Every major open-source project has its own style guide: a set of conventions (sometimes arbitrary) about how to write code for that project. It is much easier to understand a large codebase when all the code in it is in a consistent style.
This project holds the style guidelines I use for my code. If you are modifying a project that was created by me, you may be pointed to this style guide.
The style guide can be found in haskell-style.md.