Bas van Dijk writes:
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 ...
It's done that way in transformers to keep the package portable. As for doing it elsewhere, although this catch-all instance requires UndecidableInstances, I don't think it introduces non-termination.