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https://github.com/nixos/nixfmt

The official (but not yet stable) formatter for Nix code
https://github.com/nixos/nixfmt

formatter haskell nix

Last synced: 5 months ago
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The official (but not yet stable) formatter for Nix code

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README 

          
# `nixfmt`



`nixfmt` is the official formatter for Nix language code, intended to easily apply a uniform style.



![Build Status](https://github.com/NixOS/nixfmt/actions/workflows/main.yml/badge.svg?branch=master)



## State



`nixfmt` was originally developed by [Serokell](https://github.com/serokell).

It was used as the basis for the official standardised Nix formatter, as established by [RFC 166](https://github.com/NixOS/rfcs/pull/166).



The official standard differs considerably from the original implementation.

Be aware of this if you track the [`master`](https://github.com/NixOS/nixfmt/tree/master) branch.

Until the [next release](https://github.com/NixOS/nixfmt/issues/272), expect `nixfmt` to change.



A recent version of `nixfmt` is available as `pkgs.nixfmt-rfc-style` in Nixpkgs.

The original `nixfmt` is still available as `pkgs.nixfmt-classic`, but it is unmaintained and will eventually be removed.



For more details, see the [RFC implementation tracking issue](https://github.com/NixOS/nixfmt/issues/153).



## Installation



> [!NOTE]

> `nixfmt` can only process one file at a time.

> Consider using a configuration helper for [formatting a project](#in-a-project).



### In the environment



#### NixOS



To install `nixfmt` on NixOS for all users, add it to the [`environment.systemPackages`](https://search.nixos.org/options?show=environment.systemPackages) configuration option:



```nix

{ pkgs, ... }:

{

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

}

```



To install it on NixOS for a particular user, add it to the [`users.users..packages`](https://search.nixos.org/options?show=users.users.%3Cname%3E.packages) configuration option:



```

{ pkgs, ... }:

{

  users.users.example-user-name.packages = [ pkgs.nixfmt-rfc-style ];

}

```



#### Home Manager



To install `nixfmt` in Home Manager, add it adding to the [`home.packages`](https://nix-community.github.io/home-manager/options.xhtml#opt-home.packages) configuration option:



```nix

{ pkgs, ... }:

{

  home.packages = [ pkgs.nixfmt-rfc-style ];

}

```



#### Declarative shell environment



To make `nixfmt` available in a shell environment invoked with [`nix-shell`](https://nix.dev/manual/nix/2.28/command-ref/nix-shell), add it to the `packages` argument of `mkShell`:



```nix

{ pkgs }:

pkgs.mkShellNoCC {

  packages = [ pkgs.nixfmt-rfc-style ];

}

```



### In an editor



#### neovim + nixd



```lua

local nvim_lsp = require("lspconfig")

nvim_lsp.nixd.setup({

   settings = {

      nixd = {

         formatting = {

            command = { "nixfmt" },

         },

      },

   },

})

```



> [!NOTE]

> This only works when `nixfmt` is available [in the environment](#in-the-environment).



#### neovim + nil



```lua

local nvim_lsp = require("lspconfig")

nvim_lsp.nil_ls.setup({

   settings = {

      ['nil'] = {

         formatting = {

            command = { "nixfmt" },

         },

      },

   },

})

```



> [!NOTE]

> This only works when `nixfmt` is available [in the environment](#in-the-environment).



#### neovim + none-ls



```lua

local null_ls = require("null-ls")

null_ls.setup({

    sources = {

        null_ls.builtins.formatting.nixfmt,

    },

})

```



> [!NOTE]

> This only works when `nixfmt` is available [in the environment](#in-the-environment).



### In a project



#### `treefmt-nix`



[`treefmt-nix`](https://github.com/numtide/treefmt-nix) automatically configures the correct packages and formatters for [`treefmt`](https://github.com/numtide/treefmt) using the Nix language, and has native support for `nixfmt`:



```nix

{ pkgs, treefmt-nix }:

treefmt-nix.mkWrapper pkgs {

  programs.nixfmt.enable = true;

};

```



#### `treefmt`



[`treefmt`](https://github.com/numtide/treefmt) can also be used directly:



```toml

# treefmt.toml

[formatter.nixfmt-rfc-style]

command = "nixfmt"

includes = ["*.nix"]

```



> [!NOTE]

> This only works when `nixfmt` is available [in the environment](#in-the-environment).



#### `git-hooks.nix`



[`git-hooks.nix`](https://github.com/cachix/git-hooks.nix) can automatically configure Git hooks like [`pre-commit`](https://pre-commit.com/) using the Nix language, and has native support for `nixfmt`:



```nix

{ pkgs, git-hooks }:

{

  pre-commit-check = git-hooks.run {

    hooks = {

      nixfmt-rfc-style.enable = true;

    };

  };

  shell = pkgs.mkShellNoCC {

    packages = [ pre-commit-check.enabledPackages ];

    shellHook = ''

      ${pre-commit-check.shellHook}

    '';

  };

}

```



#### `pre-commit`



[`pre-commit`](https://pre-commit.com/) can also be used directly:



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



    ```console

    $ pre-commit --version

    pre-commit 3.7.1

    ```



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



    ```console

    $ cd 

    ```



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



    ```console

    $ 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:



    ```yaml

    repos:

        - repo: https://github.com/NixOS/nixfmt

          rev: 

          hooks:

                - id: nixfmt

    ```



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



#### `git mergetool`



`nixfmt` provides a mode usable by [`git mergetool`](https://git-scm.com/docs/git-mergetool)

via `--mergetool` that allows resolving formatting-related conflicts automatically in many cases.



It can be installed by any of these methods:



- For only for the current repo, run:

  ```

  git config mergetool.nixfmt.cmd 'nixfmt --mergetool "$BASE" "$LOCAL" "$REMOTE" "$MERGED"'

  git config mergetool.nixfmt.trustExitCode true

  ```

- For all repos with a mutable config file, run

  ```

  git config --global mergetool.nixfmt.cmd 'nixfmt --mergetool "$BASE" "$LOCAL" "$REMOTE" "$MERGED"'

  git config --global mergetool.nixfmt.trustExitCode true

  ```

- For all repos with a NixOS-provided config file, add this to your `configuration.nix`:

  ```nix

  programs.git.config = {

    mergetool.nixfmt = {

      cmd = "nixfmt --mergetool \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"";

      trustExitCode = true;

    };

  };

  ```

- For all repos with a home-manager-provided config file, add this to your `home.nix`:

  ```nix

  programs.git.extraConfig = {

    mergetool.nixfmt = {

      cmd = "nixfmt --mergetool \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"";

      trustExitCode = true;

    };

  };

  ```



Then, when `git merge` or `git rebase` fails, run

```

git mergetool -t nixfmt .

# or, only for some specific files

git mergetool -t nixfmt FILE1 FILE2 FILE3

```



and some `.nix` files will probably get merged automagically.



Note that files that `git` merges successfully even before `git mergetool`

will be ignored by \`git mergetool\`.



If you don't like the result, run

```

git restore --merge .

# or, only for some specific files

git restore --merge FILE1 FILE2 FILE3

```



to return back to the unmerged state.



#### `nix fmt` (experimental)



[nix fmt](https://nix.dev/manual/nix/latest/command-ref/new-cli/nix3-fmt) (part of the [`flakes` experimental feature](https://nix.dev/manual/nix/latest/development/experimental-features#xp-feature-flakes)) can be configured to use `nixfmt` by setting the `formatter` flake output to the respective package (assuming a `nixpkgs` flake input exists):



```nix

# flake.nix

{

  outputs =

    { nixpkgs, self }:

    {

      formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;

    };

}

```



## 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 :heart: by

[Serokell](https://serokell.io/). The names and logo for Serokell are trademark

of Serokell OÜ.



We love open source software! See

[our other projects](https://serokell.io/community?utm_source=github) or

[hire us](https://serokell.io/hire-us?utm_source=github) to design, develop and

grow your idea!