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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Development     Formatting    

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License BSD3 Hackage

Fourmolu is a formatter for Haskell source code. It is a fork of Ormolu, with the intention to continue to merge upstream improvements.

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.


Fourmolu looks for a fourmolu.yaml file in all parents of the current directory, followed by the XDG config directory. A complete configuration file, corresponding to Fourmolu's default options, looks like:

indentation: 4
comma-style: leading # for lists, tuples etc. - can also be 'trailing'
import-export-comma-style: leading # for module import export lists - can also be 'trailing'
record-brace-space: false # rec {x = 1} vs. rec{x = 1}
indent-wheres: false # 'false' means save space by only half-indenting the 'where' keyword
diff-friendly-import-export: true # 'false' uses Ormolu-style lists
respectful: true # don't be too opinionated about newlines etc.
haddock-style: multi-line # '--' vs. '{-'
newlines-between-decls: 1 # number of newlines between top-level declarations

A config to simulate the behaviour of Ormolu (as far as currently possible) would be:

indentation: 2
comma-style: trailing
import-export-comma-style: trailing
record-brace-space: true
indent-wheres: true
diff-friendly-import-export: false
respectful: false
haddock-style: single-line
newlines-between-decls: 1

These options can also be set on the command line (which takes precedence over config files). Run fourmolu -h to see all options.


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

$ cabal install fourmolu
$ stack install fourmolu

Building from source

$ cabal build
$ stack build


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

Use find to format a tree recursively:

$ fourmolu -i $(find . -name '*.hs')

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

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

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

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

:zap: Beware git's core.autocrlf on Windows :zap:

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.

Editor integration

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

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 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.

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

Account for .cabal files

Many cabal and stack projects use default-extensions to enable GHC language extensions in all source files. With the --cabal-default-extensions flag, Ormolu will take them into consideration during formatting.

When you format input from stdin, you can pass --stdin-input-file 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 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
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


  • 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.
  • 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 DEVELOPER.md 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.

*Note that all licence references and agreements mentioned in the fourmolu README section above are relevant to that project's source code only.