ormolu: A formatter for Haskell source code

[ bsd3, development, formatting, library, program ] [ Propose Tags ]

A formatter for Haskell source code.

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Manual Flags


Turn on development settings.


An internal ad-hoc flag that is enabled by default, Ormolu Live disables it due to missing WASM TH support.


Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info


Maintainer's Corner

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For package maintainers and hackage trustees


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Change log CHANGELOG.md
Dependencies ansi-terminal (>=0.10 && <1.2), array (>=0.5 && <0.6), base (>=4.14 && <5), binary (>=0.8 && <0.9), bytestring (>=0.2 && <0.13), Cabal-syntax (>=3.12 && <3.13), choice (>= && <0.3), containers (>=0.5 && <0.8), deepseq (>=1.4 && <1.6), Diff (>=0.4 && <1), directory (>=1.3 && <1.4), file-embed (>=0.0.15 && <0.1), filepath (>=1.2 && <1.6), ghc-lib-parser (>=9.10 && <9.11), megaparsec (>=9), MemoTrie (>=0.6 && <0.7), mtl (>=2 && <3), optparse-applicative (>=0.14 && <0.19), ormolu, syb (>=0.7 && <0.8), text (>=2.1 && <3), th-env (>=0.1.1 && <0.2) [details]
License BSD-3-Clause
Maintainer Mark Karpov <mark.karpov@tweag.io>
Category Development, Formatting
Home page https://github.com/tweag/ormolu
Bug tracker https://github.com/tweag/ormolu/issues
Source repo head: git clone https://github.com/tweag/ormolu.git
Uploaded by mrkkrp at 2024-06-16T12:59:16Z
Distributions Arch:, Debian:, Fedora:, LTSHaskell:, NixOS:, Stackage:
Reverse Dependencies 2 direct, 1 indirect [details]
Executables ormolu
Downloads 24073 total (458 in the last 30 days)
Rating (no votes yet) [estimated by Bayesian average]
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Status Docs uploaded by user
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Readme for ormolu-

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License BSD3 Hackage Stackage Nightly Stackage LTS CI

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 haskell-src-exts.
  • 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 https://ormolu-live.tweag.io! See Ormolu Live for more info.


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

You can also install using cabal or stack:

$ 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

Make sure to accept the offered Nix caches (in particular the IOG cache), otherwise building may take a very long time.

Alternatively, stack could be used as follows:

$ 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:tweag/ormolu";
  outputs = { ormolu, ... }: {
    # use ormolu.packages.${system}.default here


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

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

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

Beware git's core.autocrlf on Windows

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.tweag.io. Older versions are available at https://COMMITHASH--ormolu-live.netlify.app.

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

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

Language extensions, dependencies, and fixities

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

Here is an example of .ormolu file:

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

It uses exactly the same syntax as usual Haskell fixity declarations to make it easier for Haskellers to edit and maintain.

As of Ormolu, .ormolu files can also contain instructions about module re-exports that Ormolu should be aware of. This might be desirable because at the moment Ormolu cannot know about all possible module re-exports in the ecosystem and only few of them are actually important when it comes to fixity deduction. In 99% of cases the user won't have to do anything, especially since most common re-exports are already programmed 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 come in handy. 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 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:




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 {- ORMOLU_DISABLE -} 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.


One can ask Ormolu to format a region of input and leave the rest unformatted. This is accomplished by passing the --start-line and --end-line command line options. --start-line defaults to the beginning of the file, while --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 depended upon from other Haskell programs. For these purposes only the top Ormolu module should be considered stable. It follows PVP starting from the version Rely on other modules at your own risk.


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 from Hackage), Ormolu probably doesn't recognize that the operator is the same as the third-party one.

    Some reasons this might be the case:

    • You might have a custom Prelude that re-exports things from Prelude
    • You might have -XNoImplicitPrelude turned on

    If any of these are true, make sure to specify the reexports correctly in a .ormolu file.

You can see how Ormolu decides the fixity of operators if you use --debug.


  • 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.

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 .#hackage.<package>

Then inspect result/log.txt for possible problems. The derivation will also contain formatted .hs files for inspection and original inputs with .hs-original extension (those are with CPP dropped, 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.





Copyright © 2018–present Tweag I/O