flakefmt

A simple wrapper around treefmt, allowing you to use it in a nix flake.

You specify the formatter packages needed, and we provide a wrapper package around treefmt with them in PATH. Then you can use this as the package for the formatter flake output to format all the code in your project with nix fmt. There is also a check provided to see if all the code in your project has been properly formatted with nix flake check. Finally, there is an overlay that adds the wrapper package to nixpkgs as flakefmt, which has executable flakefmt.

Note that this tool uses the treefmt.toml configuration file (which should be at the root of your project), it does not use a configuration in nix (like the offical treefmt-nix does).

Usage

First, add flakefmt.url = "github:mkorje/flakefmt"; to the inputs of your flake. Now you can create an 'instance' of flakefmt with a list of the formatters needed (from nixpkgs), as specified in your treefmt.toml file. Note that the command for the formatter may not always actually be the name of the package in nixpkgs.

flakefmtEval = flakefmt.lib.eval [
  "taplo"
  "nixfmt-rfc-style"
  "black"
];

You can then use

• (flakefmtEval pkgs).package to get a derivation of the wrapper package created (with executable flakefmt)

• (flakefmtEval pkgs).overlay to get an overlay which adds the wrapper package created to nixpkgs as flakefmt

• (flakefmtEval pkgs).check <dir> to get a derivation which runs flakefmt on the directory provided (this will usually be the flake itself, accessible using the input self) and prints a diff.

Example

An example of what a very basic flake using this might look like.

flake.nix

{
  description = "example flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
    flakefmt.url = "github:mkorje/flakefmt";
  };

  outputs =
    {
      self,
      nixpkgs,
      flakefmt,
    }:
    let
      # list of systems
      systems = [
        "x86_64-linux"
        "aarch64-linux"
        "x86_64-darwin"
        "aarch64-darwin"
      ];

      # flakefmt 'instance'
      flakefmtEval = flakefmt.lib.eval [
        "black"
        "nixfmt-rfc-style"
        "taplo"
      ];

      # function to create attribute sets for each system
      forAllSystems =
        f:
        nixpkgs.lib.genAttrs systems (
          system:
          f (
            import nixpkgs {
              inherit system;
              # this adds the package `flakefmt` to nixpkgs
              overlays = [ (flakefmtEval nixpkgs).overlay ];
            }
          )
        );
    in
    {
      # used by `nix fmt`
      formatter = forAllSystems (pkgs: pkgs.flakefmt);
      # since we applied the overlay, we can use pkgs.flakefmt
      # otherwise, we can use the below to get the package
      formatter = forAllSystems (pkgs: (flakefmtEval pkgs).package);

      # used by `nix flake check`
      checks = forAllSystems (pkgs: {
        # we want to format our whole project (the flake/git repo), so we use
        # self to reference the flake itself
        formatting = (flakefmtEval pkgs).check self;
      });
    };
}