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 wisdom of the crowd, which comes in big part from the 2.3 million lines of code of Nixpkgs. Then we applied the feedback of developers who have used Nix on a day to day basis for several years.

• ✔️ 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:

  • Alejandra extension for Visual Studio Code

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 with Flakes:

  $ nix profile install github:kamadorueda/alejandra/1.0.0

• Nix stable:

  Pick one depending on your platform:

  $ nix-env -ivA aarch64-darwin -f https://github.com/kamadorueda/alejandra/tarball/1.0.0
  $ nix-env -ivA aarch64-linux -f https://github.com/kamadorueda/alejandra/tarball/1.0.0
  $ nix-env -ivA x86_64-darwin -f https://github.com/kamadorueda/alejandra/tarball/1.0.0
  $ nix-env -ivA x86_64-linux -f https://github.com/kamadorueda/alejandra/tarball/1.0.0

Then run Alejandra with:

$ alejandra --help

NixOS installation

• Nix with Flakes:

  {
    inputs = {
      nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
  
      alejandra.url = "github:kamadorueda/alejandra/1.0.0";
      alejandra.inputs.nixpkgs.follows = "nixpkgs";
    };
  
    outputs = {alejandra, nixpkgs, ...}: {
      nixosConfigurations = {
        example = nixpkgs.lib.nixosSystem rec {
          # We support: aarch64-linux, aarch64-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
            # ...
          ];
        };
      };
    };
  }

• Nix stable:

  let
    alejandra =
      (import (builtins.fetchTarball {
        url = "https://github.com/kamadorueda/alejandra/tarball/1.0.0";
        sha256 = "0000000000000000000000000000000000000000000000000000";
      }))
      # Pick one from: aarch64-linux, aarch64-linux, x86_64-darwin, x86_64-linux
      .x86_64-linux
      .outPath;
  in {
    environment.systemPackages = [alejandra];
  }

Do I need to configure anything?

• No.

Discussion

• RFC 0101 - Nix formatting

Cool libraries

• rnix-parser

Alternatives

• nixpkgs-fmt
• nixfmt

See why Alejandra was created and a comparison between alternatives 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 see: CHANGELOG.md.

Footnotes

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 -qaP --json --drv-path > before
      

   2. Now format with Alejandra and run:

      $ nix-env -qaP --json --drv-path > after
      

   As of 2022-02-21, there are 46 differences in a set of 36314 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/157760
   ↩