On 28 October 2011 16:59, Ertugrul Soeylemez
I'm not sure whether this will work well. You will get overlapping instances, and I don't see a way to hide instances when importing. Perhaps the OverlappingInstances extension could help here.
You're right. I didn't get an overlapping instances error when building transformers with this change (Note I didn't remove the custom MonadIO instances). However when *using* liftIO I did get it:
runReaderT (liftIO $ putStrLn "Hello World!") (10 :: Int)
<interactive>:0:13: Overlapping instances for MonadIO (ReaderT Int m0) arising from a use of `liftIO' Matching instances: instance MonadIO m => MonadIO (ReaderT r m) -- Defined at Control/Monad/Trans/Reader.hs:128:10-45 instance (MonadTrans t, Monad (t m), MonadIO m) => MonadIO (t m) -- Defined at Control/Monad/IO/Class.hs:43:10-64 Enabling the OverlappingInstances extension does fix it. However I don't want to force users to use it so I keep the custom instances. Thanks, Bas