[xmonad] Allow xmonad to get input from spawned applications

[Ivan Miljenovic]

I wanted to try and get a pop-up message shown on a key-press showing
the current mpd information.  Preferably, I'd like to keep everything
inside xmonad rather than relying on external scripts.

Now, I could have something like "mpc | head -1 | dzen"... but I
wanted to use something like notify-send (part of libnotify) to do so,
as its less obtrusive and handles non-Latin characters better (i.e.
using fontconfig, etc.).

However, since notify-send doesn't seem to want to take piped inputs,
I'd first have to read and parse the mpc data before I could call
notify-send.  This would work well... if only xmonad let me read data
from the processes it spawns.  As such, I think there should be a new
operation that lets you obtain the stdout output from the programs
that are spawned... something like "spawnOutput :: String -> m
String".

Is such a thing feasible/possible/good?  I _could_ just have a
seperate bash script to do this, but I'd like it to have my settings
as self-contained as possible.
-- 
Ivan Lazar Miljenovic