Michael Orlitzky
(replying to no one in particular)
This problem isn't specific to Haskell. In every other language, you have projects that support major versions of toolkits, compilers, libraries and whatnot. And there's already a tool for it: git.
Instead of using #ifdef to handle four different compilers, keep a branch for each. Git is designed to make this easy, and it's usually trivial to merge changes from the master branch back into e.g. the ghc-7.8 branch. That way the code in your master branch stays clean. When you want to stop supporting an old GHC, delete that branch.
I don't find this option terribly appealing. As a maintainer I would far prefer maintaining a bit of CPP than a proliferation of branches, with all of the cherry-picking and potential code divergence that they bring. Really though, the point I've been trying to make throughout is that I think that much of the CPP that we are currently forced to endure isn't even strictly necessary. With a few changes to our treatment of warnings and some new pragmas (aimed at library authors), we can greatly reduce the impact that library interface changes have on users. Cheers, - Ben