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# From an end-user configuration file (`configuration'), build a NixOS
# configuration object (`config') from which we can retrieve option
# values.
{ system ? builtins.currentSystem
, nixpkgs ? import ./from-env.nix "NIXPKGS" /etc/nixos/nixpkgs
, pkgs ? null
, baseModules ? import ../modules/module-list.nix
, extraArgs ? {}
, modules
let extraArgs_ = extraArgs; pkgs_ = pkgs; in
rec {
# These are the NixOS modules that constitute the system configuration.
configComponents = modules ++ baseModules;
# Merge the option definitions in all modules, forming the full
# system configuration. This is called "configFast" because it's
# not checked for undeclared options.
configFast =
pkgs.lib.definitionsOf configComponents extraArgs;
# These are the extra arguments passed to every module. In
# particular, Nixpkgs is passed through the "pkgs" argument.
extraArgs = extraArgs_ // {
inherit pkgs optionDeclarations;
modulesPath = ../modules;
# Import Nixpkgs, allowing the NixOS option nixpkgs.config to
# specify the Nixpkgs configuration (e.g., to set package options
# such as firefox.enableGeckoMediaPlayer, or to apply global
# overrides such as changing GCC throughout the system). This is
# tricky, because we have to prevent an infinite recursion: "pkgs"
# is passed as an argument to NixOS modules, but the value of "pkgs"
# depends on config.nixpkgs.config, which we get from the modules.
# So we call ourselves here with "pkgs" explicitly set to an
# instance that doesn't depend on nixpkgs.config.
pkgs =
if pkgs_ != null
then pkgs_
else import nixpkgs {
inherit system;
config =
(import ./eval-config.nix {
inherit system nixpkgs extraArgs modules;
# For efficiency, leave out most NixOS modules; they don't
# define nixpkgs.config, so it's pointless to evaluate them.
baseModules = [ ../modules/misc/nixpkgs.nix ];
pkgs = import nixpkgs { inherit system; config = {}; };
# "fixableDeclarationsOf" is used instead of "declarationsOf" because some
# option default values may depends on the definition of other options.
# !!! This seems inefficent. Didn't definitionsOf already compute
# the option declarations?
optionDeclarations =
pkgs.lib.fixableDeclarationsOf configComponents extraArgs configFast;
# Optionally check wether all config values have corresponding
# option declarations.
config = pkgs.checker configFast
optionDeclarations configFast;