nixpkgs/lib/eval-config.nix

# From an end-user configuration file (`configuration'), build a NixOS
# configuration object (`config') from which we can retrieve option
# values.

{ configuration
, system ? builtins.currentSystem
, nixpkgs ? import ./from-env.nix "NIXPKGS" /etc/nixos/nixpkgs
, pkgs ? import nixpkgs {inherit system;}
, extraArgs ? {}
, extraModules ? []
}:

let extraArgs_ = extraArgs; in

rec {
  inherit nixpkgs pkgs;

  configComponents =
    [ configuration
      ./check-config.nix
    ]
    ++ extraModules
    ++ (import ../modules/module-list.nix);

  extraArgs = extraArgs_ // {
    inherit pkgs optionDeclarations;
    modulesPath = ../modules;
  };

  config_ =
    pkgs.lib.definitionsOf configComponents extraArgs;

  # "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 config_;

  # Optionally check wether all config values have corresponding
  # option declarations.
  config = pkgs.checker config_
    config_.environment.checkConfigurationOptions
    optionDeclarations config_;
}