{ config, pkgs, ... }:

with pkgs.lib;

let
  isConfig = x:
    builtins.isAttrs x || builtins.isFunction x;

  optCall = f: x:
    if builtins.isFunction f
    then f x
    else f;

  mergeConfig = lhs_: rhs_:
    let
      lhs = optCall lhs_ { inherit pkgs; };
      rhs = optCall rhs_ { inherit pkgs; };
    in
    lhs // rhs //
    optionalAttrs (lhs ? packageOverrides) {
      packageOverrides = pkgs:
        optCall lhs.packageOverrides pkgs //
        optCall (attrByPath ["packageOverrides"] ({}) rhs) pkgs;
    };

  configType = mkOptionType {
    name = "nixpkgs config";
    check = traceValIfNot isConfig;
    merge = fold mergeConfig {};
  };

in

{
  options = {

    nixpkgs.config = mkOption {
      default = {};
      example = literalExample
        ''
          { firefox.enableGeckoMediaPlayer = true;
            packageOverrides = pkgs: {
              firefox60Pkgs = pkgs.firefox60Pkgs.override {
                enableOfficialBranding = true;
              };
            }; 
          }
        '';
      type = configType;
      description = ''
        The configuration of the Nix Packages collection.  (For
        details, see the Nixpkgs documentation.)  It allows you to set
        package configuration options, and to override packages
        globally through the <varname>packageOverrides</varname>
        option.  The latter is a function that takes as an argument
        the <emphasis>original</emphasis> Nixpkgs, and must evaluate
        to a set of new or overriden packages.
      '';
    };

    nixpkgs.system = mkOption {
      default = "";
      description = ''
        Specifies the Nix platform type for which NixOS should be built.
        If unset, it defaults to the platform type of your host system
        (<literal>${builtins.currentSystem}</literal>).
        Specifying this option is useful when doing distributed
        multi-platform deployment, or when building virtual machines.
      '';
    };

  };
}