Hi, I am trying to use an ErrorT monad inside some Persist code. I wrote the following instance: newtype OcrError = OE String ... instance PersistBackend m => PersistBackend (ErrorT OcrError m) where replace k v = lift $ replace k v update k l = lift $ update k l updateWhere f u = lift $ updateWhere f u delete = lift . delete insert = lift . insert deleteBy = lift . deleteBy deleteWhere = lift . deleteWhere get = lift . get getBy = lift . getBy count = lift . count selectKeys f = selectKeys f selectEnum f o lim off = selectEnum f o lim off It seems like there is a problem with selectEnum though. In GHCI, this: l :: [PersistKV OcrRefMap] <- rs $ selectEnum [] [] 0 0 works fine, while this: l :: Either OcrError [PersistKV OcrRefMap] <- rs $ runErrorT $ selectEnum [] [] 0 0 hangs. rs is a small function to run persist calls: rs :: (MonadControlIO m) => SqlPersist m a -> m a rs f = do let conn = DT.pack "user=hachicode password=password host=localhost port=5432 dbname=hachicode" withPostgresqlConn conn $ runSqlConn f and you need -XScopedTypeVariables for the ghci commands. Any advice? Thanks, Max