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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 certain situations 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.
  • 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.
  • Be well-tested and robust so that the formatter can be used in large projects.

Try it out in your browser at! See Ormolu Live for more info.


The release page has binaries for Linux, macOS and Windows.

You can also install using

$ cabal install ormolu
$ stack install ormolu

Ormolu is also included in several package repositories. E.g., on Arch Linux, one can use the package on AUR:

$ yay -S ormolu

Building from source

The easiest way to build the project is with Nix:

$ nix-build -A ormolu

Note that you will need to add IOHK Hydra binary cache, otherwise building may take a very long time.


could be used as follows:
$ stack build # to build
$ stack install # to install

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

  pkgs = import  { };
  source = pkgs.fetchFromGitHub {
      owner = "tweag";
      repo = "ormolu";
      rev = "c1d8a8083cf1492545b8deed342c6399fe9873ea"; # update as necessary
      # do not forget to update the hash:
      sha256 = "sha256-3XxKuWqZnFa9s3mY7OBD+uEn/fGxPmC8jdevx7exy9o=";
in (import source {  }).ormoluExe # this is e.g. the executable derivation


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')

Or find all files in a project with

git ls-files
$ ormolu --mode inplace $(git ls-files '*.hs')

To check if files are are already formatted (useful on CI):

$ ormolu --mode check $(find . -name '*.hs')

:zap: Beware git's
on Windows :zap:

Ormolu's output always uses LF line endings. In particular,

ormolu --mode check
will fail if its input is correctly formatted except that it has CRLF line endings. This situation can happen on Windows when checking out a git repository without having set

Ormolu Live

On every new commit to

, Ormolu Live is deployed to Older versions are available at

Editor integration

We know of the following editor integrations:

Haskell Language Server

Haskell Language Server has built-in support for using Ormolu as a formatter.

GitHub actions

is the recommended way to ensure that a project is formatted with Ormolu.

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 fragments where Ormolu is enabled must be parseable on their own. Because of that the magic comments cannot be placed arbitrarily, but rather must enclose independent top-level definitions.

Account for .cabal files

Many cabal and stack projects use

to enable GHC language extensions in all source files. With the
flag, Ormolu will take them into consideration during formatting.

When you format input from stdin, you can pass

which will give Ormolu the location of the Haskell source file that should be used as the starting point for searching for a suitable .cabal file.

Exit codes

Exit code

0 Success
1 General problem
2 CPP used (deprecated)
3 Parsing of original input failed
4 Parsing of formatted code failed
5 AST of original and formatted code differs
6 Formatting is not idempotent
7 Unrecognized GHC options
8 Cabal file parsing failed
9 Missing input file path when using stdin input and accounting for .cabal files
100 In checking mode: unformatted files
101 Inplace mode does not work with stdin
102 Other issue (with multiple input files)


  • 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.

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).





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