fourmolu: A formatter for Haskell source code

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A formatter for Haskell source code.

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


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Dependencies aeson (>=1.0 && <3.0), ansi-terminal (>=0.10 && <1.1), array (>=0.5 && <0.6), base (>=4.14 && <5), binary (>=0.8 && <0.9), bytestring (>=0.2 && <0.13), Cabal-syntax (>=3.10 && <3.11), containers (>=0.5 && <0.7), deepseq (>=1.4 && <1.6), Diff (>=0.4 && <1), directory (>=1.3.3 && <1.4), file-embed (>=0.0.15 && <0.1), filepath (>=1.2 && <1.5), fourmolu, ghc-lib-parser (>=9.8 && <9.9), megaparsec (>=9), MemoTrie (>=0.6 && <0.7), mtl (>=2 && <3), optparse-applicative (>=0.14 && <0.19), scientific (>=0.3.2 && <1), syb (>=0.7 && <0.8), text (>=2 && <3), th-env (>=0.1.1 && <0.2), yaml (>= && <1) [details]
License BSD-3-Clause
Maintainer Matt Parsons <> George Thomas <> Brandon Chinn <>
Category Development, Formatting
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Source repo head: git clone
Uploaded by brandonchinn178 at 2024-02-19T21:50:33Z
Distributions Arch:, LTSHaskell:, NixOS:
Reverse Dependencies 1 direct, 1 indirect [details]
Executables fourmolu
Downloads 14309 total (441 in the last 30 days)
Rating 1.25 (votes: 1) [estimated by Bayesian average]
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Readme for fourmolu-

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License BSD3 Hackage CI

Fourmolu is a formatter for Haskell source code. It is a fork of Ormolu, with upstream improvements continually merged.

We share all bar one of Ormolu's goals:

  • 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.
  • That 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.
  • Implementing one “true” formatting style which admits no configuration. We allow configuration of various parameters, via CLI options or config files. We encourage any contributions which add further flexibility.




To install the latest release from Hackage, simply install with Cabal or Stack:

$ cabal install fourmolu
$ stack install fourmolu

Building from source

$ cabal build -fdev
$ stack build --flag fourmolu:dev

The dev flag may be omitted in your local workflow as you work, but CI may not pass if you only build without the dev flag.


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

$ fourmolu Module.hs

Add -i (or --mode inplace) to replace the contents of the input file with the formatted output.

$ fourmolu -i Module.hs

Specify a directory to recursively process all of its .hs files:

$ fourmolu -i src

Or find all files in a project with git ls-files:

$ fourmolu --mode inplace $(git ls-files '*.hs')
# Or to avoid hitting command line length limits and enable parallelism (12-way here):
$ git ls-files -z '*.hs' | xargs -P 12 -0 fourmolu --mode inplace

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

$ fourmolu --mode check src

Beware git's core.autocrlf on Windows

Fourmolu's output always uses LF line endings. In particular, fourmolu --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.

Web app

See to try Fourmolu in your browser. This is re-deployed on every new commit to main, so will use the latest version of Fourmolu, potentially including unreleased changes.

Editor integration

Fourmolu can be integrated with your editor via the Haskell Language Server. Just set haskell.formattingProvider to fourmolu (instructions).

GitHub actions

[run-fourmolu][] is the recommended way to ensure that a project is formatted with Fourmolu.

Language extensions, dependencies, and fixities

Fourmolu 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 the fixities configuration option in fourmolu.yaml. When the input comes from stdin, one can pass --stdin-input-file which will give Fourmolu the location that should be used as the starting point for searching for .cabal files.

Here is an example of the fixities configuration:

  - 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.

fourmolu.yaml can also contain instructions about module re-exports that Fourmolu should be aware of. This might be desirable because at the moment Fourmolu 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 Fourmolu. (You are welcome to open PRs to make Fourmolu aware of more re-exports by default.) However, when the fixity of an operator is not inferred correctly, making Fourmolu 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

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 files can be disabled by passing --no-cabal.

Magic comments

Fourmolu 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 {- FOURMOLU_DISABLE -} at the very top. Note that for Fourmolu to work the fragments where Fourmolu 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.

{- ORMOLU_DISABLE -} and {- ORMOLU_ENABLE -}, respectively, can be used to the same effect, and the two styles of magic comments can be mixed.


One can ask Fourmolu 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.

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)
400 Failed to load Fourmolu configuration file

Using as a library

The fourmolu 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.
  • Various minor idempotence issues, most of them are related to comments or column limits.
  • Fourmolu is in a fairly early stage of development. The implementation should be as stable as Ormolu, as it only makes minimal changes, and is extensively tested. But the default configuration style may change in some minor ways in the near future, as we make more options available. It will always be possible to replicate the old default behaviour with a suitable fourmolu.yaml.


If there are any options you'd like to see, let us know. If it's not too complicated to implement (and especially if you implement it yourself!) then we'll probably add it.

See for documentation.



Copyright © 2018–2020 Tweag I/O, 2020-present Matt Parsons


The vast majority of work here has been done by the Ormolu developers, and thus they deserve almost all of the credit. This project is simply intended as a haven for those of us who admire their work, but can't quite get on board with some of their decisions when it comes down to the details.