[Haskell-cafe] A catch-all MonadIO instance
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
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Re: [Haskell-cafe] A catch-all MonadIO instance
On Fri, Oct 28, 2011 at 9:11 AM, Bas van Dijk v.dijk@gmail.com wrote:
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
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Re: [Haskell-cafe] A catch-all MonadIO instance
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.
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Re: [Haskell-cafe] A catch-all MonadIO instance
On 28 October 2011 16:23, Antoine Latter aslat...@gmail.com wrote: 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 r.pater...@city.ac.uk wrote: 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A catch-all MonadIO instance
Bas van Dijk v.dijk@gmail.com wrote: 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/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A catch-all MonadIO instance
On 28 October 2011 16:59, Ertugrul Soeylemez e...@ertes.de wrote: 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
