Example of using `callCabal2nix` to set up xmonad with a cabalized configuration

mrkgnaow-xmonad.nix

{ pkgs }:

{
  services.windowManager.xmonad = {
    enable = true;
    haskellPackages = pkgs.haskellPackages.override {
      overrides = self: super: {
        # /path/to/mrkgnaow-xmonad is a path that contains `mrkgnaow-xmonad.cabal`
        # you can also put an invocation of `fetchgit` or `fetchFromGitHub` there.
        mrkgnaow-xmonad = self.callCabal2nix "mrkgnaow-xmonad" /path/to/mrkgnaow-xmonad {};
      };
    };
  };
}

# Then, supposing that your mrkgnaow-xmonad cabal package exposes a module called
# `XMonad.Config.Mrkgnaow`, just put the following in your `~/.config/xmonad/xmonad.hs`:
#
# module Main (main) where
# import qualified XMonad
# import qualified XMonad.Config.Mrkgnaow
# main :: IO ()
# main = XMonad.xmonad XMonad.Config.Mrkgnaow.config