Flakes: Difference between revisions

'''Nix flakes''' are an [https://nix.dev/manual/nix/stable/development/experimental-features experimental feature] first introduced in the 2.4 [[Nix]] release,{{Cite manual|nix|development/experimental-features|number=13.8|title=Experimental Features|subsection=xp-feature-flakes|subtitle=flakes}}{{Cite manual|nix|release-notes/rl-2.4|number=14.27|title=Release 2.4 (2021-11-01)}} aiming to address a number of areas of improvement for the Nix ecosystem: they provide a uniform structure for Nix projects, allow for pinning specific versions of each dependency, and sharing these dependencies via lock files, and overall make it more convenient to write reproducible Nix expressions.

Flakes also allow for locking references and versions, which can then be queried and updated programmatically via the inputs {{cite manual|nix|command-ref/new-cli/nix3-flake-lock|number=7.5.19|title=nix flake lock}}{{cite manual|nix|command-ref/new-cli/nix3-flake-info|number=7.5.17|title=nix flake info}}. Additionally, an experimental CLI utility accepts flake references for expressions that build, run, and deploy packages.{{Cite manual|nix|command-ref/new-cli/nix|number=8.5.1|title=nix}}

Minimally, a flake file contains a description of the flake, a set of input dependencies and an output. You can generate a very basic flake file after [[#Setup]] using <code>nix flake init</code>. This will populate the current directory with a file called flake.nix that will contain something akin to:

--experimental-features 'nix-command flakes'

Add the following to the [[NixOS system configuration#Usage|NixOS configuration]]:

nix.settings.experimental-features = [ "nix-command" "flakes" ];

nix.settings.experimental-features = [ "nix-command" "flakes" ];

{{Note | The  [https://github.com/DeterminateSystems/nix-installer Determinate Nix Installer] enables flakes by default, but installs the proprietary Determinate Nix.}}

This flake produces a single flake output <code>packages</code>. And within that, <code>x86_64-linux</code> is a system-specific attribute set. And within that, two package [[derivations]] <code>default</code> and <code>hello</code>. You can find outputs with the {{Nix Manual|name=show command|anchor=command-ref/new-cli/nix3-flake-show}} of a flake as shown below:

         ├───default: package 'hello-2.12.3'

         └───hello: package 'hello-2.12.3'

* <code>nixConfig</code> is an attribute set of values which reflect the [https://nix.dev/manual/nix/stable/command-ref/conf-file.html values given to nix.conf]. This can extend the normal behavior of a user's nix experience by adding flake-specific configuration, such as a [[Binary Cache|binary cache]].

[https://nix.dev/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-inputs The nix flake inputs manual].

[https://nix.dev/manual/nix/stable/command-ref/new-cli/nix3-flake.html#flake-references The nix flake references manual].

inputs.self.submodules = true;

The output schema is described the [https://nix.dev/manual/nix/stable/command-ref/new-cli/nix3-flake-check.html#evaluation-checks nix flake check manual page].

  inputs = {

  outputs = { self, nixpkgs }: let

      systems = [ "x86_64-linux" "aarch64-linux" ];

      forAllSystems = f: builtins.listToAttrs (map (system: {

        name = system;

        value = f system;

      }) systems);

       packages = forAllSystems (system: let

        hello = pkgs.hello;

        default = pkgs.hello;

      });

    };

To allow for [[Unfree software|unfree software]] in a flake project, you need to explicitly allow it by setting <code>config.allowUnfree = true;</code> when importing Nixpkgs.

       pkgs = import nixpkgs { inherit system; config.allowUnfree = true; };

<syntaxhighlight lang=console>

$ nix build github:nixos/nixpkgs?ref=pull/<PR_NUMBER>/head#<PACKAGE>

</syntaxhighlight>

<syntaxhighlight lang=console>

$ git fetch upstream pull/<PR_NUMBER>/head && git checkout FETCH_HEAD && nix build .#PACKAGE

</syntaxhighlight>

<syntaxhighlight lang=console>

$ git add --intent-to-add extra/flake.nix

$ git update-index --skip-worktree --assume-unchanged extra/flake.nix

</syntaxhighlight>

<syntaxhighlight lang=console>

$ cd ~/libdep-src-checkout/

$ nix develop # Or `nix-shell` if applicable.

$ export prefix="./install" # configure nix to install it here

$ buildPhase  # build it like nix does

$ installPhase # install it like nix does

</syntaxhighlight>

<syntaxhighlight lang=console>

$ cd ~/consumexe-src-checkout/

$ nix develop --redirect libdep ~/libdep-src-checkout/install

$ echo $buildInputs | tr " " "\n" | grep libdep

$ # Output should show ~/libdep-src-checkout/ so you know it worked

</syntaxhighlight>

* [https://nix.dev/manual/nix/stable/command-ref/new-cli/nix3-flake.html Nix flake command reference manual] - Many additional details about flakes, and their parts.