Hi Magnus,
I'm under the impression that there already are rather a lot of packages that are updated to deal with changes in 7.2. I don't have any numbers on that though, are there any such numbers ready available?
I am not aware of any statistics that estimate the percentage of packages broken with GHC 7.2.x, unfortunately, but even if we'd have those kind of statistics, the numbers might be deceptive. For instance, one of the packages that is broken is 'zlib'. zlib itself seems to compile fine, but the resulting package won't link when used by others. Now, that's just one package, but the effect of that particular problem is quite significant, because it cascades down the dependency chain. See http://hydra.nixos.org/job/nixpkgs/trunk/haskellPackages_ghc722.yesod for a concrete example. Furthermore, the problem exists the other way round, too. There are packages that build fine with 7.2.x, but won't compile with older GHC versions, such as 'yap' or the latest version of 'repa'. Whether that is a problem or not depends on what you're trying to do, obviously. Personally, I have come to prefer NixOS http://nixos.org/ for Haskell development, because it allows me to install any number of different GHC versions simultaneously. Take care, Peter