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fix: polish by chatgpt

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Ryan Yin 2023-07-04 14:00:32 +08:00
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docs/nixos-with-flakes/modularize-the-configuration.md
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@ -83,96 +83,6 @@ For example, [ryan4yin/nix-config/v0.0.2](https://github.com/ryan4yin/nix-config
└── 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;
# ......
```
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`