$ nix build
copying path '/nix/store/izrapklxwba42ncw8ssfmdpajmwx53v7-distribution-nixpkgs-1.7.1-data' from 'https://cache.nixos.org'...
copying path '/nix/store/dkpi8hi25fc2g4lqsagkbzsawdgah1dl-cabal2nix-2.19.1' from 'https://cache.nixos.org'...
building '/nix/store/5i3v21ys61s1bq0523436frl3wa65iiz-cabal2nix-xmonadConfig.drv'...
building '/nix/store/pd8msq4vyy7sfisp0ym5sf58y53xyhi0-cabal2nix-xmonad.drv'...
building '/nix/store/4zdsdl9sh94nmcpgyazkjwhy82lddg5k-cabal2nix-xmonad-contrib.drv'...
these 2 derivations will be built:
  /nix/store/1h15nwhqymsrl1wdffmckn8d426qaldq-haskell-generic-builder-test-wrapper.sh.drv
  /nix/store/cqwm7isfwgzhg87k0w7jwqh0fg0zlv1v-xmonadConfig-0.1.0.0.drv
this path will be fetched (0.00 MiB download, 0.00 MiB unpacked):
  /nix/store/2klclm31nr44ikrjaqmyqr9hzsxxsz7v-remove-references-to
copying path '/nix/store/2klclm31nr44ikrjaqmyqr9hzsxxsz7v-remove-references-to' from 'https://cache.nixos.org'...
building '/nix/store/1h15nwhqymsrl1wdffmckn8d426qaldq-haskell-generic-builder-test-wrapper.sh.drv'...
building '/nix/store/cqwm7isfwgzhg87k0w7jwqh0fg0zlv1v-xmonadConfig-0.1.0.0.drv'...