tomberek / alejandra

The Uncompromising Nix Code Formatter

Alejandra 💅

The Uncompromising Nix Code Formatter

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 optimize for the wisdom of the crowd, which comes in big part from the 2.3 million lines of code of Nixpkgs.

• ✔️ Transparent

  You won't notice the formatter after a while.

  Humans care about the content, machines about the style!

Getting started

Let's get Alejandra on our systems:

• Nix with Flakes:

  $ nix run github:kamadorueda/alejandra -- --help

• Nix stable:

  Pick one depending on your platform:

  $ 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 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.

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

   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
   ↩