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

[Johan Tibell]

On Thu, Nov 4, 2010 at 6:05 PM, Ross Paterson <ross at soi.city.ac.uk> wrote:
> 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.)

I agree! I meant, if libraries would follow the PVP they won't
immediately break when a new version comes out, giving the maintainers
some time to upgrade.

> % 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.

Diamond dependencies like these are indeed a problem. I don't have a
better way to solve them except patching the libraries. The
alternative would be to never make any breaking changes.

Johan