Fourmolu action


This Fourmolu 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.

Example usage

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

- uses: fourmolu/[email protected]

The @v1 after fourmolu-action should be replaced with the version of the Fourmolu Action you want to use. See Releases for all versions available. Each version of the Fourmolu Action generally has a corresponding version of fourmolu. Make sure you pick a Fourmolu Action version that uses the version of fourmolu you use locally.

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

- uses: fourmolu/[email protected]
    # Only check the format of .hs in the src/ directory.
    pattern: |

    # 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"

Example Usage with Build Matrix

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

    runs-on: ubuntu-latest
      - uses: actions/[email protected]
      - uses: fourmolu/[email protected]
    runs-on: ubuntu-latest
    needs: fourmolu

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


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


If you are running a workflow on Windows, be wary of Git’s core.autocrlf. Fourmolu always converts CRLF endings to LF endings which may result in spurious diffs, so you probably want to disable core.autocrlf:

$ git config --global core.autocrlf false

Hacking on this repo

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: 14.19.1
  • npm: 6.14.16

Next, clone this repo:

$ git clone [email protected]:fourmolu/fourmolu-action.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.


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