[[ch-nix-flakes]]
== Nix Flakes

:nixos-wiki-flakes: https://nixos.wiki/wiki/Flakes

Home Manager includes a `flake.nix` file for compatibility with {nixos-wiki-flakes}[Nix Flakes].
The support is still experimental and may change in backwards incompatible ways.

[[sec-flakes-prerequisties]]
=== Prerequisites

* Install Nix 2.4 or later, or have it in `nix-shell`.

* Enable experimental features `nix-command` and `flakes`.
+
** When using NixOS, add the following to your `configuration.nix` and rebuild your system.
+
[source,nix]
nix = {
  package = pkgs.nixFlakes;
  extraOptions = ''
    experimental-features = nix-command flakes
  '';
};
+
** If you are not using NixOS, add the following to `nix.conf` (located at `~/.config/nix/` or `/etc/nix/nix.conf`).
+
[source,bash]
experimental-features = nix-command flakes
+
You may need to restart the Nix daemon with, for example, `sudo systemctl restart nix-daemon.service`.
+
** Alternatively, you can enable flakes on a per-command basis with the following additional flags to `nix` and `home-manager`:
+
[source,console]
----
$ nix --extra-experimental-features "nix-command flakes" <sub-commands>
$ home-manager --extra-experimental-features "nix-command flakes" <sub-commands>
----

* Prepare your Home Manager configuration (`home.nix`).
+
Unlike the channel-based setup,
`home.nix` will be evaluated when the flake is built,
so it must be present before bootstrap of Home Manager from the flake.
See <<sec-usage-configuration>> for introduction about
writing a Home Manager configuration.

[[sec-flakes-standalone]]
=== Standalone setup

1. Set up a flake with a `flake.nix` as follows:
+
[source,nix]
----
{
  description = "Home Manager configuration of Jane Doe";

  inputs = {
    # Specify the source of Home Manager and Nixpkgs.
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    home-manager = {
      url = "github:nix-community/home-manager";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = { nixpkgs, home-manager, ... }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
    in {
      homeConfigurations.jdoe = home-manager.lib.homeManagerConfiguration {
        inherit pkgs;

        # Specify your home configuration modules here, for example,
        # the path to your home.nix.
        modules = [
          ./home.nix
        ];

        # Optionally use extraSpecialArgs
        # to pass through arguments to home.nix
      };
    };
}
----
+
[NOTE]
====
* The above example tracks the master branch of Home Manager
and nixos-unstable branch of Nixpkgs.
If you would like to use the `release-22.11` branch,
change the `home-manager` input url to `github:nix-community/home-manager/release-22.11`
and `nixpkgs` url to `github:NixOS/nixpkgs/nixos-22.11`.

* The Home Manager library is exported by the flake under
`lib.hm`.

* You can use the above `flake.nix` as a template in `~/.config/nixpkgs` by
[source,console]
$ nix flake new ~/.config/nixpkgs -t github:nix-community/home-manager
====

2. Install Home Manager and apply the configuration by
+
[source,console]
----
$ nix build --no-link <flake-uri>#homeConfigurations.jdoe.activationPackage
$ "$(nix path-info <flake-uri>#homeConfigurations.jdoe.activationPackage)"/activate
----
+
Substitute `<flake-uri>` with the flake URI of the configuration flake.
If `flake.nix` resides in `~/.config/nixpkgs`,
`<flake-uri>` may be `~/.config/nixpkgs`
as a Git tree or `path:~/.config/nixpkgs` if not.

3. Since the release `21.05`,
building a flake-based configuration is as simple as
+
[source,console]
$ home-manager switch --flake '<flake-uri>#jdoe'
+
once home-manager is installed.
+
Here, `jdoe` is a configuration specified in the flake file,
and `<flake-uri>#jdoe` will be expanded to
`<flake-uri>#homeConfigurations.jdoe.activationPackage`
and be built by Nix.

[NOTE]
====
The flake inputs are not upgraded automatically when switching.
The analogy to the command `home-manager --update ...` is `nix flake update`.

If updating more than one input is undesirable,
the command `nix flake lock --update-input <input-name>` can be used.

You can also pass flake-related options
such as `--recreate-lock-file` or `--update-input [input]`
to `home-manager` when building/switching,
and these options will be forwarded to `nix build`.
See the {nixos-wiki-flakes}[NixOS Wiki page] for detail.
====

[[sec-flakes-nixos-module]]
=== NixOS module

To use Home Manager as a NixOS module,
a bare-minimum `flake.nix` would be as follows:

[source,nix]
----
{
  description = "NixOS configuration";

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

  outputs = inputs@{ nixpkgs, home-manager, ... }: {
    nixosConfigurations = {
      hostname = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          ./configuration.nix
          home-manager.nixosModules.home-manager
          {
            home-manager.useGlobalPkgs = true;
            home-manager.useUserPackages = true;
            home-manager.users.jdoe = import ./home.nix;

            # Optionally, use home-manager.extraSpecialArgs to pass
            # arguments to home.nix
          }
        ];
      };
    };
  };
}
----

The Home Manager configuration is then part of the NixOS configuration
and is automatically rebuilt with the system when using the appropriate command
for the system, such as `nixos-rebuild switch --flake <flake-uri>`.

You can use the above `flake.nix` as a template in `/etc/nixos` by

[source,console]
$ nix flake new /etc/nixos -t github:nix-community/home-manager#nixos

[[sec-flakes-nix-darwin-module]]
=== nix-darwin module

The flake-based setup of the Home Manager nix-darwin module
is similar to that of NixOS. The `flake.nix` would be:

[source,nix]
----
{
  description = "Darwin configuration";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    darwin.url = "github:lnl7/nix-darwin";
    darwin.inputs.nixpkgs.follows = "nixpkgs";
    home-manager.url = "github:nix-community/home-manager";
    home-manager.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = inputs@{ nixpkgs, home-manager, darwin, ... }: {
    darwinConfigurations = {
      hostname = darwin.lib.darwinSystem {
        system = "x86_64-darwin";
        modules = [
          ./configuration.nix
          home-manager.darwinModules.home-manager
          {
            home-manager.useGlobalPkgs = true;
            home-manager.useUserPackages = true;
            home-manager.users.jdoe = import ./home.nix;

            # Optionally, use home-manager.extraSpecialArgs to pass
            # arguments to home.nix
          }
        ];
      };
    };
  };
}
----

and it is also rebuilt with the nix-darwin generations.
The rebuild command here may be `darwin-rebuild switch --flake <flake-uri>`.

You can use the above `flake.nix` as a template in `~/.config/darwin` by

[source,console]
$ nix flake new ~/.config/darwin -t github:nix-community/home-manager#nix-darwin