This run-fourmolu GitHub Action helps to ensure that your Haskell project is formatted with Fourmolu. The action tries to find all Haskell source code files in your repository and fails if any of them are not formatted. In case of failure it prints the diff between the actual contents of the file and its formatted version.

In the simple case all you need to do is to add this step to your job:

- uses: haskell-actions/run-fourmolu@v9
  with:
    version: "0.13.0.0"

The 0.13.0.0 should be replaced with the version of Fourmolu you want to use. See Fourmolu releases for all Fourmolu versions. If you don't specify this Fourmolu version input, the latest version of Fourmolu will be used.

The @v9 after haskell-actions/run-fourmolu should be replaced with the version of the run-fourmolu Action you want to use. See run-fourmolu releases for all run-fourmolu versions available.

You can use the latest version of Fourmolu by specifying version: "latest":

- uses: haskell-actions/run-fourmolu@v9
  with:
    version: "latest"

Alternatively, if you leave out the version input, then "latest" will be assumed.

It is recommended that users always specify the exact version of Fourmolu they want to use, since new releases of Fourmolu are often not backwards compatible. Your CI may break when there is a new release of Fourmolu.

Here's a full YAML file you can copy and paste into your repo that runs run-fourmolu. Add this as a file like .github/workflows/fourmolu.yaml. This Workflow will run everytime you push to a branch.

name: fourmolu
on: push
jobs:
  format:
    runs-on: ubuntu-latest
    steps:
      # Note that you must checkout your code before running haskell-actions/run-fourmolu
      - uses: actions/checkout@v3
      - uses: haskell-actions/run-fourmolu@v9
        with:
          version: "0.13.0.0"

Here's a more complicated example that shows more options being used:

- uses: haskell-actions/run-fourmolu@v9
  with:
    # Use fourmolu-0.13.0.0.  If you don't specify this, then the latest
    # release of fourmolu will be used.
    version: "0.13.0.0"

    # Only check the format of .hs in the src/ directory
    # except src/Main.hs.
    pattern: |
      src/**/*.hs
      !src/Main.hs

    # Don't follow symbolic links to .hs files.
    follow-symbolic-links: false

    # Extra args to pass to fourmolu on the command line.
    extra-args: "--indent-wheres true"

    # Change to the ./my-haskell-code/ directory before running this action.
    working-directory: "./my-haskell-code"

See docs on pattern syntax.

If you are using a build matrix, then it is more efficient to have a separate job for checking of formatting:

jobs:
  fourmolu:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: haskell-actions/run-fourmolu@v9
        with:
          version: "0.13.0.0"
  build:
    runs-on: ubuntu-latest
    needs: fourmolu
    ...

Here, the build job depends on the fourmolu job and will not run unless the fourmolu job passes.

The available options are defined in ./action.yml. See that file for more explanation.

In order to do development on this repo, you first need to install NodeJS and npm to your system. This has been confirmed to work with the following versions, but newer (or slightly older) versions may work as well:

• NodeJS: v20.3.1
• npm: 9.6.7

Next, clone this repo:

$ git clone [email protected]:haskell-actions/run-fourmolu.git

Then, within this repo, install all dependencies defined in package.json:

$ npm install

Now, you should be setup to start development. Development consists of hacking on the ./index.js file. After making changes to this file, you'll need to regenerate the files in ./dist/. You can do that with the following command:

$ npm run prepare

When sending a PR, make sure to run npm run prepare and commit the changes to ./dist whenever you make a change in ./index.js.

Do the following steps to make a new release:

1. Add entries to CHANGELOG.md for all changes since the last release.
2. Manually create a GitHub release on https://github.com/haskell-actions/run-fourmolu/releases/new.