Help test XDG support in cabal-install

Cabal, or more precisely cabal-install which is mostly known to Haskellers as the cabal command line program ("cabal" means four things), recently merged a pull request that makes cabal support the XDG Basedir Specification. Previously, cabal would put all of its files in the ~/.cabal directory. Now the files are spread over multiple directories:

• $XDG_CONFIG_HOME/cabal for the main configuration file. On Unix, this defaults to ~/.config/cabal.

• $XDG_CACHE_HOME/cabal for downloaded packages and script executables. Defaults to ~/.cache/cabal on Unix.

• $XDG_STATE_HOME/cabal for compiled libraries and other stateful artifacts. Defaults to ~/.local/state/cabal on Unix.

• ~/.local/bin for executables installed with cabal install.

The advantage is mostly that cabal now behaves slightly more like other modern Unix tools. For example, it is easier to put all configuration files under version control if they're all in the same subdirectory, and it is easier to delete all program caches when you're low on disk space.

This is obviously a rather invasive change. Does it mean that the next version of cabal will break your workflow? Maybe! But if so, it is not intentional. This change comes with what I hope is rather thorough backwards compatibility behaviour. Basically, if ~/.cabal exists, or $CABAL_DIR is set, the old behaviour of using a single unified directory will be maintained. And of course, most paths can still be configured manually in the configuration file.

Still, because of the delicacy of a change like this, we'd like some external confirmation that cabal is still usable. This requires human trials. Therefore, if you are a human who uses cabal, please try installing the latest development version and see if it still works for you. If you want to try out the new XDG future, you can delete your ~/.cabal directory (possibly copying ~/.cabal/config to ~/.config/cabal/config first). I've been dogfooding this support for a over month, but I have no illusions about my usage covering the full feature space.

Beyond whether cabal remains at all functional, I am personally curious whether the XDG simulacrum that is implemented on Windows is at all useful for Windows users, or whether it would be better for cabal to retain a single unified directory on that platform.

Finally, on a meta note, this is my first contribution to Cabal. I previously heard horror stories about its code complexity, but I don't think think they are warranted, at least not in the corner that I was touching. You certainly find relics of a long development process, including code necessary to support obsolete features (v1-build and sandboxes), but for a project its age and scope, I found the code both well structured and reasonably well documented. XDG support was added entirely by modifying cabal client code, without touching the Cabal library at all.