by tweag

tweag / ormolu

A formatter for Haskell source code

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License BSD3 Hackage Stackage Nightly Stackage LTS Build status

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
  • 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 accommodate 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.


The easiest way to build the project is with Nix:

$ nix-build -A ormolu

Or with

from the Nix shell:
$ nix-shell --run "cabal new-build"


could be used with a
file as follows.
$ cat stack.yaml
resolver: lts-16.0
- '.'

$ stack build # to build $ stack install # to install

To use Ormolu directly from GitHub with Nix, this snippet may come in handy:

# This overlay adds Ormolu straight from GitHub.
self: super:

let source = super.fetchFromGitHub { owner = "tweag"; repo = "ormolu"; rev = "de279d80122b287374d4ed87c7b630db1f157642"; # update as necessary sha256 = "0qrxfk62ww6b60ha9sqcgl4nb2n5fhf66a65wszjngwkybwlzmrv"; # as well }; ormolu = import source { pkgs = self; }; in { haskell = super.haskell // { packages = super.haskell.packages // { "${ormolu.ormoluCompiler}" = super.haskell.packages.${ormolu.ormoluCompiler}.override { overrides = ormolu.ormoluOverlay; }; }; }; }


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

$ ormolu Module.hs


--mode inplace
to replace the contents of the input file with the formatted output.
$ ormolu --mode inplace Module.hs


to format a tree recursively:
$ ormolu --mode inplace $(find . -name '*.hs')

Magic comments

Ormolu understands two magic comments:




This allows us to disable formatting selectively for code between these markers or disable it for the entire file. To achieve the latter, just put

at the very top. Note that for Ormolu to work the source code must still be parseable even when the disabled regions are omitted. Because of that the magic comments cannot be placed arbitrarily, but rather must enclose independent top-level definitions.

Current limitations

  • CPP support is experimental. CPP is virtually impossible to handle correctly, so we process them as a sort of unchangeable snippets. This works only in simple cases when CPP conditionals surround top-level declarations. See the CPP section in the design notes for a discussion of the dangers.
  • Input modules should be parsable by Haddock, which is a bit stricter criterion than just being valid Haskell modules.
  • Various minor idempotence issues, most of them are related to comments.

Running on Hackage

It's possible to try Ormolu on arbitrary packages from Hackage. For that execute (from the root of the cloned repo):

$ nix-build -A hackage.

Then inspect

for possible problems. The derivation will also contain formatted
files for inspection and original inputs with
extension (those are with CPP dropped, exactly what is fed into Ormolu).

Editor integration

We know of the following editor integrations:

Arch Linux

To install Ormolu on Arch Linux, one can use the package on AUR:

yay -S ormolu





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