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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
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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.


Available options

Defaults are in bold.

Configuration option Valid options Description
indentation any non-negative integer (4) How many spaces to use as an indent
function-arrows trailing, leading, leading-args Where to place arrows in type signatures
comma-style leading, trailing Where to place commas in lists, tuples, etc.
import-export-style leading, trailing, diff-friendly How to format multiline import/export lists (diff-friendly lists have trailing commas but keep the opening parenthesis on the same line as import)
indent-wheres true, false Use an extra level of indentation vs only half-indent the where keyword
record-brace-space true, false rec {x = 1} vs rec{x = 1}
newlines-between-decls any integer (1) Number of newlines between top-level declarations
haddock-style single-line, multi-line, multi-line-compact Use `-- \
haddock-style-module same as haddock-style haddock-style, but specifically for the module docstring (not specifying anything = use the same setting as haddock-style)
let-style inline, newline, auto, mixed How to style let blocks (auto uses newline if there's a newline in the input and inline otherwise, and mixed uses inline only when the let has exactly one binding)
in-style left-align, right-align How to align the in keyword with respect to let
unicode always, detect, never Output Unicode syntax. With detect we output Unicode syntax exactly when the extension is seen to be enabled. When using always, make sure to have the UnicodeSyntax extension enabled, or Fourmolu will throw errors.
respectful true, false Be less aggressive in reformatting input, e.g. keep empty lines in import list
fixities list of strings ([]) See the "Language extensions, dependencies, and fixities" section below

For examples of each of these options, see the test files.

Specifying configuration

Configuration options may be specified in either a fourmolu.yaml file or via command-line options. 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
function-arrows: trailing
comma-style: leading
import-export-style: diff-friendly
indent-wheres: false
record-brace-space: false
newlines-between-decls: 1
haddock-style: multi-line
let-style: auto
in-style: right-align
respectful: true
fixities: []
unicode: never

The configuration that most closely matches Ormolu's styling is:

indentation: 2
function-arrows: trailing
comma-style: trailing
import-export-style: trailing
indent-wheres: true
record-brace-space: true
newlines-between-decls: 1
haddock-style: single-line
let-style: inline
in-style: right-align
respectful: false
fixities: []
unicode: never

Command-line options override options in a configuration file. Run fourmolu --help 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 -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:
$ git ls-files -z '*.hs' | xargs -0 fourmolu --mode inplace

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

$ fourmolu --mode check src
: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).

Language extensions, dependencies, and fixities

Fourmolu automatically locates the Cabal file that corresponds to a given source code file. When input comes from stdin, one can pass --stdin-input-file which will give Fourmolu the location of the Haskell source file that should be used as the starting point for searching for a suitable Cabal 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 with the fixities configuration option in fourmolu.yaml, e.g.

  - 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.

Besides, 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.

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


  • 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.