nixfmt-tree

A zero-setup treefmt starter to get started using the official Nix formatter.

• For nix fmt to format all Nix files, add this to the flake.nix outputs:

  formatter.${system} = nixpkgs.legacyPackages.${system}.nixfmt-tree;
  

• The same can be done more efficiently with the treefmt command, which you can get in nix-shell/nix develop by extending mkShell using

  mkShell {
    packages = [ pkgs.nixfmt-tree ];
  }
  

  You can then also use treefmt in a pre-commit/pre-push Git hook and with your editor's format-on-save feature.

• To check formatting in CI, run the following in a checkout of your Git repository:

  treefmt --ci
  

For more flexibility, you can customise this package using

pkgs.nixfmt-tree.override {
  settings = { /* additional treefmt config */ };
  runtimeInputs = [ /* additional formatter packages */ ];
}

You can achieve similar results by manually configuring treefmt:

pkgs.treefmt.withConfig {
  runtimeInputs = [ pkgs.nixfmt-rfc-style ];

  settings = {
    # Log level for files treefmt won't format
    on-unmatched = "info";

    # Assume the tree is a Git repository, will fail if it's not
    tree-root-file = ".git/index";

    # Configure nixfmt for .nix files
    formatter.nixfmt = {
      command = "nixfmt";
      includes = [ "*.nix" ];
    };
  };
}

Alternatively you can switch to the more fully-featured treefmt-nix.