# Modularize Your NixOS Configuration At this point, the skeleton of the entire system is configured. The current configuration structure in `/etc/nixos` should be as follows: ``` $ tree . ├── flake.lock ├── flake.nix ├── home.nix └── configuration.nix ``` The functions of these four files are: - `flake.lock`: An automatically generated version-lock file that records all input sources, hash values, and version numbers of the entire flake to ensure reproducibility. - `flake.nix`: The entry file that will be recognized and deployed when executing `sudo nixos-rebuild switch`. See [Flakes - NixOS Wiki](https://nixos.wiki/wiki/Flakes) for all options of flake.nix. - `configuration.nix`: Imported as a Nix module in flake.nix, all system-level configuration is currently written here. See [Configuration - NixOS Manual](https://nixos.org/manual/nixos/unstable/index.html#ch-configuration) for all options of configuration.nix. - `home.nix`: Imported by Home-Manager as the configuration of the user `ryan` in flake.nix, containing all of `ryan`'s configuration and managing `ryan`'s home folder. See [Appendix A. Configuration Options - Home-Manager](https://nix-community.github.io/home-manager/options.html) for all options of home.nix. By modifying these files, you can declaratively change the system and home directory status. However, as the configuration grows, relying solely on `configuration.nix` and `home.nix` can lead to bloated and difficult-to-maintain files. A better solution is to use the Nix module system to split the configuration into multiple Nix modules and write them in a classified manner. The Nix module system provides a parameter, `imports`, which accepts a list of `.nix` files and merges all the configuration defined in these files into the current Nix module. Note that `imports` will not simply overwrite duplicate configuration but handle it more reasonably. For example, if `program.packages = [...]` is defined in multiple modules, then `imports` will merge all `program.packages` defined in all Nix modules into one list. Attribute sets can also be merged correctly. The specific behavior can be explored by yourself. > I only found a description of `imports` in [Nixpkgs-Unstable Official Manual - evalModules Parameters](https://nixos.org/manual/nixpkgs/unstable/#module-system-lib-evalModules-parameters): `A list of modules. These are merged together to form the final configuration.` It's a bit ambiguous... With the help of `imports`, we can split `home.nix` and `configuration.nix` into multiple Nix modules defined in different `.nix` files. For example, [ryan4yin/nix-config/v0.0.2](https://github.com/ryan4yin/nix-config/tree/v0.0.2) is the configuration of my previous NixOS system with the i3 window manager. Its structure is as follows: ```shell ├── flake.lock ├── flake.nix ├── home │ ├── default.nix # here we import all submodules by imports = [...] │ ├── fcitx5 # fcitx5 input method's configuration │ │ ├── default.nix │ │ └── rime-data-flypy │ ├── i3 # i3 window manager's configuration │ │ ├── config │ │ ├── default.nix │ │ ├── i3blocks.conf │ │ ├── keybindings │ │ └── scripts │ ├── programs │ │ ├── browsers.nix │ │ ├── common.nix │ │ ├── default.nix # here we import all modules in programs folder by imports = [...] │ │ ├── git.nix │ │ ├── media.nix │ │ ├── vscode.nix │ │ └── xdg.nix │ ├── rofi # rofi launcher's configuration │ │ ├── configs │ │ │ ├── arc_dark_colors.rasi │ │ │ ├── arc_dark_transparent_colors.rasi │ │ │ ├── power-profiles.rasi │ │ │ ├── powermenu.rasi │ │ │ ├── rofidmenu.rasi │ │ │ └── rofikeyhint.rasi │ │ └── default.nix │ └── shell # shell/terminal related configuration │ ├── common.nix │ ├── default.nix │ ├── nushell │ │ ├── config.nu │ │ ├── default.nix │ │ └── env.nu │ ├── starship.nix │ └── terminals.nix ├── hosts │ ├── msi-rtx4090 # My main machine's configuration │ │ ├── default.nix # This is the old configuration.nix, but most of the content has been split out to modules. │ │ └── hardware-configuration.nix # hardware & disk related configuration, autogenerated by nixos │ └── nixos-test # my test machine's configuration │ ├── default.nix │ └── hardware-configuration.nix ├── modules # some common NixOS modules that can be reused │ ├── i3.nix │ └── system.nix └── wallpaper.jpg # wallpaper ``` For more details, you can refer to the [ryan4yin/nix-config/v0.0.2](https://github.com/ryan4yin/nix-config/tree/v0.0.2) repository. ## `lib.mkOverride`, `lib.mkDefault`, and `lib.mkForce` In Nix, some people use `lib.mkDefault` and `lib.mkForce` to define values. These functions are designed to set default values or force values of options. You can explore the source code of `lib.mkDefault` and `lib.mkForce` by running `nix repl -f ''` and then entering `:e lib.mkDefault`. To learn more about `nix repl`, type `:?` for the help information. Here's the source code: ```nix # ...... mkOverride = priority: content: { _type = "override"; inherit priority content; }; mkOptionDefault = mkOverride 1500; # priority of option defaults mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default mkImageMediaOverride = mkOverride 60; # image media profiles can be derived by inclusion into host config, hence needing to override host config, but do allow user to mkForce mkForce = mkOverride 50; mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’ # ...... ``` In summary, `lib.mkDefault` is used to set default values of options with a priority of 1000 internally, and `lib.mkForce` is used to force values of options with a priority of 50 internally. If you set a value of an option directly, it will be set with a default priority of 1000, the same as `lib.mkDefault`. The lower the `priority` value, the higher the actual priority. As a result, `lib.mkForce` has a higher priority than `lib.mkDefault`. If you define multiple values with the same priority, Nix will throw an error. Using these functions can be very helpful for modularizing the configuration. You can set default values in a low-level module (base module) and force values in a high-level module. For example, in my configuration at [ryan4yin/nix-config/blob/main/modules/nixos/core-server.nix#L30](https://github.com/ryan4yin/nix-config/blob/main/modules/nixos/core-server.nix#L30), I define default values like this: ```nix{6} { lib, pkgs, ... }: { # ...... nixpkgs.config.allowUnfree = lib.mkDefault false; # ...... } ``` Then, for my desktop machine, I override the value in [ryan4yin/nix-config/blob/main/modules/nixos/core-desktop.nix#L15](https://github.com/ryan4yin/nix-config/blob/main/modules/nixos/core-desktop.nix#L15) like this: ```nix{10} { lib, pkgs, ... }: { # import the base module imports = [ ./core-server.nix ]; # override the default value defined in the base module nixpkgs.config.allowUnfree = lib.mkForce true; # ...... } ``` ## `lib.mkOrder`, `lib.mkBefore`, and `lib.mkAfter` In addition to `lib.mkDefault` and `lib.mkForce`, there are also `lib.mkBefore` and `lib.mkAfter`, which are used to set the merge order of \*\*list -type options\*\*. These functions further contribute to the modularization of the configuration. As mentioned earlier, when you define multiple values with the same **override priority**, Nix will throw an error. However, by using `lib.mkOrder`, `lib.mkBefore`, or `lib.mkAfter`, you can define multiple values with the same override priority, and they will be merged in the order you specify. To examine the source code of `lib.mkBefore`, you can run `nix repl -f ''` and then enter `:e lib.mkBefore`. To learn more about `nix repl`, type `:?` for the help information: ```nix # ...... mkOrder = priority: content: { _type = "order"; inherit priority content; }; mkBefore = mkOrder 500; mkAfter = mkOrder 1500; # The default priority for things that don't have a priority specified. defaultPriority = 100; # ...... ``` Therefore, `lib.mkBefore` is a shorthand for `lib.mkOrder 500`, and `lib.mkAfter` is a shorthand for `lib.mkOrder 1500`. To test the usage of `lib.mkBefore` and `lib.mkAfter`, let's create a simple Flake project: ```shell{16-29} # Create flake.nix with the following content › cat < :lf . Added 9 variables. # Check the order of systemPackages nix-repl> outputs.nixosConfigurations.nixos-test.config.environment.systemPackages [ «derivation /nix/store/0xvn7ssrwa0ax646gl4hwn8cpi05zl9j-git-2.40.1.drv» «derivation /nix/store/7x8qmbvfai68sf73zq9szs5q78mc0kny-curl-8.1.1.drv» «derivation /nix/store/bly81l03kh0dfly9ix2ysps6kyn1hrjl-nixos-container.drv» ...... ...... «derivation /nix/store/qpmpv q5azka70lvamsca4g4sf55j8994-vim-9.0.1441.drv» ] ``` As you can see, the order of `systemPackages` is `git -> curl -> default packages -> vim`, which matches the order we defined in `flake.nix`. > Although adjusting the order of `systemPackages` may not be useful in practice, it can be helpful in other scenarios. ## References - [Nix modules: Improving Nix's discoverability and usability](https://cfp.nixcon.org/nixcon2020/talk/K89WJY/) - [Module System - Nixpkgs](https://github.com/NixOS/nixpkgs/blob/nixos-unstable/doc/module-system/module-system.chapter.md)