install

Additional install commands

macOS

Homebrewverified · 100%

brew install fourmolu

local Homebrew formula metadata

Linux

Nixverified · 92%

nix profile install nixpkgs#fourmolu

nixpkgs package indexes · pkgs/by-name/fo/fourmolu/package.nix · source: api.github.com

overview

Package summary

Formatter for Haskell source code

Project links

Commands and aliases

fourmolu

history

Project history and usage

Fourmolu is a formatter for Haskell source code. It is an Ormolu fork that keeps Ormolu's parser-driven, idempotent formatting goals while adding configurable style options through CLI flags and `fourmolu.yaml`.

Project history

The repository was created in 2020 and describes itself as a fork of Ormolu with upstream improvements continually merged. The official README frames the project around one key departure from Ormolu: replacing the no-configuration style rule with configurable parameters.

The changelog shows a pattern of adding Fourmolu-specific configuration while also importing upstream Ormolu changes, including parser updates, formatting fixes, and support for new GHC language features.

Adoption history

Fourmolu is adopted by Haskell projects that want automatic formatting but need policy knobs that Ormolu intentionally avoids. The README documents installation through GHCup, Cabal, Stack, release notes, editor integration through Haskell Language Server, and CI through the recommended `run-fourmolu` GitHub Action.

How it is used

Users run `fourmolu Module.hs` to print formatted output, `fourmolu -i` to rewrite files, or `fourmolu --mode check` in CI. Configuration can be written in `fourmolu.yaml`, discovered from parent directories and the XDG config directory, or passed as command-line options.

Why package nerds care

Fourmolu is a package-nerd case study in formatter governance. It forked an intentionally opinionated tool not to change the parser architecture, but to change the social contract around style configuration, while still tracking upstream Ormolu.

Timeline

2020: The public `fourmolu/fourmolu` repository was created.
2023: v0.13.x and v0.14.x releases continued the configurable formatter line.
2024: v0.16.x added explicit configuration-file selection and upstream parser updates.
2025: v0.19.0.0 added options such as `haddock-location-signature` and `if-style`.
2026: v0.20.0.0 made Fourmolu installable with GHCup and added more import-grouping and record-style configuration.

Related projects

Fourmolu is directly related to Ormolu, GHC, ghc-lib-parser, Cabal, Stack, GHCup, Haskell Language Server, Hackage, and the `run-fourmolu` GitHub Action.

Sources

security posture

Risk level: green

narrow executable package without higher-risk signals.

Risk classifier

green risk · low confidence · appliance

Why

narrow executable package without higher-risk signals

Signals

metadata:no-higher-risk-signals

Install behavior

No Homebrew post-install hook is recorded in formula metadata.
Homebrew bottle metadata is available for 6 platform targets.
Installs with 1 runtime dependencies.
Build metadata lists 2 build dependencies.

Recommended review

Before unattended agent use, check whether the tool reads plaintext credentials, writes remote state, publishes artifacts, or shells out to plugins.

local files

Configuration and credential file locations

These source-backed paths show where this package keeps local settings or durable credentials. Automic Vault can use them as review targets for secret scanning, migration, and command approval.

Configuration files

Config paths the tool may read or write during local use.

Unix: fourmolu.yaml

executables

Installed executables

Command	Kind	Exposure	Note
`fourmolu`	cli	global executable

freshness

Version and freshness

These signals separate page generation age, package-manager activity, and upstream release comparison. Version lag is warned only when an evidence URL and comparable versions are present.

page generated2026-07-08

manager version0.20.0.0

manager updated2026-06-28

local dataok

upstreamnot checked

latest detectednot detected

https://fourmolu.github.io/

infoRelease/tag comparison is only available for GitHub repositories.https://fourmolu.github.io/none confidence

install metadata

Package metadata

Package key	brew:fourmolu
Version	0.20.0.0
Package manager	Homebrew
Package manager page	https://formulae.brew.sh/formula/fourmolu
Homepage	https://fourmolu.github.io/
Repository	https://github.com/fourmolu/fourmolu
Upstream docs	https://fourmolu.github.io/
License	BSD-3-Clause
Source archive	https://hackage.haskell.org/package/fourmolu-0.20.0.0/fourmolu-0.20.0.0.tar.gz
Last updated	2026-06-28T12:05:17-04:00
Pulse	updated
Dependencies	gmp
Build dependencies	cabal-install, ghc
Uses from macOS	libffi
Bottle	available (on arm64_linux, arm64_sequoia, arm64_sonoma, arm64_tahoe, sonoma, x86_64_linux)
Homebrew post-install	not defined
Service	none declared

registry facts

Source database details

Source Database	Homebrew formula API
Tap	homebrew/core
Full Name	fourmolu
Version Scheme	0
Revision	0
Head Version	HEAD
Bottle Stable Root URL	https://ghcr.io/v2/homebrew/core
Deprecated	no
Disabled	no
Keg Only	no
URL Keys	head stable

source database matches

Other package-manager records

Matches are pulled from external package-manager indexes and kept separate from local Automic Vault package links.

Nix95%

fourmolu

nix profile install nixpkgs#fourmolu

normalized package name match
Matched by: Fourmolu

nixpkgs package indexes · api.github.com · nixpkgs package indexes: pkgs/by-name/fo/fourmolu/package.nix from https://api.github.com/repos/NixOS/nixpkgs/git/trees/master?recursive=1

package graph

Links come from deterministic package relationships, av.db category and tag curation, ecosystem matches, and package hub membership.

source trail

Generated from repository data

This page is generated by av-web from the private package SQLite artifact built by scripts/generate-pkg-sqlite.py.

Combined YAML source

View the package source record on GitHub.

combined/fourmolu.yml

Used sources

Geiger risk classifier
Nucleus package database
av.db category and tag curation
cross-ecosystem install command graph
curated configuration and credential file locations
curated package history
external package-manager database matches
package relationship graph
package version freshness
package-page enrichment

Install fourmolu with Homebrew, Nix