[Haskell-cafe] A catch-all MonadIO instance

[Bas van Dijk]

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