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
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
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.
On 28 October 2011 16:23, Antoine Latter
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 actually have never seen a MonadIO instance for a `t m` where 't' is
not a MonadTrans instance.
On 28 October 2011 16:24, Paterson, Ross
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.
I understand, portability is important for transformers. For monad-control it's less of an issue because I already use other language extensions (incl. RankNTypes). So I think I go ahead and add a catch-all instance for MonadControlIO to monad-control. Thanks for your responses, Bas
Bas van Dijk
I understand, portability is important for transformers. For monad-control it's less of an issue because I already use other language extensions (incl. RankNTypes).
So I think I go ahead and add a catch-all instance for MonadControlIO to monad-control.
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. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/
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
participants (4)
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Antoine Latter -
Bas van Dijk -
Ertugrul Soeylemez -
Paterson, Ross