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
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