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

Johan Tibell johan.tibell at gmail.com
Thu Nov 4 13:11:27 EDT 2010

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- It requires mtl ==2.*
> For the dependency on mtl ==2.* there are these packages: mtl- However
> none of them are available.
> mtl- was excluded because haskeline- 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.


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