Add MonadBase IO instances

[chrismwendt]

This makes it easier to interface with lifted-async, lifted-base, and others.

The adds a dependency on transformers-base, which seems pretty small (107 LOC) and stable (~3 patch releases in 4 years).

[chrismwendt]

I stumbled upon #404 (comment) and discovered unliftio mostly does the trick 🎉

However, there are so called "control" functions like withMVar which require MonadUnliftIO, but there's no instance for MonadUnliftIO ConduitT i o m, so apparently it's impossible to use withMVar inside of a ConduitT. Could this be implemented? What's preventing it from working? It might be hard to track the values being emitted from the callback's conduit, but it seems possible (although I haven't tried it).

As a fallback, could there be hybrid functions with plain IO callbacks like:

withMVarHybrid :: MonadIO m => MVar a -> (a -> IO b) -> m b
withMVarHybrid v f = liftIO $ withMVar v f

Here's the unliftio implementation for comparison:

withMVar :: MonadUnliftIO m => MVar a -> (a -> m b) -> m b
withMVar var f = withRunInIO $ \run -> M.withMVar var (run . f)

[snoyberg]

You cannot set up "control" functions with ConduitT since it is continuation based. This is a deep topic, and I've written quite a bit on it previously. One starting point is:

https://github.com/snoyberg/conduit#resource-allocation

Adding MonadBase will not help with this, MonadIO already does the same thing and is insufficient for what you're trying to do.

[chrismwendt]

Got it, I'll read up more before diving further into this.