Recently the Cats project has started using codecov.io to track test coverage of our project. As of writing our tests cover about 48% of the core subproject.
One of the benefits that we get from pure functional programming is that there are fewer ways to make mistakes. Nonetheless, we value having a robust test suite which exercises and verifies the properties of our code.
Writing tests is a great way to get involved with the Cats project! The project currently marks test-related issues with a testing label, and we use an in-progress label to signal that someone is working on the issue.