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nixos-and-flakes-book/docs/nixos-with-flakes/nixos-flake-and-module-system.md
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The combination ability of Flakes and Nixpkgs module system

Nixpkgs Module Structure Explained

The detailed workings of this module system will be introduced in the following Modularizing NixOS Configuration section. Here, we'll just cover some basic knowledge.

You might be wondering why the /etc/nixos/configuration.nix configuration file adheres to the Nixpkgs Module definition and can be referenced directly within the flake.nix.

This is because the Nixpkgs repository contains a significant amount of NixOS implementation source code, primarily written in Nix. To manage and maintain such a large volume of Nix code and to allow users to customize various functions of their NixOS systems, a modular system for Nix code is essential.

This modular system for Nix code is also implemented within the Nixpkgs repository and is primarily used for modularizing NixOS system configurations. However, it is also widely used in other contexts, such as nix-darwin and home-manager. Since NixOS is built on this modular system, it is only natural that its configuration files, including /etc/nixos/configuration.nix, are Nixpkgs Modules.

Before delving into the subsequent content, it's essential to have a basic understanding of how this module system operates.

Here's a simplified structure of a Nixpkgs Module:

{lib, config, options, pkgs, ...}:
{
  # Importing other Modules
  imports = [
    # ...
    ./xxx.nix
  ];
  for.bar.enable = true;
  # Other option declarations
  # ...
}

The definition is actually a Nix function, and it has five automatically generated, automatically injected, and declaration-free parameters provided by the module system:

  1. lib: A built-in function library included with nixpkgs, offering many practical functions for operating Nix expressions.
  2. config: A set of all options' values in the current environment, which will be used extensively in the subsequent section on the module system.
  3. options: A set of all options defined in all Modules in the current environment.
  4. pkgs: A collection containing all nixpkgs packages, along with several related utility functions.
    • At the beginner stage, you can consider its default value to be nixpkgs.legacyPackages."${system}", and the value of pkgs can be customized through the nixpkgs.pkgs option.
  5. modulesPath: A parameter available only in NixOS, which is a path pointing to nixpkgs/nixos/modules.

Passing Non-default Parameters to Submodules

If you need to pass other non-default parameters to submodules, you will need to use some special methods to manually specify these non-default parameters.

The Nixpkgs module system provides two ways to pass non-default parameters:

  1. The specialArgs parameter of the nixpkgs.lib.nixosSystem function
  2. Using the _module.args option in any module to pass parameters

The official documentation for these two parameters is buried deep and is vague and hard to understand. If readers are interested, I will include the links here:

  1. specialArgs: There are scattered mentions related to it in the NixOS Manual and the Nixpkgs Manual.
  2. _module.args:

In short, specialArgs and _module.args both require an attribute set as their value, and they serve the same purpose, passing all parameters in the attribute set to all submodules. The difference between them is:

  1. The _module.args option can be used in any module to pass parameters to each other, which is more flexible than specialArgs, which can only be used in the nixpkgs.lib.nixosSystem function.
  2. _module.args is declared within a module, so it must be evaluated after all modules have been evaluated before it can be used. This means that if you use the parameters passed through _module.args in imports = [ ... ];, it will result in an infinite recursion error. In this case, you must use specialArgs instead.

The NixOS community generally recommends prioritizing the use of the _module.args option and resorting to specialArgs only when _module.args cannot be used.

Suppose you want to pass a certain dependency to a submodule for use. You can use the specialArgs parameter to pass the inputs to all submodules:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    another-input.url = "github:username/repo-name/branch-name";
  };

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

      # Set all inputs parameters as special arguments for all submodules,
      # so you can directly use all dependencies in inputs in submodules
      specialArgs = { inherit inputs; };
      modules = [
        ./configuration.nix
      ];
    };
  };
}

Or you can achieve the same effect using the _module.args option:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    another-input.url = "github:username/repo-name/branch-name";
  };
  outputs = inputs@{ self, nixpkgs, another-input, ... }: {
    nixosConfigurations.my-nixos = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
        {
          # Set all inputs parameters as special arguments for all submodules,
          # so you can directly use all dependencies in inputs in submodules
          _module.args = { inherit inputs; };
        }
      ];
    };
  };
}

Choose one of the two methods above to modify your configuration, and then you can use the inputs parameter in /etc/nixos/configuration.nix. The module system will automatically match the inputs defined in specialArgs and inject it into all submodules that require this parameter:

# Nix will match by name and automatically inject the inputs
# from specialArgs/_module.args into the third parameter of this function
{ config, pkgs, inputs, ... }:
{
  # ...
}

The next section will demonstrate how to use specialArgs/_module.args to install system software from other flake sources.

Installing System Software from Other Flake Sources

The most common requirement for managing a system is to install software, and we have already seen in the previous section how to install packages from the official nixpkgs repository using environment.systemPackages. These packages all come from the official nixpkgs repository.

Now, we will learn how to install software packages from other flake sources, which is much more flexible than installing directly from nixpkgs. The main use case is to install the latest version of a software that is not yet added or updated in Nixpkgs.

Taking the Helix editor as an example, here's how to compile and install the master branch of Helix directly.

First, add the helix input data source to flake.nix:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";

    # helix editor, use the master branch
    helix.url = "github:helix-editor/helix/master";
  };

  outputs = inputs@{ self, nixpkgs, ... }: {
    nixosConfigurations.my-nixos = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      specialArgs = { inherit inputs; };
      modules = [
        ./configuration.nix

        # This module works the same as the `specialArgs` parameter we used above
        # choose one of the two methods to use
        # { _module.args = { inherit inputs; };}
      ];
    };
  };
}

Next, you can reference this flake input data source in configuration.nix:

{ config, pkgs, inputs, ... }:
{
  # ...
  environment.systemPackages = with pkgs; [
    git
    vim
    wget
    curl
    # Here, the helix package is installed from the helix input data source
    inputs.helix.packages."${pkgs.system}".helix
  ];
  # ...
}

Make the necessary changes and deploy with sudo nixos-rebuild switch. The deployment will take much longer this time because Nix will compile the entire Helix program from source.

After deployment, you can directly test and verify the installation using the hx command in the terminal.

Additionally, if you just want to try out the latest version of Helix and decide whether to install it on your system later, there is a simpler way to do it in one command (but as mentioned earlier, compiling from source will take a long time):

nix run github:helix-editor/helix/master

We will go into more detail on the usage of nix run in the following section Usage of the New CLI.

Leveraging Features from Other Flakes Packages

In fact, this is the primary functionality of Flakes — a flake can depend on other flakes, allowing it to utilize the features they provide. It's akin to how we incorporate functionalities from other libraries when writing programs in TypeScript, Go, Rust, and other programming languages.

The example above, using the latest version from the official Helix Flake, illustrates this functionality. More use cases will be discussed later, and here are a few examples referenced for future mention:

  • Getting Started with Home Manager: This introduces the community's Home-Manager as a dependency, enabling direct utilization of the features provided by this Flake.
  • Downgrading or Upgrading Packages: Here, different versions of Nixpkgs are introduced as dependencies, allowing for flexible selection of packages from various versions of Nixpkgs.

More Flakes Tutorials

Up to this point, we have learned how to use Flakes to configure NixOS systems. If you have more questions about Flakes or want to learn more in-depth, please refer directly to the following official/semi-official documents: