Alejandra 💅

The Uncompromising Nix Code Formatter

Try it on your browser! here

Features

• ✔️ Fast

  It's written in Rust and formats Nixpkgs in just a few seconds. ¹

• ✔️ Powerful

  We define a comprehensive style for all possible combinations of the Nix expression language.

• ✔️ Reliable

  High coverage, battle tested.

  From Nix's eyes, code is just the same. ²

• ✔️ Beautiful

  Beauty is subjective, right?

  We started from the original style of Nixpkgs, and then we applied the feedback of developers who have used Nix at scale for several years, producing a very well-grounded style guide.

• ✔️ Transparent

  You won't notice the formatter after a while.

  Humans care about the content, machines about the style!

• ✔️ Native

  We integrate with common code editors and workflows:

  • Visual Studio Code
  • Neovim
  • GNU Emacs
  • Doom Emacs
  • Pre-commit
  • Pre-commit-hooks.nix

Getting started

On the web editor

Please visit: kamadorueda.github.io/alejandra.

Prebuilt binaries

You can download a binary for your platform:

• aarch64-unknown-linux-musl
• armv6l-unknown-linux-musleabihf
• armv7l-unknown-linux-musleabihf
• i686-unknown-linux-musl
• x86_64-unknown-linux-musl

Make it executable ($ chmod +x) and run Alejandra with:

$ ./alejandra --help

or:

$ /path/to/alejandra --help

From Nixpkgs

Please visit: search.nixos.org/packages?query=alejandra.

Nix installation

• Nix stable:

  $ nix-env -ivf https://github.com/kamadorueda/alejandra/tarball/2.0.0
  

• Nix with Flakes:

  $ nix profile install github:kamadorueda/alejandra/2.0.0
  

Then run Alejandra with:

$ alejandra --help

NixOS installation

• Nix stable:

  let
    alejandra =
      (import (builtins.fetchTarball {
        url = "https://github.com/kamadorueda/alejandra/tarball/2.0.0";
        sha256 = "0000000000000000000000000000000000000000000000000000";
      }) {})
      .outPath;
  in {
    environment.systemPackages = [alejandra];
  }
  

• Nix with Flakes:

  {
    inputs = {
      nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
  
      alejandra.url = "github:kamadorueda/alejandra/2.0.0";
      alejandra.inputs.nixpkgs.follows = "nixpkgs";
    };
  
    outputs = {alejandra, nixpkgs, ...}: {
      nixosConfigurations = {
        example = nixpkgs.lib.nixosSystem rec {
          # We support: aarch64-darwin, aarch64-linux, i686-linux, x86_64-darwin, x86_64-linux
          system = "x86_64-linux";
  
          modules = [
            {
              environment.systemPackages = [alejandra.defaultPackage.${system}];
            }
            # Import your other modules here
            # ./path/to/my/module.nix
            # ...
          ];
        };
      };
    };
  }
  

Do I need to configure anything?

• No.

Discussion

• RFC 0101 - Nix formatting

Cool libraries

• NixEL
• Santiago
• rnix-parser

Alternatives

• nixpkgs-fmt
• nixfmt

See why Alejandra was created and a comparison between alternatives here.

Alternatively, checkout the code examples of the different formatters here.

Versioning

We use semver to version Alejandra.

Our public API consists of:

• The formatting rules (a.k.a. the style).
• The CLI tool ($ alejandra), command line flags, positional arguments, exit codes, and stdout.

Changelog

Please read: CHANGELOG.

Contributors

The following people have helped improving Alejandra.

Thank you ❤️

• Kevin Amado ~ Email, Patreon.
• Thomas Bereknyei.
• Piegames.
• Joachim Ernst.
• David Arnold.
• David Hauer.
• Ryan Mulligan.
• Fabian Möller.
• Rok Garbas.
• Yorick van Pelt.
• Rehno Lindeque.
• Jörg Thalheim.
• Vincent Ambo.
• Jamie Quigley.
• Loïc Reynier.
• Mr Hedgehog.
• Tristan Maat.
• Norbert Melzer.
• Patrick Stevens.
• Connor Baker.
• Florian Finkernagel.

Footnotes

---

1. Running on a machine with:

   • CPU: 8 physical x Intel(R) Core(TM) i7-10700K, 16 logical
   • MHz: 3800.00
   • BogoMips: 7599.80
   • Cache Size: 16384 KB

   Using:

   # x86_64-unknown-linux-gnu
   $ time alejandra --threads $threads /path/to/nixpkgs
   

   Results:

   $threads	Seconds
   1	13.4
   2	6.9
   4	3.6
   8	2.6
   16	3.1

   ↩︎

2. The methodology to claim this is:

   1. Checkout Nixpkgs and run:

      $ nix-env -qaf . --drv-path --xml > before
      

   2. Now format with Alejandra and run:

      $ nix-env -qaf . --drv-path --xml > after
      

   As of 2022-06-22, there are 41 differences in a set of 38109 derivations because of things like this:

   goDeps = ./deps.nix;
   

   Since ./deps.nix was also formatted you get a semantical difference.

   This is something that should be solved on Nixpkgs and not a bug in Alejandra. For example:

   • https://github.com/NixOS/nixpkgs/pull/178378
   • https://github.com/NixOS/nixpkgs/pull/157760
   ↩︎