Hello all, As you may have noticed, I've been knocking around GHC and Cabal the past few weeks. One of the tasks that has been on my list is essentially reimplementing Philipp Schuster's 2012 GSoC, with a few small but important architectural differences. Here is a status report of what is going on. In my copy of GHC and Cabal [1,2], you can now install multiple copies of a package with differing dependencies to the package database, i.e. q-1.0 compiled against p-1.0, and against p-2.0. The packages in the database are distinguished via a *package key*, which is an md5 hash of the package id (e.g. q-1.0) and the sorted list of the package IDs of the transitive closure of dependencies (e.g. p-1.0 or p-2.0). The package key is used to generate linker symbols for packages, so it's possible to link together both copies of q in the same program, as long as you rename the modules appropriately (of course, the redefined types are considered unequal). When registering a package, ghc-pkg now checks and removes duplicate package keys, as opposed to package IDs. I've also implemented many of the necessary niceties for making it pleasant to deal with duplicated package IDs: for example, GHC's output logic has been adjusted to only qualify a package ID with the package key when there are multiples of the same package ID exposed in the database. I've also adjusted GHC's build system to use package keys rather than package IDs to refer to packages when building. Like the original GSoC project, we still need to have decisions on some of the major design choices. I think the most pressing one is "simplistic dependency resolution" when you use, e.g. ghc -package foo-1.0 or ./Setup configure --with-constraint="foo==1.0". Right now, I pick dependencies in an unspecified manner, check if they are consistent, and bail out if they are not. As it turns out, both GHC and ./Setup configure will already compute the transitive closure of dependencies, so I suspect we might be able to do something clever here. Less pressing but eventually necessary is clueing in cabal-install. Comments would be especially appreciated. Cheers, Edward [1] https://phabricator.haskell.org/D80 [2] https://github.com/haskell/cabal/pull/2002