Ormolu is a formatter for #haskell source code. The project was created with the following goals in mind:

- Using GHC's own parser to avoid parsing problems caused by haskell-src-exts.
- Let some whitespace be programmable. The layout of the input influences the layout choices in the output. This means that the choices between single-line/multi-line layouts in each particular situation are made by the user, not by an algorithm. This makes the implementation simpler and leaves some control to the user while still guaranteeing that the formatted code is stylistically consistent.
- Writing code in such a way so it's easy to modify and maintain.
- Implementing one “true” formatting style which admits no configuration.
- That formatting style aims to result in minimal diffs while still remaining very close to “conventional” Haskell formatting people use.
- Choose a style compatible with modern dialects of Haskell. As new Haskell extensions enter broad use, we may change the style to accomodate them.
- Idempotence: formatting already formatted code doesn't change it.
- Be well-tested and robust to the point that it can be used in large projects without exposing unfortunate, disappointing bugs here and there.

https://github.com/tweag/ormolu