I am trying to get xmobar running on GNOME3. However, xmonad starts the
process, then it quits. I am not sure why, but I know that it quits
because I changed the output handler (ppOutput) to first use writeFile
then to the xmobar Handle. I saw the contents that would have been piped
to xmobar in the file.
If I start xmobar manually after xmonad has started, it starts fine,
Do you ever see the xmobar appear? It occurs to me that this may be the usual $PATH issue: $PATH is set in terminals, so you can find xmobar easily, but the $PATH passed to xmonad itself is /bin:/usr/bin:/usr/local/bin or whatever your system uses as a default $PATH and xmobar installed via cabal or (some ways) git won't be found.
This would show up as errors in ~/.xsession-errors on many, but not all, systems.