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
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.
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 ];
}
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 (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 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 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 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
toolIf 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:
Make sure that you have the pre-commit
command:
$ pre-commit --version
pre-commit 3.7.1
Make sure that you’re in your Git repo:
$ cd <path-to-git-repo>
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
If you don’t already have one, then create a .pre-commit-config.yaml
file.
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.
Try to commit a badly formatted Nix file in order to make sure that everything works.
[!WARNING]
nixfmt
’s integration with thepre-commit
tool is relatively new. At the moment, none of the stable releases ofnixfmt
can be used with thepre-commit
tool. You’ll have to use an unstable version for the time being.
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).
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).
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).
Haskell dependencies will be built by Nix.
nix-shell
cabal new-build
Haskell dependencies will be built by Cabal.
cabal new-build
nixfmt < input.nix
– reads Nix code from stdin
, formats it, and outputs to stdout
nixfmt file.nix
– format the file in placenixfmt
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