Xmonad/ghci
XMonad can run in ghci. This allows using the ghci debugger, which allows stepping through execution and inspecting values / exceptions that can occur. Inspecting or modifying IORefs used in xmonad from ghci's main thread is another option, which requires xmonad's code to be modified slightly:
If we have a copy of core patched to put the XState into xstateRef :: IORef XState and contrib unpacked into ~/.xmonad,
darcs get http://code.haskell.org/XMonadContrib
darcs get http://code.haskell.org/~aavogt/xmonad/iorefState
We can then load up everything into ghci with:
ghci -iXMonadContrib -iiorefState/src -ilib xmonad.hs
Then start your usual main as a separate thread, so that the repl can still work:
import Control.Concurrent
import Data.IORef
import System.Environment
xm <- forkIO (withArgs ["--replace"] main)
Then we can do things like
readIORef xstateRef
killThread xm -- does not release resources properly, so a second call to main will fail