Haskell Platform call for consensus: add transformers and revise the mtl package to depend on it

[Ross Paterson]

On Thu, Nov 04, 2010 at 05:39:29PM +0100, Johan Tibell wrote:
> So all these libraries are missing an upper bound on their mtl dependencies?

I think it would be better if they were upgraded to mtl-2, or made to
work with both versions.  (If the problem is a Functor constraint, this
can be done by just adding such a constraint in addition to the Monad
constraint, as was just done to the cgi package.)

> > A further concern is that we now have 138 packages whose dependencies
> > exclude mtl-2 and 8 (and rising) whose dependencies require it.
> > That will prevent an increasing number of packages from building.
> 
> Is that really the case? Libraries can build against older versions of
> another library? This gives library authors time to upgrade their
> dependencies and fix compilation errors. Problems should only show up
> when libraries don't follow the PVP.

% cabal install --dry-run atomo-0.2 
Resolving dependencies...
cabal: cannot configure monads-fd-0.1.0.3. It requires mtl ==2.*
For the dependency on mtl ==2.* there are these packages: mtl-2.0.0.0. However
none of them are available.
mtl-2.0.0.0 was excluded because haskeline-0.6.3.1 requires mtl ==1.1.*

This will become more common as more packages are upgraded to mtl-2.