This is Ormolu action that helps to ensure that your Haskell project is formatted with Ormolu.

• 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@v1

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@v2
  build:
    runs-on: ubuntu-latest
    needs: ormolu
    ...

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