Ormolu
A formatter for Haskell source 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. The formatting style aims to result in minimal diffs. Choose a style compatible with modern dialects of Haskell. As new Haskell extensions enter broad use, we may change the style to accommodate them. Idempotence: formatting already formatted code doesn't change it.