nixos-rebuild switch --show-trace

build output

fetching list of Nix archives at `http://nixos.org/releases/nixpkgs/channels/nixpkgs-unstable/MANIFEST.bz2'...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
31610 store paths in manifest
building Nix...
building the system configuration...
error: while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `system' at `/etc/nixos/nixos/modules/system/activation/top-level.nix:146:5':
while evaluating the derivation attribute `buildCommand' at `/etc/nixos/nixos/modules/system/activation/top-level.nix:147:5':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `etc' at `/etc/nixos/nixos/modules/system/etc/etc.nix:41:5':
while evaluating the derivation attribute `sources' at `/etc/nixos/nixos/modules/system/etc/etc.nix:46:5':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `upstart-xserver.conf' at `/etc/nixos/nixpkgs/pkgs/build-support/trivial-builders.nix:10:14':
while evaluating the derivation attribute `jobText' at `/etc/nixos/nixos/modules/system/upstart/upstart.nix:113:43':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/strings.nix:18:25':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/lists.nix:17:19':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/strings.nix:14:32':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/lists.nix:17:19':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/strings.nix:14:32':
while evaluating the builtin function `head':
while evaluating the function at `/etc/nixos/nixos/modules/system/upstart/upstart.nix:46:31':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `slim.cfg' at `/etc/nixos/nixpkgs/pkgs/build-support/trivial-builders.nix:10:14':
while evaluating the derivation attribute `text' at `/etc/nixos/nixpkgs/pkgs/build-support/trivial-builders.nix:21:29':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `xsession' at `/etc/nixos/nixpkgs/pkgs/build-support/trivial-builders.nix:10:14':
while evaluating the derivation attribute `text' at `/etc/nixos/nixpkgs/pkgs/build-support/trivial-builders.nix:21:29':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/strings.nix:18:25':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/lists.nix:17:19':
while evaluating the function at `/etc/nixos/nixpkgs/pkgs/lib/strings.nix:14:32':
while evaluating the builtin function `head':
while evaluating the function at `/etc/nixos/nixos/modules/services/x11/display-managers/default.nix:98:29':
while evaluating the attribute `start' at `/etc/nixos/nixos/modules/services/x11/desktop-managers/default.nix:41:13':
while evaluating the attribute `start' at `/etc/nixos/nixos/modules/services/x11/desktop-managers/xfce.nix:29:9':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `system-path' at `/etc/nixos/nixpkgs/pkgs/build-support/trivial-builders.nix:10:14':
while evaluating the derivation attribute `paths' at `/etc/nixos/nixpkgs/pkgs/build-support/buildenv/default.nix:29:12':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `firefox-4.0b7' at `/etc/nixos/nixpkgs/pkgs/applications/networking/browsers/firefox/4.0.nix:110:5':
while evaluating the derivation attribute `buildNativeInputs' at `/etc/nixos/nixpkgs/pkgs/stdenv/generic/default.nix:73:15':
undefined variable `p'

configuration.nix

# Edit this configuration file which defines what would be installed on the
# system.  To Help while choosing option value, you can watch at the manual
# page of configuration.nix or at the last chapter of the manual available
# on the virtual console 8 (Alt+F8).

{config, pkgs, ...}:

{
  require = [
    # Include the configuration for part of your system which have been
    # detected automatically.  In addition, it includes the same
    # configuration as the installation device that you used.
    ./hardware-configuration.nix
  ];

  time.timeZone = "America/New_York";

  environment.systemPackages = [ pkgs.vim ];

  environment.x11Packages = [ pkgs.firefox40Pkgs.firefox ];

  nix.extraOptions = "build-cores = 8";

  boot.initrd.availableKernelModules = [
    "ext4"
  ];

  boot.kernelPackages = pkgs.linuxPackages_2_6_37;

  boot.loader.grub = {
    # Use grub 2 as boot loader.
    enable = true;
    version = 2;

    # Define on which hard drive you want to install Grub.
    device = "/dev/sda";
  };

  networking = {
    hostName = "nixos";
    interfaceMonitor.enable = true; # Watch for plugged cable.
    enableWLAN = false;
  };

  # Add file system entries for each partition that you want to see mounted
  # at boot time.  You can add filesystems which are not mounted at boot by
  # adding the noauto option.
  fileSystems = [
    { mountPoint = "/";
      device = "/dev/sda3";
      fsType = "ext4";
    }

    { mountPoint = "/boot";
      device = "/dev/sda1";
      fsType = "ext2";
    }
  ];

  swapDevices = [
    { device = "/dev/sda2"; }
  ];

  # Select internationalisation properties.
  # i18n = {
  #   consoleFont = "lat9w-16";
  #   consoleKeyMap = "us";
  #   defaultLocale = "en_US.UTF-8";
  # };

  # List services that you want to enable:

  services.openssh.enable = true;

  # Add CUPS to print documents.
  # services.printing.enable = true;

  services.xserver = {
    enable = true;
    layout = "us";
    autorun = false;
    desktopManager = {
      xfce.enable = true;
      default = "xfce";
    };
  };

  # Add the NixOS Manual on virtual console 8
  services.nixosManual.showManual = true;
}