Ormolu is a modern Haskell formatter by Tweag I/O. It implements exactly one formatting style, respects the layout choice (single-line vs multi-line), and allows no other configuration.
Ormolu is already quite usable and can handle the complete GHC Haskell syntax. But its style is not set in stone, and some corner cases are still handled less gracefully than the others. So, we have adapted Ormolu for web to get more community feedback.
This live version runs entirely in the browser and does not send your code anywhere. Play with it and post your suggestions and issues to the Ormolu issue tracker.