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Ormolu

Ormolu is a formatter for Haskell source code. The project was created with the following features 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 influence 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. Roughly, it means that the project follows the path of hindent and is very much about printing AST in a particular way.
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.
Idempotency: 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.

Building

The easiest way to build the project is with Nix:

$ nix-build

Or with cabal-install from the Nix shell:

$ nix-shell --run "cabal new-build"

Alternatively, stack could be used with a stack.yaml file as follows.

$ cat stack.yaml
resolver: lts-13.19
packages:
- '.'

$ stack build

The following will print the formatted output to the standard output.

$ ormolu Module.hs

Add --mode inplace to replace the contents of the input file with the formatted output.

$ ormolu --mode inplace Module.hs

A configuration file can specify the language extensions to use by default.

$ cat ormolu.yaml
ghc-opts:
  - -XLambdaCase
unsafe: False
debug: False

$ ormolu -c ormolu.yaml Module.hs