Provide a GitHub Action

[grahamc]

It'd be really great if I could hook up a GitHub Action with nixfmt so it provided annotations to the code about reformatting that needs doing.

[Lucus16]

I think that would be cool too, but I don't really know anything about Github actions. There is already the -c flag to check if files are formatted, for use in CI. If you just want a list of files that need formatting, it should be pretty trivial and I'd appreciate a PR. If github supports annotations per line, I think that would be easiest to achieve with a separate diff tool since nixfmt does not remember source locations after parsing.

[omkumar312]

I suggest adding a GitHub Action to check Nix formatting using nixfmt and annotate issues in pull requests.

Create a workflow: <-add a file named->.github/workflows/nixfmt.yml

`name: Check Nix Formatting

on: [pull_request, push]

jobs:
nixfmt:
runs-on: ubuntu-latest

steps:
  - uses: actions/checkout@v3
  - run: nix-env -iA nixpkgs.nixfmt
  - run: nixfmt --check .
  - run: nixfmt --diff . | sed 's/^/::error file=/' || true`

This workflow installs nixfmt, checks formatting, and uses GitHub's problem matcher to annotate any reformatting needed directly in the pull request.

[omkumar312]

steps workflow

1. Checkout Code: Pulls the latest code from the repository.
2. Install nixfmt: Installs the nixfmt tool.
3. Check Formatting: Verifies if the Nix files are correctly formatted.
4. Annotate Issues: Adds inline annotations to the pull request for any formatting issues.

The action ensures code the highlights formatting issues directly in pull requests.

[dasJ]

I don't think this is implementable anymore with the removal of recursive mode. A GH action in treefmt would probably be more appropriate.

[infinisil]

Briefly discussed in the meeting today, agreed with @dasJ

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/formatting-team-meeting-2024-10-01/53763/1

[cdepillabout]

While I can certainly understand if the nixfmt considers this out of scope, I disagree with @dasJ:

I don't think this is implementable anymore with the removal of recursive mode.

It is certainly still possible to implement a GitHub Action that finds all the .nix files in a repo, runs nixfmt on them, and fails if any of them are not formatted.

I'm currently in the process of switching my company over to using nixfmt-rfc-style, and not having an out-of-the-box, easy-to-use GitHub Action makes it just slightly harder to switch over.

[infinisil]

I'd like to reconsider this. Being able to have a simple nixfmt check with GHA would be nice. Some brainstorming on that:

Could have an interface like this:

# Put this in your repos .github/workflows/nixfmt.yml
name: "Check Nix formatting"
on:
  pull_request:
  push:
jobs:
  check:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v4
    - uses: cachix/install-nix-action@v30
    - uses: NixOS/treefmt-nix-action@v1
      with:
        nixpkgs: https://github.com/NixOS/nixpkgs/archive/f1c5d6d23fd57011b78cb2424f5c73e68c9e8879.tar.gz
        treefmt-nix: https://github.com/numtide/treefmt-nix/3d0579f5cc93436052d94b73925b48973a104204.tar.gz
        module: |
          {
            # See https://github.com/numtide/treefmt-nix/tree/main/programs for more options 
            programs.nixfmt-rfc-style.enable = true;
          }
        # As an alternative to the above:
        # file: treefmt.nix
        # flake: .#treefmt

In the back this would:

• Create a treefmt.nix file like this:

  {
    # Alternatively, get pinned versions from npins, flakes, niv, gridlock or so
    nixpkgs ? fetchTarball {
      url = "https://github.com/NixOS/nixpkgs/archive/f1c5d6d23fd57011b78cb2424f5c73e68c9e8879.tar.gz";
      # sha256 = "";
    },
    treefmt-nix ? fetchTarball {
      url = "https://github.com/numtide/treefmt-nix/3d0579f5cc93436052d94b73925b48973a104204.tar.gz";
      # sha256 = "";
    },
    system ? builtins.currentSystem,
    pkgs ? import nixpkgs {
      config = { };
      overlays = [ ];
      inherit system;
    };
  }
  let
    treefmtEval = (import treefmt-nix).evalModule pkgs
      {
        # See https://github.com/numtide/treefmt-nix/tree/main/programs for more options 
        programs.nixfmt-rfc-style.enable = true;
      }
      ;
  };
  in
  {
    # Use this with `mkShell { inputsFrom = [ treefmt.shell ]; packages = [ ... ]; }
    # shell = treefmtEval.config.build.devShell;
    pkg = treefmtEval.config.build.wrapper;
  };

• Run nix-build treefmt.nix -A pkg
• Clone the users repo
• Run result/bin/treefmt . in the repo

The action should also tell the user that they can copy that generated file and put it in treefmt.nix to allow more reuse with other Nix code. Or alternatively use flake.nix.

This could be implemented using a composite GitHub Action.

[MattSturgeon]

If this is added, it'd be great for it to have PR annotation message support, as-per #76 (comment). Ideally it'd also have line-wise annotations, not just per-file. See workflow commands docs.

Somewhat related treefmt issues:

• GitHub workflow PR annotation messages numtide/treefmt#526
• github action formatter numtide/treefmt#312

EDIT: I was convinced by discussion on numtide/treefmt#526 that inline PR annotations aren't suitable when using a treewide auto-formatter. They could still make sense if files are being formatted on a per-file basis, as is currently the case in nixpkgs (although that's soon to change: NixOS/nixpkgs#380990).