nixfmt

nixfmt is a formatter for Nix code, intended to easily apply a uniform style.

You are encouraged to test this out on your code and submit any undesirable formatting you find as an issue

State

nixfmt will form the basis for the initial official standard Nix formatter, as established by RFC 166.

The established standard Nix formatting differs considerably from the original one. Be aware of this if you track the main branch. Until the first new release the main branch should be considered very unstable.

For more details, see the RFC implementation tracking issue.

Installation And Usage Instructions

nixpkgs/NixOS

nixfmt was used as the basis for the official Nix formatter with a standardized formatting. The new formatting differs considerably from the original one. A recent nixfmt version is available as pkgs.nixfmt-rfc-style in Nixpkgs. The formatting of this version differs considerably from the original nixfmt that was used as the basis for the standardised official formatter, which is also still available as pkgs.nixfmt-classic for now, though unmaintained.

So installing this nixfmt is as simple as adding to the system packages:

{ pkgs, ... }:

{
  environment.systemPackages = [ pkgs.nixfmt-rfc-style ];
}

From the repository

It's also possible to install nixfmt directly from the repository using nix-env. Also, updates are not done automatically (as it would with the system packages).

$ nix-env -f https://github.com/NixOS/nixfmt/archive/master.tar.gz -i

nix fmt

nix fmt (which is a flakes-only feature) can be configured by adding the following to flake.nix (assuming a nixpkgs input exists):

{
  outputs =
    { nixpkgs, self }:
    {
      formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;
    };
}

treefmt

treefmt can be used to format repositories consisting of different languages with one command. A possible configuration for nixfmt in treefmt.toml looks like this:

[formatter.nixfmt-rfc-style]
command = "nixfmt"
includes = ["*.nix"]

This only works when nixfmt-rfc-style is installed (see above for installation instructions).

treefmt can be integrated into text editors and CI to ensure consistent formatting for all filetypes.

treefmt-nix

treefmt-nix automatically configures the correct packages and formatters using a Nix configuration and has native support for nixfmt:

# ...
treefmt-nix.mkWrapper nixpkgs {
  # ... other options ...
  programs.nixfmt-rfc-style.enable = true;
}

More information about configuration can be found in the README.

git-hooks.nix

git-hooks.nix can automatically configure git hooks like pre-commit using nix configuration and has native support for nixfmt:

{
  pre-commit-check = nix-pre-commit-hooks.run {
    # ... other options ...
    hooks = {
      # ... other hooks ...
      nixfmt-rfc-style.enable = true;
    };
  };
}

pre-commit tool

If you have Nix files in a Git repo and you want to make sure that they’re formatted with nixfmt, then you can use the pre-commit tool from pre-commit.com:

1. Make sure that you have the pre-commit command:

   $ pre-commit --version
   pre-commit 3.7.1

2. Make sure that you’re in your Git repo:

   $ cd <path-to-git-repo>

3. Make sure that the pre-commit tool is installed as a Git pre-commit hook:

   $ pre-commit install
   pre-commit installed at .git/hooks/pre-commit

4. If you don’t already have one, then create a .pre-commit-config.yaml file.

5. Add an entry for the nixfmt hook to your .pre-commit-config.yaml file:

   repos:
       - repo: https://github.com/NixOS/nixfmt
         rev: <version>
         hooks:
               - id: nixfmt

   If you want to use a stable version of nixfmt, then replace <version> with a tag from this repo. If you want to use an unstable version of nixfmt, then replace <version> with a commit hash from this repo.

6. Try to commit a badly formatted Nix file in order to make sure that everything works.

Warning

nixfmt’s integration with the pre-commit tool is relatively new. At the moment, none of the stable releases of nixfmt can be used with the pre-commit tool. You’ll have to use an unstable version for the time being.

neovim + nixd

local nvim_lsp = require("lspconfig")
nvim_lsp.nixd.setup({
   settings = {
      nixd = {
         formatting = {
            command = { "nixfmt" },
         },
      },
   },
})

This only works when nixfmt-rfc-style is installed (see above for installation instructions).

neovim + nil

local nvim_lsp = require("lspconfig")
nvim_lsp.nil_ls.setup({
   settings = {
      ['nil'] = {
         formatting = {
            command = { "nixfmt" },
         },
      },
   },
})

This only works when nixfmt-rfc-style is installed (see above for installation instructions).

neovim + none-ls

local null_ls = require("null-ls")
null_ls.setup({
    sources = {
        null_ls.builtins.formatting.nixfmt,
    },
})

This only works when nixfmt-rfc-style is installed (see above for installation instructions).

Development

With Nix

Haskell dependencies will be built by Nix.

• Enter nix-shell
• Build with cabal new-build

Without Nix

Haskell dependencies will be built by Cabal.

• Build with cabal new-build

Usage

• nixfmt < input.nix – reads Nix code from stdin, formats it, and outputs to stdout
• nixfmt file.nix – format the file in place

About Serokell

nixfmt is maintained and funded with ❤️ by Serokell. The names and logo for Serokell are trademark of Serokell OÜ.

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