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Fourmolu

Free#23 of 34 in Code Formatting Tools

Fourmolu: A focused Haskell formatter with YAML configuration, CI checks, and editor integration. Ranked #23 of 34 in Code Formatting Tools by our editors (7.0/10); pricing: Open source; best for configurable Haskell formatting.

7.0/10Editor score

Visit Fourmolu

Fourmolu7.0 Visit Fourmolu

At a glance

Editor score
7.0 / 10
Pricing
Open source
Best for
Configurable Haskell formatting
Paid from
Free
Supported languages
Haskell
Facts checked
21 Sep 2026

Where it wins
- YAML and CLI options support configurable formatting
- Check mode supports detecting unformatted files in CI
- Integrates with Haskell Language Server and GitHub Actions
Where it doesn't
- Supports Haskell rather than multiple programming languages
- Platform coverage centers on web and Windows
- Configuration adds choices for teams seeking fixed formatting rules

Our verdict on Fourmolu

Fourmolu is an open-source formatter for Haskell source code, intended for individual developers and teams that want consistent formatting with control over project rules. It uses GHC-based parsing and can format files, directories, or standard input. Developers can apply changes in place, while teams can define formatting through fourmolu.yaml files or command-line options. Its browser-based formatting application also provides a web option for working with Haskell code.

Its strongest distinction is configurable workflow coverage. Fourmolu can format Git-tracked Haskell files, restrict formatting to selected start and end lines, and use magic comments to disable or re-enable formatting in specific regions. Check mode is suited to CI workflows because it detects unformatted files without applying changes. GitHub Actions integration supports that automation path, while Haskell Language Server integration connects formatting with the editor environment. The formatter can also be used as a library from other Haskell programs, which suits teams building Haskell-based tooling around a shared formatting rule set.

Fourmolu is a focused choice for Haskell teams that need more control than a fixed formatter provides, especially when repository configuration, selective formatting, editor support, and CI checks are important. Its scope is deliberately narrow: the supported language is Haskell, and its documented platform coverage is web and Windows rather than a broad cross-platform matrix. Teams seeking a formatter for several programming languages, or a tool centered on a wider editor and operating-system ecosystem, should consider a broader alternative. For Haskell projects, Fourmolu fits best when configurable rules and multiple formatting workflows matter.

Your product? Our editors can test Fourmolu in depth and publish a review, labelled as paid · from $149/mo →

Fourmolu pricing

Plans Open sourceFree Free to use — no paid tier required for the core job.

See plans on fourmolu.github.io

Fourmolu fact sheet

Free plan	Not verified
Paid from	Free
Supported languages	Haskell
Supported platforms	web, windows
Editor integration	Yes
Ignore files	Not verified
Changed-lines formatting	Not verified
Library API	Yes
License model	Open_source
Deployment	Cloud, Self-hosted
Platforms	Web, Windows
Support	Docs
Built for	Solo, Small business, Mid-market (editorial estimate)
Integrations	2 integrations: Haskell Language Server, GitHub Actions
Pricing	Open source
Website	fourmolu.github.io
Facts checked	21 Sep 2026

Fourmolu integrations

Fourmolu lists 2 integrations on its own site.

Haskell Language Server
GitHub Actions

Alternatives to Fourmolu

See all Fourmolu alternatives →

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Featured on iTechGuides

Fourmolu is listed in our Code Formatting Tools directory. Add the badge to your site — it links back to this page.

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Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026

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Last updated 7 Oct 2026 · How we research and update