and an instance that uses persistent and sqlite .. instance (MonadIO m) => DB (SqlPersistT m) where query ano = get (AccountKey ano) .. Now I want to move this to using an effect system - Polysemy .. data DB m a where Query :: Text -> DB m (Maybe Account) ... makeSem ''DB which generates the functions .. I need some help figuring out the type of the function that interpretes the above .. An initial intuition is this .. runDB :: Member SqlPersistT r => Sem (DB ': r) a -> Sem r a runDB = interpret $ \case Query ano -> get (AccountKey ano) which doesn't compile .. errors out with .. Couldn't match type ‘Control.Monad.Trans.Reader.ReaderT backend0 m0 (Maybe Account)’ with ‘Sem r (Maybe Account)’ Expected type: Sem r x Actual type: Control.Monad.Trans.Reader.ReaderT backend0 m0 (Maybe Account) • In the expression: get (AccountKey accountNo) I think I am missing something here. The type of interpret is shown as interpret :: (forall x (m :: * -> *). DB m x -> Sem r x) -> Sem (DB : r) a -> Sem r a But how do I translate the SqlPersistT part ? Any help will be appreciated. regards. -- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg