# Install fourmolu with Homebrew, Nix Formatter for Haskell source code. Version 0.20.0.0 via Homebrew; verified 2026-06-28. ## Install ```sh sudo av install brew:fourmolu ``` Additional install commands: ### macOS - Homebrew (100%): ```sh brew install fourmolu ``` Evidence: local Homebrew formula metadata ### Linux - Nix (92%): ```sh nix profile install nixpkgs#fourmolu ``` Evidence: nixpkgs package indexes: pkgs/by-name/fo/fourmolu/package.nix from https://api.github.com/repos/NixOS/nixpkgs/git/trees/master?recursive=1 ## Package facts - **Package key:** brew:fourmolu - **Package manager:** Homebrew - **Package manager page:** - **Version:** 0.20.0.0 - **Source summary:** Formatter for Haskell source code - **Homepage:** - **Repository:** - **Upstream docs:** - **License:** BSD-3-Clause - **Source archive:** - **Last updated:** 2026-06-28T12:05:17-04:00 - **Generated:** 2026-07-08T18:08:21+00:00 ## Executables - fourmolu (cli) - fourmolu (alias) ## Dependencies - gmp ## Build dependencies - cabal-install - ghc ## Uses from macOS - libffi ## Install behavior - Post-install hook: not defined - Bottle: available on arm64_linux, arm64_sequoia, arm64_sonoma, arm64_tahoe, sonoma, x86_64_linux ## Freshness - Page generated: 2026-07-08 - Package-manager version: 0.20.0.0 - Package-manager updated: 2026-06-28 - Local data: ok - Upstream repository: https://fourmolu.github.io/ - info: Release/tag comparison is only available for GitHub repositories. ## 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 Notes narrow executable package without higher-risk signals. - **Geiger risk:** green / low - narrow executable package without higher-risk signals ## 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 - Unix: fourmolu.yaml ## 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:** - **Deprecated:** no - **Disabled:** no - **Keg Only:** no - **URL Keys:** head, stable ## Other Package-Manager Records - Nix - fourmolu: normalized package name match | nixpkgs package indexes: pkgs/by-name/fo/fourmolu/package.nix from https://api.github.com/repos/NixOS/nixpkgs/git/trees/master?recursive=1 ## Related links - [Source-control packages](https://www.automicvault.com/pkg/source-control-tools/) - Belongs to a source-control command family. - [Terminal utility packages](https://www.automicvault.com/pkg/terminal-utilities/) - Matched terminal and command-line workflow metadata. - [Text processing packages](https://www.automicvault.com/pkg/text-processing-tools/) - Matched text, document, or structured-data processing metadata. - [Developer build packages](https://www.automicvault.com/pkg/developer-build-tools/) - Matched build, compiler, generator, or developer workflow metadata. - [cabal-install](https://www.automicvault.com/pkg/brew/cabal-install/) - Build dependency declared by Homebrew. - [ghc](https://www.automicvault.com/pkg/brew/ghc/) - Build dependency declared by Homebrew. - [dfmt](https://www.automicvault.com/pkg/brew/dfmt/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters, source-code. - [fnlfmt](https://www.automicvault.com/pkg/brew/fnlfmt/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters, source-code. - [fprettify](https://www.automicvault.com/pkg/brew/fprettify/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters, source-code. - [stylish-haskell](https://www.automicvault.com/pkg/brew/stylish-haskell/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters, haskell. - [beautysh](https://www.automicvault.com/pkg/brew/beautysh/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters. - [buildifier](https://www.automicvault.com/pkg/brew/buildifier/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters. - [cljfmt](https://www.automicvault.com/pkg/brew/cljfmt/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters. - [dockerfmt](https://www.automicvault.com/pkg/brew/dockerfmt/) - Shares av.db curated category or tags: cli, developer-tools, formatter, formatters. - [hindent](https://www.automicvault.com/pkg/brew/hindent/) - Both packages touch the same language runtime or ecosystem. Shared terms: cli, code, developer, developer-tools, formatter. - [ormolu](https://www.automicvault.com/pkg/brew/ormolu/) - Both packages touch the same language runtime or ecosystem. Shared terms: cli, code, developer, developer-tools, formatter. ## Combined YAML source View the package source record on GitHub. [combined/fourmolu.yml](https://github.com/automic-vault/db/blob/main/combined/fourmolu.yml) ## Sources - Nucleus package database - Geiger risk classifier - package-page enrichment - curated configuration and credential file locations - curated package history - package version freshness - av.db category and tag curation - package relationship graph - external package-manager database matches - cross-ecosystem install command graph