Hello, I'm trying to use SqlPersistT funcs with MonadError, but am failing in writing the runDB correctly. I tried several things, but always getting: • Couldn't match type ‘IOException’ with ‘AppException’ arising from a functional dependency between: constraint ‘MonadError AppException IO’ arising from a use of ‘f1’ instance ‘MonadError IOException IO’ at <no location info> • In the second argument of ‘runPool’, namely ‘f1’ In the second argument of ‘($)’, namely ‘runPool pgConf f1 pgPool’ In a stmt of a 'do' block: liftIO $ runPool pgConf f1 pgPool I understand there already is "instance MonadError IOException IO" and fundep says it can be only one for IO. How to make it compile? Best regards, vlatko Here is the minimal reproducible code: module Test where import Prelude import Control.Exception.Base import Control.Monad.Except import Control.Monad.Trans.Reader import Database.Persist.Postgresql data AppException = ExcText String | ExcIO IOException deriving (Show) type AppM m = ExceptT AppException (ReaderT App m) data App = App { pgConf :: PostgresConf, pgPool :: ConnectionPool} runDB :: (MonadIO m, MonadError AppException m) => AppM m Bool runDB = do App{..} <- lift ask liftIO $ runPool pgConf *f1* pgPool -- error -- liftIO $ runPool pgConf *f2* pgPool -- OK f1 :: (MonadIO m, MonadError AppException m) => SqlPersistT m Bool f1 = throwError $ ExcText "exception" f2 :: MonadIO m => SqlPersistT m Bool f2 = return True