Ormolu

• Installation
• Building from source
• Usage
  • Ormolu Live
  • Editor integration
  • Haskell Language Server
  • GitHub Actions
  • Language extensions, dependencies, and fixities
  • Magic comments
  • Regions
  • Exit codes
  • Using as a library
• Troubleshooting
  • Operators are being formatted weirdly!
• Limitations
• Running on Hackage
• Forks and modifications
• Contributing
• License

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

• Use GHC's own parser to avoid the parsing problems caused by haskell-src-exts.
• Make some whitespace programmable. The layout of the input influences the layout choices in the output, so the choice between single-line and multi-line layouts is made by the user rather than by an algorithm. This keeps the implementation simpler and leaves some control to the user while still guaranteeing that the formatted code is stylistically consistent.
• Implement one “true” formatting style that admits no configuration.
• Produce minimal diffs.
• Choose a style compatible with modern dialects of Haskell. As new Haskell extensions enter broad use, we may adjust the style to accommodate them.
• Guarantee idempotence: formatting already formatted code doesn't change it.
• Stay well-tested and robust, so that the formatter can be used in large projects.

Try it out in your browser at https://ormolu-live.markkarpov.com! See Ormolu Live for more info.

Installation

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

You can also install Ormolu with cabal or stack:

$ cabal install ormolu
$ stack install ormolu

Ormolu is also included in several package repositories. For example, on Arch Linux you can use the package on AUR:

$ yay -S ormolu

Building from source

The easiest way to build the project is with Nix:

$ nix build

Make sure to accept the offered Nix binary caches, otherwise building may take a very long time. The flake declares the relevant caches (the IOG cache and the project's own ormolu.cachix.org, which is populated by CI) via its nixConfig, but Nix uses them only if you allow it to. The simplest way is to pass --accept-flake-config:

$ nix build --accept-flake-config

To avoid repeating the flag, add the following to your Nix configuration (/etc/nix/nix.conf, or nix.settings on NixOS):

extra-substituters = https://cache.iog.io https://ormolu.cachix.org
extra-trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= ormolu.cachix.org-1:0L9Y4A+6dGpvfGtaeaq5w44pgX0AVRivKMfi2fiOzYE=

Alternatively, you can use stack:

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

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

{
  inputs.ormolu.url = "github:mrkkrp/ormolu";
  outputs = { ormolu, ... }: {
    # use ormolu.packages.${system}.default here
  };
}

Usage

The following prints 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

Use find 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 whether files are already formatted (useful on CI):

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

:zap: Beware git's core.autocrlf 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 core.autocrlf to false.

Ormolu Live

On every new commit to master, Ormolu Live is deployed to https://ormolu-live.markkarpov.com. Older versions are available at https://COMMITHASH--ormolu.netlify.app, where COMMITHASH is the hash of the commit you want.

Editor integration

We know of the following editor integrations:

• Emacs
• VS Code
• Vim: neoformat, vim-ormolu

Haskell Language Server

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

GitHub Actions

run-ormolu is the recommended way to ensure that a project stays formatted with Ormolu.

Language extensions, dependencies, and fixities

Ormolu automatically locates the Cabal file that corresponds to a given source file. Cabal files are used to extract both default extensions and dependencies. Default extensions directly affect the behavior of the GHC parser, while dependencies are used to determine the fixities of operators that appear in the source code. Fixities can also be overridden via an .ormolu file, which should be located higher in the file system hierarchy than the source file being formatted. When the input comes from stdin, you can pass --stdin-input-file to tell Ormolu which location to use as the starting point when searching for .cabal and .ormolu files.

Here is an example of an .ormolu file:

infixr 9  .
infixr 5  ++
infixl 4  <$
infixl 1  >>, >>=
infixr 1  =<<
infixr 0  $, $!
infixl 4 <*>, <*, *>, <**>

infixr 3 >~<
infixr 3.3 |~|
infixr 3.7 <~>

It uses exactly the same syntax as ordinary Haskell fixity declarations, which makes it easier for Haskellers to edit and maintain. Since Ormolu 0.7.8.0, fractional precedences are supported for more precise control over the formatting of complex operator chains.

As of Ormolu 0.7.0.0, .ormolu files can also contain instructions about module re-exports that Ormolu should be aware of. This can be useful because Ormolu cannot know about every possible module re-export in the ecosystem, and only a few of them actually matter for fixity deduction. In 99% of cases you won't have to do anything, especially since the most common re-exports are already built into Ormolu. (You are welcome to open PRs to make Ormolu aware of more re-exports by default.) However, when the fixity of an operator is not inferred correctly, making Ormolu aware of a re-export may help. Here is an example:

module Control.Lens exports Control.Lens.At
module Control.Lens exports "lens" Control.Lens.Lens

Module re-export declarations can be mixed freely with fixity overrides, as long as each declaration is on its own line. As of Ormolu 0.7.1.0 explicit package names are allowed in re-export declarations (see the example above).

Finally, all of the above-mentioned parameters can be controlled from the command line:

• Language extensions can be specified with the -o or --ghc-opt flag.
• Dependencies can be specified with the -p or --package flag.
• Fixities can be specified with the -f or --fixity flag.
• Re-exports can be specified with the -r or --reexport flag.

Searching for .cabal and .ormolu files can be disabled by passing --no-cabal and --no-dot-ormolu respectively.

Magic comments

Ormolu understands two magic comments:

{- ORMOLU_DISABLE -}

and

{- ORMOLU_ENABLE -}

These let you disable formatting selectively for the code between the two markers, or for the entire file. To disable formatting for the whole file, just put {- ORMOLU_DISABLE -} at the very top. Note that the fragments where Ormolu is enabled must be parseable on their own. Because of this, the magic comments cannot be placed arbitrarily; they must enclose independent top-level definitions.

Regions

You can ask Ormolu to format a region of the input and leave the rest unformatted by passing the --start-line and --end-line command line options. --start-line defaults to the beginning of the file, and --end-line defaults to the end.

Note that the selected region needs to be parseable Haskell code on its own.

Exit codes

Exit code	Meaning
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
10	Parse error while parsing fixity overrides
100	In checking mode: unformatted files
101	Inplace mode does not work with stdin
102	Other issue (with multiple input files)

Using as a library

The ormolu package can also be used as a dependency from other Haskell programs. For this purpose, only the top-level Ormolu module should be considered stable. It follows the PVP starting from version 0.5.3.0. Rely on other modules at your own risk.

Troubleshooting

Operators are being formatted weirdly!

This can happen when Ormolu doesn't know or can't determine the fixity of an operator.

• If this is a custom operator, see the instructions in the Language extensions, dependencies, and fixities section to specify the correct fixities in a .ormolu file.

• If this is a third-party operator (e.g. from base or some other package on Hackage), Ormolu probably doesn't recognize that the operator is the same as the third-party one.

  Some possible reasons for this:

  • You have a custom Prelude that re-exports things from the standard Prelude.
  • You have -XNoImplicitPrelude turned on.

  If either of these applies, make sure to specify the re-exports correctly in a .ormolu file.

You can see how Ormolu decides the fixity of operators by using --debug.

Limitations

• CPP support is experimental. CPP is virtually impossible to handle correctly, so Ormolu treats CPP sections as unchangeable snippets. This works only in simple cases, where CPP conditionals surround top-level declarations. See the CPP section of the design notes for a discussion of the dangers.

Running on Hackage

You can try Ormolu on arbitrary packages from Hackage. To do so, run the following from the root of the cloned repo:

$ nix build .#hackage.<package>

Then inspect result/log.txt for possible problems. The derivation also contains the formatted .hs files for inspection, along with the original inputs under the .hs-original extension (these have CPP dropped and are exactly what is fed into Ormolu).

Forks and modifications

We know of the following actively maintained forks:

• Fourmolu, which uses 4-space indentation and allows arbitrary configuration.

Contributing

Contributions of all kinds are welcome, from bug reports and documentation fixes to new features. Please see CONTRIBUTING.md to get started.

License

See LICENSE.md.

Copyright © 2018–2026 Tweag I/O, 2026–present Mark Karpov