nixos-and-flakes-book/docs/other-usage-of-flakes/module-system.md

Module System and Custom Options

In our previous NixOS configurations, we set various values for options to configure NixOS or Home Manager. These options are actually defined in two locations:

If you are still using nix-darwin, its configuration is similar, and its module system is implemented in nix-darwin/modules.

• NixOS: nixpkgs/nixos/modules, where all NixOS options visible on https://search.nixos.org/options are defined.
• Home Manager: home-manager/modules, where you can find all its options at https://nix-community.github.io/home-manager/options.xhtml.

The foundation of the aforementioned NixOS Modules and Home Manager Modules is a universal module system implemented in Nixpkgs, found in lib/modules.nix. The official documentation for this module system is provided below (even for experienced NixOS users, understanding this can be a challenging task):

• Module System - Nixpkgs(Module System - Nixpkgs)

Because the documentation for Nixpkgs' module system is lacking, it directly recommends reading another writing guide specifically for NixOS module system, which is clearer but might still be challenging for newcomers:

• Writing NixOS Modules - Nixpkgs(Writing NixOS Modules - Nixpkgs)

In summary, the module system is implemented by Nixpkgs and is not part of the Nix package manager. Therefore, its documentation is not included in the Nix package manager's documentation. Additionally, both NixOS and Home Manager are based on Nixpkgs' module system implementation.

What is the Purpose of the Module System?

As ordinary users, using various options implemented by NixOS and Home Manager based on the module system is sufficient to meet most of our needs. So, what are the benefits of delving into the module system for us?

In the earlier discussion on modular configuration, the core idea was to split the configuration into multiple modules and then import these modules using imports = [ ... ];. This is the most basic usage of the module system. However, using only imports = [ ... ]; allows us to import configurations defined in the module as they are without any customization, which limits flexibility. In simple configurations, this method is sufficient, but if the configuration is more complex, it becomes inadequate.

To illustrate the drawback, let's consider an example. Suppose I manage four NixOS hosts, A, B, C, and D. I want to achieve the following goals while minimizing configuration repetition:

• All hosts (A, B, C, and D) need to enable the Docker service and set it to start at boot.
• Host A should change the Docker storage driver to btrfs while keeping other settings the same.
• Hosts B and C, located in China, need to set a domestic mirror in Docker configuration.
• Host C, located in the United States, has no special requirements.
• Host D, a desktop machine, needs to set an HTTP proxy to accelerate Docker downloads.

If we purely use imports, we might have to split the configuration into several modules like this and then import different modules for each host:

› tree
.
├── docker-default.nix  # Basic Docker configuration, including starting at boot
├── docker-btrfs.nix    # Imports docker-default.nix and changes the storage driver to btrfs
├── docker-china.nix    # Imports docker-default.nix and sets a domestic mirror
└── docker-proxy.nix    # Imports docker-default.nix and sets an HTTP proxy

Doesn't this configuration seem redundant? This is still a simple example; if we have more machines with greater configuration differences, the redundancy becomes even more apparent.

Clearly, we need other means to address this redundant configuration issue, and customizing some of our own options is an excellent choice.

Before delving into the study of the module system, I emphasize once again that the following content is not necessary to learn and use. Many NixOS users have not customized any options and are satisfied with simply using imports to meet their needs. If you are a newcomer, consider learning this part when you encounter problems that imports cannot solve. That's completely okay.

Basic Structure and Usage

The basic structure of modules defined in Nixpkgs is as follows:

{ config, pkgs, ... }:

{
  imports =
    [ # import other modules here
    ];

  options = {
    # ...
  };

  config = {
    # ...
  };
}

Among these, we are already familiar with imports = [ ... ];, but the other two parts are yet to be explored. Let's have a brief introduction here:

• options = { ... };: Similar to variable declarations in programming languages, it is used to declare configurable options.
• config = { ... };: Similar to variable assignments in programming languages, it is used to assign values to the options declared in options.

The most typical usage is to, within the same Nixpkgs module, set values for other options in config = { .. }; based on the current values declared in options = { ... };. This achieves the functionality of parameterized configuration.

It's easier to understand with a direct example:

{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.programs.foo;
in {
  options.programs.foo = {
    enable = mkEnableOption "the foo program";

    package = mkOption {
      type = types.package;
      default = pkgs.jq;
      defaultText = literalExpression "pkgs.foo";
      description = "foo package to use.";
    };

    extraConfig = mkOption {
      default = "";
      example = ''
        foo bar
      '';
      type = types.lines;
      description = ''
        Extra settings for foo.
      '';
    };
  };

  config = mkIf cfg.enable {
    home.packages = [ cfg.package ];
    xdg.configFile."foo/foorc" = mkIf (cfg.extraConfig != "") {
      text = ''
        # Generated by Home Manager.

        ${cfg.extraConfig}
      '';
    };
  };
}

The module defined above introduces three options:

• programs.foo.enable: Used to control whether to enable this module.
• programs.foo.package: Allows customization of the foo package, such as using different versions, setting different compilation parameters, and so on.
• programs.foo.extraConfig: Used for customizing the configuration file of foo.

Then, in the config section, based on the values declared in these three variables in options, different settings are applied:

• If programs.foo.enable is false or undefined, no settings are applied.
  • This is achieved using lib.mkIf.
• Otherwise,
  • Add programs.foo.package to home.packages to install it in the user environment.
    • Write the value of programs.foo.extraConfig to ~/.config/foo/foorc.

This way, we can import this module in another Nix file and achieve custom configuration for foo by setting the options defined here. For example:

{ config, lib, pkgs, ... }:

{
  imports = [
    ./foo.nix
  ];

  programs.foo ={
    enable = true;
    package = pkgs.foo;
    extraConfig = ''
      foo baz
    '';
  };
}

In the example above, the way we assign values to options is actually a kind of abbreviation. When a module declares only options without config (and other special parameters of the module system), we can omit the config prefix and directly use the name of options for assignment.

Assignment and Lazy Evaluation in the Module System

The module system takes full advantage of Nix's lazy evaluation feature, which is crucial for achieving parameterized configuration.

Let's start with a simple example:

{
  description = "NixOS Flake for Test";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    home-manager = {
      url = "github:nix-community/home-manager/release-23.11";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = {nixpkgs, ...}: {
    nixosConfigurations = {
      "test" = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          ({
            config,
            lib,
            ...
          }: {
            options = {
              foo = lib.mkOption {
                default = false;
                type = lib.types.bool;
              };
            };

            # Scenario 1 (works fine)
            config.warnings = if config.foo then ["foo"] else [];

            # Scenario 2 (error: infinite recursion encountered)
            # config = if config.foo then { warnings = ["foo"];} else {};

            # Scenario 3 (works fine)
            # config = lib.mkIf config.foo {warnings = ["foo"];};
          })
        ];
      };
    };
  };
}

In the examples 1, 2, and 3 of the above configuration, the value of config.warnings depends on the value of config.foo, but their implementation methods are different. Save the above configuration as flake.nix, and then use the command nix eval .#nixosConfigurations.test.config.warnings to test examples 1, 2, and 3 separately. You will find that examples 1 and 3 work correctly, while example 2 results in an error: error: infinite recursion encountered.

Let's explain each case:

1. Example 1 evaluation flow: config.warnings => config.foo => config

   1. First, Nix attempts to compute the value of config.warnings but finds that it depends on config.foo.
   2. Next, Nix tries to compute the value of config.foo, which depends on its outer config.
   3. Nix attempts to compute the value of config, and since the contents not genuinely used by config.foo are lazily evaluated by Nix, there is no recursive dependency on config.warnings at this point.
   4. The evaluation of config.foo is completed, followed by the assignment of config.warnings, and the computation ends.

2. Example 2: config => config.foo => config

   1. Initially, Nix tries to compute the value of config but finds that it depends on config.foo.
   2. Next, Nix attempts to compute the value of config.foo, which depends on its outer config.
   3. Nix tries to compute the value of config, and this loops back to step 1, leading to an infinite recursion and eventually an error.

3. Example 3: The only difference from example 2 is the use of lib.mkIf to address the infinite recursion issue.

The key lies in the function lib.mkIf. When using lib.mkIf to define config, it will be lazily evaluated by Nix. This means that the calculation of config = lib.mkIf ... will only occur after the evaluation of config.foo is completed.

The Nixpkgs module system provides a series of functions similar to lib.mkIf for parameterized configuration and intelligent module merging:

1. lib.mkIf: Already introduced.
2. lib.mkOverride / lib.mkDefault / lib.mkForce: Previously discussed in Modularizing NixOS Configuration.
3. lib.mkOrder, lib.mkBefore, and lib.mkAfter: As mentioned above.
4. Check Option Definitions - NixOS for more functions related to option assignment (definition).

Option Declaration and Type Checking

While assignment is the most commonly used feature of the module system, if you need to customize some options, you also need to delve into option declaration and type checking. I find this part relatively straightforward; it's much simpler than assignment, and you can understand the basics by directly referring to the official documentation. I won't go into detail here.

• Option Declarations - NixOS
• Options Types - NixOS

References

• Best resources for learning about the NixOS module system? - Discourse
• NixOS modules - NixOS Wiki
• NixOS: config argument - NixOS Wiki
• Module System - Nixpkgs
• Writing NixOS Modules - Nixpkgs