Editing Xmonad/Frequently asked questions (section)

=== A manage hook is having no effect ===

If a manage hook that checks an attribute of a window(e.g. resource =? "foo"
--> bar) doesn't work even though the property is the same as reported by
xprop, it can be caused by a program setting the corresponding property after
the window has been created.  To debug whether or not the term on the right
hand side is at fault, it is useful to log which resource (or other property)
xmonad sees:

<haskell>
(resource >>= io . appendFile "/home/<youruser>/xmonad_debug" >> idHook)
</haskell>

If opening the desired window causes xmonad to produce a log entry with the
resource you were interested in, it means that the term on the right-hand side
of --> is not working.