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

• pattern Glob pattern that are used to find source files to format. It is possible to specify several patterns by putting each on a new line.
• follow-symbolic-links Whether to follow symbolic links.
• extra-args Extra arguments to pass to Ormolu.

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

- uses: mrkkrp/ormolu-action@v3

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

jobs:
  ormolu:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: mrkkrp/ormolu-action@v3
  build:
    runs-on: ubuntu-latest
    needs: ormolu
    ...

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