Implements a --replace behavior outside of core.

You must run the replace action before starting xmonad proper, this results in xmonad replacing the currently running WM regardless of the arguments it is run with:

import XMonad
import XMonad.Util.Replace
main = do
   replace
   xmonad $ def { .... }

This doesn't seem to work for replacing WMs that have been started from within xmonad, such as with restart "openbox" False, but no other WMs that implements --replace manage this either. replace works for replacing metacity when the full gnome-session is started at least.

You can use getArgs to watch for an explicit --replace flag:

import XMonad
import XMonad.Util.Replace (replace)
import Control.Monad (when)
import System.Environment (getArgs)

main = do
   args <- getArgs
   when ("--replace" `elem` args) replace
   xmonad $ def { .... }

Note that your ~/.xmonad/xmonad-$arch-$os binary is not run with the same flags as the xmonad binary that calls it. You may be able to work around this by running your ~/.xmonad/xmonad-$arch-$os binary directly, which is otherwise not recommended.