Fourmolu review
A focused Haskell formatter with YAML configuration, CI checks, and editor integration.
Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026
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.
Fourmolu pros and cons
- 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
Fourmolu fact sheet, pricing and score →
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