[jfischoff]

I think for modules to help here, the interface would have to not change between versions, useful for ByteString, not so much for Yesod.

As it stands now, cabal's constraint solver will prevent the installation of a package that indirectly depends on two different versions of the same package. This is too bad, because it is not necessarily true the package will fail to install.

Ideally cabal could somehow know which types are exported and which are not. In lieu of that, I personally I would prefer to have a chance of a successful install, and get a compiler error if things really do conflict, instead of a conservative constraint solver error.

[lomnakkus]

Certainly it's not going to be a panacea, but if you are familiar with O'Caml or SML you can hopefully sort of see how it's going to be of immense help when dealing with "trivial" package upgrades and such.

Hopefully Yesod will also (if/when this Backpack thing comes to fruition) expose some sort of stable interface for, say, the 5.x series. I think Michael has been very good about keeping things compatible, given the current constraints on the ecosystem, so I imagine he'd be very interested in declaring a stable interface explicitly if that were possible.