nixos-and-flakes-book/docs/nixos-with-flakes/modularize-the-configuration.md

# Modularize Your NixOS Configuration

At this point, the skeleton of the entire system is configured. The current configuration structure in `/etc/nixos` should be:

```
$ 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.

As the configuration increases, it becomes difficult to maintain the configuration by relying solely on `configuration.nix` and `home.nix`. Therefore, a better solution is to use the Nix module system to split the configuration into multiple 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 i3 window manager. Its structure is:

```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, see [ryan4yin/nix-config/v0.0.2](https://github.com/ryan4yin/nix-config/tree/v0.0.2).

## `lib.mkOverride`, `lib.mkDefault`, and `lib.mkForce`

Some people use `lib.mkDefault` and `lib.mkForce` to define values in Nix files. As their names suggest, `lib.mkDefault` and `lib.mkForce` are used to set default values or force values of options.

You can read the source code of `lib.mkDefault` and `lib.mkForce` by running `nix repl -f '<nixpkgs>'` and then entering `:e lib.mkDefault`. To learn the basic usage of `nix repl`, type `:?` to see 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’

  # ......
```

`lib.mkDefault` is used to set default values of options with a priority of 1000 internally, while `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. Therefore, `lib.mkForce` has a higher priority than `lib.mkDefault`. If you define multiple values with the same priority, Nix will throw an error.

These functions are useful for modularizing the configuration. You can set default values in a low-level module (base module) and force values in a high-level module.

As an example, in my configuration <https://github.com/ryan4yin/nix-config/blob/main/modules/nixos/core-server.nix#L30>, I define default values here:

```nix
{ lib, pkgs, ... }:

{
  # ......

  nixpkgs.config.allowUnfree = lib.mkDefault false;

  # ......
}
```

And for my dekstop machine, I force the values to another value in <https://github.com/ryan4yin/nix-config/blob/main/modules/nixos/core-desktop.nix#L15>:

```nix
{ 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`

`lib.mkBefore` and `lib.mkAfter` are used to set the merge order of **list-type options**. Like `lib.mkDefault` and `lib.mkForce`, they're also useful for modularizing the configuration.

As I mentioned earlier, if you define multiple values with the same **override priority**, Nix will throw an error. However, with `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 defined.

To take a look at the source code of `lib.mkBefore`, run `nix repl -f '<nixpkgs>'` and then enter `:e lib.mkBefore`. To learn the basic usage of `nix repl`, type `:?` to see 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;

  # ......
```

So `lib.mkBefore` is a shortcut for `lib.mkOrder 500`, and `lib.mkAfter` is a shortcut for `lib.mkOrder 1500`.

To test the usage of `lib.mkBefore` and `lib.mkAfter`, let's create a simple Flake project:

```shell
# create flake.nix with the following content
› cat <<EOF | sudo tee flake.nix
{
  description = "Ryan's NixOS Flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
  };

  outputs = { self, nixpkgs, ... }@inputs: {
    nixosConfigurations = {
      "nixos-test" = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";

        modules = [
          # demo module 1, insert git at the head of list
          ({lib, pkgs, ...}: {
            environment.systemPackages = lib.mkBefore [pkgs.git];
          })

          # demo module 2, insert vim at the tail of list
          ({lib, pkgs, ...}: {
            environment.systemPackages = lib.mkAfter [pkgs.vim];
          })

          # demo module 3, just add curl to the list normally
          ({lib, pkgs, ...}: {
            environment.systemPackages = with pkgs; [curl];
          })
        ];
      };
    };
  };
}
EOF

# create flake.lock
› nix flake update

# enter nix repl environment
› nix repl
Welcome to Nix 2.13.3. Type :? for help.

# load the flake we just created
nix-repl> :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/qpmpvq5azka70lvamsca4g4sf55j8994-vim-9.0.1441.drv» ]
```

As we can see, the order of `systemPackages` is `git -> curl -> default packages -> vim`, which is the same as the order we defined in `flake.nix`.

> Though it's useless to adjust the order of `systemPackages`, it may be helpful at some other places...

## 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)