Flakes/fr: Difference between revisions

'''Nix flakes''' are an [https://nix.dev/manual/nix/stable/contributing/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 dependencies, and sharing these dependencies via lock files, and overall make it more convenient to write reproducible Nix expressions.

   inputs = {

   outputs = { self, nixpkgs }: {

}</nowiki>|name=flake.nix|lang=nix}}

<div lang="en" dir="ltr" class="mw-content-ltr">

In the example above, you can see the description, the input specified as a GitHub repository with a specific branch (here <code>nixos/nixpkgs</code> on the <code>nixos-unstable</code> branch), and an output that makes use of the input. The output simply specifies that the flake contains one package for the x86_64 architecture called <code>hello</code>. Even if your flake's output wouldn't use its input (however, in practice, that is highly unlikely), the output still needs to be a Nix function.

<div lang="en" dir="ltr" class="mw-content-ltr">

{{Note|Flakes require you to specify its outputs for each architecture separately. For more information, read the related section below.}}

<div lang="en" dir="ltr" class="mw-content-ltr">

=== Nix configuration ===

}</nowiki>|name=flake.nix|lang=nix}}

 

{{Note|If you are used to configuring your Nix settings via the NixOS configuration, these options are under <code>nix.settings</code> and not <code>nix</code>. For example, you cannot specify the automatic storage optimisation under <code>nix.optimisation.enable</code>.}}

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

   inputs.nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";

   outputs = { self, nixpkgs }:

Running <code>nix build</code> will look in the <code>legacyPackages</code> and <code>packages</code> output attributes for the corresponding [[derivations|derivation]] and then your system architecture and build the default output. If you want to specify a build attribute in a flake repository, you can run <code>nix build .#<attr></code>. In the example above, if you wanted to build the <code>packages.x86_64-linux.hello</code> attribute, run:

Using curly brackets (<code>{}</code>), we can shorten all of this and put it in a table. The code will look something like this:

The output schema is described the [https://nix.dev/manual/nix/2.33/command-ref/new-cli/nix3-flake-check.html#evaluation-checks nix flake check manual page].

<syntaxhighlight lang="nix">

   apps."<system>".default = { type = "app"; program = "..."; meta = {description = "..."; inherit otherMetaAttrs; }; };

   # Formatter (alejandra, nixfmt, treefmt-nix or nixpkgs-fmt)

</syntaxhighlight>

* <code>builtins.currentSystem</code> is non-hermetic and impure as it reflects the host system performing the evaluation. This can usually be avoided by passing the system (i.e., x86_64-linux) explicitly to derivations requiring it.

</syntaxhighlight>