Hello, Is it unsafe to add the following catch-all MonadIO instance to transformers' Control.Monad.IO.Class module? {-# LANGUAGE FlexibleInstances, FlexibleContexts, UndecidableInstances #-} instance (MonadTrans t, Monad (t m), MonadIO m) => MonadIO (t m) where liftIO = lift . liftIO It could get rid of all the similarly looking instances: instance (MonadIO m) => MonadIO (ReaderT r m) where liftIO = lift . liftIO instance (MonadIO m) => MonadIO (StateT s m) where liftIO = lift . liftIO instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where liftIO = lift . liftIO ... The reason I ask is that I want to do something similar for monad-control's MonadControlIO type class. But I'm not sure if I don't introduce any undecidability in the type-checker. Regards, Bas