On Wed, Nov 18, 2009 at 06:04:24PM +0000, Duncan Coutts wrote:
Ross, if the plan is to have the new combined package be mtl-2.x then you might as well go ahead and modify the preferred-versions file to add:
mtl < 2
Then cabal-install will then use the old mtl for all the old packages that do not use an upper bound.
We should also add mtl to the list packages to check in Distribution/PackageDescription/Check.hs. At the moment it's hard-coded to be a list of 1 (base), so that'd need doing too. This would eventually be replaced by a generic way of opting into the regime.
Or, if you all think the amount of breakage is not that great, and it'd be simpler just to fix the packages affected, then do nothing :-)
I've just realized that it's not going to be as painless as we thought, and I'm not sure this is something preferred-versions can help with. (But I still think the transition is worth doing.) My original plan was to refactor mtl as transformers + monads-fd, so that together they'd offer a bunch of modules that were near-compatible with the modules of the same name in mtl. In only a tiny minority of cases would it be necessary to change code, but everyone would have to replace their dependencies on mtl with dependencies on transformers and monads-fd (though the latter wouldn't always be needed). Then Ganesh whipped up an mtl compatibility package, that re-exported modules from the two packages. Almost all the packages in hackage would compile unchanged if this package was mtl-2.0. But there's a problem: even if we rename the modules in monads-fd, we still have two modules in transformers that have the same names as modules in mtl, namely Control.Monad.Identity and Control.Monad.Trans. Renaming isn't an option: at least the first of those is the Right Name for the module concerned. People who build with cabal are fine: only the dependent packages are exposed, and all packages use the same type constructors, so the interfaces are compatible. But people who build with ghci or ghc --make will get the error message at the top of this thread: Could not find module `Control.Monad.Identity': it was found in multiple packages: mtl-2.0 transformers-0.1.4.0 So I think we're back at the original plan: a big bang replacement of dependencies on mtl with dependencies on transformers and monads-fd. (Unless we introduce virtual packages.) After that we could rename the FD-using modules slowly using the usual deprecation procedure. But we should rename the modules in monads-tf now -- all that needs is for someone to suggest good names.
If you are modifying the preferred-versions file, would you mind sticking in:
cabal-install < 0.8
done