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Fourmolu

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:

Configuration

See https://fourmolu.github.io/config/

Installation

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.

Usage

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

: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.

Web app

See https://fourmolu.github.io/ 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][https://github.com/haskell-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:

fixities:
  - 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:

reexports:
  - 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:

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

Magic comments

Fourmolu understands two magic comments:

{- FOURMOLU_DISABLE -}

and

{- FOURMOLU_ENABLE -}

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.

Regions

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.

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

Exit codes

Exit codeMeaning
0Success
1General problem
2CPP used (deprecated)
3Parsing of original input failed
4Parsing of formatted code failed
5AST of original and formatted code differs
6Formatting is not idempotent
7Unrecognized GHC options
8Cabal file parsing failed
9Missing input file path when using stdin input and accounting for .cabal files
10Parse error while parsing fixity overrides
100In checking mode: unformatted files
101Inplace mode does not work with stdin
102Other issue (with multiple input files)
400Failed 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 0.10.2.0. 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.

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

Limitations

Breaking changes policy

Fourmolu is still in a relatively early stage of development, but it is in wide enough use that stability is a desirable property. Fourmolu aims to uphold the following principles:

  1. It will always be possible to replicate Ormolu's formatting with a suitable fourmolu.yaml

  2. Breaking changes will be avoided where possible, but may still occur in the following circumstances:

Contributing

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.

License

See LICENSE.md.

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

Acknowledgements

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.