Could iteratee depend on mtl instead of transformers?

[Ross Paterson]

On Tue, Nov 24, 2009 at 10:26:56AM -0000, Sittampalam, Ganesh wrote:
> Ross Paterson wrote:
> >     Could not find module `Control.Monad.Identity':
> >       it was found in multiple packages: mtl-2.0 transformers-0.1.4.0
> 
> This is annoying, but can be worked around with -hide-package. We could
> also deprecate mtl as part of the migration, encouraging people to move
> away from it as quickly as possible. After a bit it could be removed
> from the platform, which would mean that people would only end up with
> it on their systems if they install it or something that still depends
> on it.

So the proposal is that HP should contain mtl-2.0, monads-fd and
transformers, but with mtl hidden.  Could the HP do that?

> > (Unless we introduce virtual packages.) 
> 
> What functionality would virtual packages have? Presumably in this
> instance they'd need to provide some mechanism for GHC to recognise that
> you get the same result by importing the modules from either source.

I was imagining something that would be implemented in Cabal, so you
could say somewhere something like

mtl >= 2  =  transformers > 0.1.4.0, monads-fd >= 0.0.0.1

and Cabal would build packages with an unversioned dependency on mtl
using the other two instead.  That would still leave old versions of
mtl needing hiding, though.