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 from the releases section, make it executable and have fun!

Alternatively there is an automated method. It needs: curl, sh and chmod:

• On any x86_64 Linux:

  $ curl https://raw.githubusercontent.com/kamadorueda/alejandra/main/installers/x86_64-linux.sh | sh
  

• On any AArch64/ARM64 Linux:

  $ curl https://raw.githubusercontent.com/kamadorueda/alejandra/main/installers/aarch64-linux.sh | sh
  

Then run Alejandra with:

$ ./alejandra --help

or:

$ /path/to/alejandra --help

Nix installation

• Nix with Flakes:

  $ nix profile install github:kamadorueda/alejandra
  

• Nix stable:

  Pick one depending on your platform:

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

Then run Alejandra with:

$ alejandra --help

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, and stdout.

Changelog

Please see: CHANGELOG.md.

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

   Results:

   Logical Cores	Seconds
   1	45
   2	22
   4	14
   8	10
   16	11

   ↩︎

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 2022-02-08, there are 47 differences in a set of 36278 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
   ↩︎