ForeignPtr-related signatures

[Simon Marlow]

> Simon Marlow wrote:
> > [...] There was this, from Dean Herrington:
> >
> >   http://haskell.cs.yale.edu/pipermail/ffi/2002-October/000940.html
> >
> > I don't have any strong feelings (I rarely do, where names are
> > concerned).
> 
> I would be happy with his proposal. We can leave the IORef 
> stuff in IOExts
> as it is and use the new signatures and functions in 
> Data.IORef. The only
> annoying thing is the signature of Data.IORef.modifyIORef, 
> but we could
> declare the old signature as a bug...  :-]

Actually, looking at the proposal again, I don't think I like the
changes to the MVar interface.  atomicModifyMVar seems strange, because
modifyMVar is already atomic.  We have to be careful about trying to
smooth over differences between IORef and MVar where those differences
are real and important.

I don't mind changing modifyIORef to have a signature which is more
similar to modifyMVar, and I think adding atomicModifyIORef_ is a good
idea (although it needs a new primitive if we're to take advantage of
its efficiency over atomicModifyIORef).  Summary:

 - make modifyIORef's type match modifyMVar
 - add modifyIORef_
 - add atomicModifyIORef and atomicModifyIORef_

Cheers,
	Simon