On Mon, Feb 6, 2012 at 10:38 AM,
The core xmonad code goes to great lengths to keep the bulk of its functionality pure, with a smallish wrapper to communicate with X, and provides a fairly comprehensive QuickCheck test suite. (See the "tests" directory in the repository.) Perhaps you can take some inspiration from there.
~d
The "wrapper to communicate with X" is unfortunately the majority of the code. It should be possible to change the IO functions to ones that manipulate a haskell model of what the X does. Most of the effort would end up going into making sure the model is correct. Some bugs you might be able to find with that approach are situations where xmonad tells X to go through an excess number of steps to get to the final state. This is probably a problem for configs using lots of contrib modules (NoBorders, Decoration, etc.). Adam