[xmonad] xmonad and ghc 6.12

[Brent Yorgey]

So, let's talk about plans for xmonad and ghc 6.12.  I am currently
building xmonad and xmonad-contrib on ghc 6.12.1rc2 and it works fine,
except I get a lot of warnings, which fall into three main categories:

  * Warnings about using base-3 instead of base-4.  Currently xmonad
    doesn't build with base-4 because of the Exception
    vs. OldException thing.  What's our plan for dealing with this?
    We should switch to using the extensible exception stuff, but how
    do we continue to support base-3? (*Should* we continue to support
    it?)

  * Warnings about redundant imports; apparently 6.12 is more
    aggressive about noticing these.  These can be cleaned up, which
    is probably a good idea anyway.  Or, if people don't like this for
    some reason, we can disable this warning.

  * Lots of warnings about actions which return something other than
    () but whose results are neither bound nor explicitly discarded.
    As discussed on IRC the right thing to do here is probably to just
    pass -fno-warn-unused-do-bind to ghc (as opposed to "fixing" all
    of these).

Thoughts?

-Brent