On Fri, Oct 28, 2011 at 9:11 AM, 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 ...
I would then need OverlappingInstances to declare a MonadIO instance for any similar looking instance head (that is `t m`) where 't' was not a proper MonadTrans instance, which sounds like a common enough things to do. I usually don't bother writing a MonadTrans instance in my own apps, and I try to avoid using OverlappingInstances unless there's no other way to do something. Although I don't have a better solution to offer for the exploding instance problem with mtl-like libraries. Antoine