[Haskell-cafe] Re: [Haskell] Top Level <-

[Dan Doel]

On Thursday 28 August 2008 2:28:35 pm David Roundy wrote:
> On Thu, Aug 28, 2008 at 01:17:29PM -0400, Dan Doel wrote:
> > On Thursday 28 August 2008 12:26:27 pm Adrian Hey wrote:
> > > As I've pointed out several times already you can find simple examples
> > > in the standard haskell libs. So far nobody has accepted my challenge
> > > to re-implement any of these "competantly" (I.E. avoiding the use of
> > > global variables).
> > >
> > > Why don't you try it with Data.Unique and find out :-)
> >
> > Here's a first pass:
> >
> > -- snip --
> >
> > {-# LANGUAGE Rank2Types, GeneralizedNewtypeDeriving #-}
> >
> > module Unique where
>
> If you want this to actually provide any guarantees, of course, you'll
> have to provide an export list.

Yes, quite right. I didn't spend a lot of time on it. I believe U and unU 
would need to be hidden to prevent people from doing bad things.

Another problem I thought of after the fact is that if you need to extend the 
IO monad in any other similar way, you're out of luck. However, I think you 
can modify things to something like:

    newtype UT r m a = UT { unUT :: ReaderT (MVar Integer) m a } ...

    runUT :: MonadIO m => (forall r. UT r m a) -> m a
    runUT m = liftIO (newMVar 0) >>= runReaderT (unUT m)

    ...

Or if you want to get really fancy, maybe you could invent an entire new 
sectioned, composable IO monad like Datatypes a la Carte. But that's a fair 
amount more work.

Cheers.
-- Dan