Fourmolu

Fourmolu is a free, open-source formatter for Haskell, built for developers who want project-aware formatting they can tune. It suits individual Haskell programmers and teams standardizing a codebase; its Cabal awareness and editor options are useful strengths, while experimental CPP handling and fixed LF output narrow its fit.

Overview

As a configurable fork of Ormolu, Fourmolu formats Haskell using GHC’s parser. The project aims for small diffs, repeatable results and robustness on large projects, qualities that matter when formatting becomes part of regular development: focused changes are easier to review, and idempotent output avoids recurring churn. It also aims to support modern Haskell dialects.

Settings can be kept in fourmolu.yaml or passed as command-line options. Fourmolu finds the Cabal file relevant to a source file and uses its default extensions and dependencies to help resolve operator fixities. That project context is valuable for Cabal-based codebases, though it gives the formatter less relevance to projects that do not use Cabal.

Key features

• Detailed style controls: Options cover indentation, line length, import grouping, record layout, Haddock comments, Unicode output and operator fixities. Teams can set conventions around specific Haskell constructs rather than accept a single inflexible style.
• Selective formatting: Magic comments can exempt selected code, and command-line options can target a line region. This is useful when only part of a file should be reformatted, but selective scope means the project still needs a broader policy for consistent formatting.
• Browser demo: The interactive web app supports configurable formatting and program options, giving users a way to explore output without first setting up a local workflow.
• Editor and CI workflows: The README describes integration through Haskell Language Server by setting haskell.formattingProvider to fourmolu, and recommends the run-fourmolu GitHub Action to keep projects formatted. These options suit teams that want formatting available in their editor and enforced in continuous integration.
• Installation choices: GHCup is the recommended route, with Cabal, Stack and dotslash also documented. That range accommodates different Haskell toolchain setups, although users must choose and maintain an installation path that fits their environment.

Pricing

Fourmolu is free and open source: the Fourmolu plan costs nothing. There is no paid tier or trial to weigh. That makes it a straightforward choice for individual developers and teams that want a Haskell formatter without a software charge, though the project’s open-source model is not a commercial support plan.

Platforms

Fourmolu is listed for web and Windows, with Linux also identified as a platform. The browser demo provides a web option, while the installation guidance covers command-line use. Its supported language is Haskell, so it is not a general-purpose formatter for mixed-language projects.

Who it's for

Fourmolu is best for Haskell developers who want adjustable formatting, project-aware operator fixities and a route into editor or CI automation. Its Cabal lookup makes it particularly relevant to Cabal projects. It is a weaker fit where preprocessing is central: CPP support is experimental and works only in simple cases where conditionals surround top-level declarations. Projects with CRLF line endings also need to account for its LF-only output, since check mode can fail even when the file is otherwise formatted.

Pros and cons

• Pro: Cabal-aware fixity handling uses extensions and dependencies from the relevant project file, which can help formatting reflect project context.
• Pro: A broad set of style controls, selective formatting and editor/CI integration support both local preferences and shared team workflows.
• Con: CPP support is limited to simple top-level conditional cases, making it a risky choice for code that relies on more complex preprocessing.
• Con: LF-only output can cause check mode to fail on CRLF files, a practical drawback for projects that require Windows-style line endings.

Alternatives

Ormolu is the closest alternative to consider if you want a free, open-source Haskell formatter; Fourmolu is itself a configurable fork of Ormolu. For Python, choose Black; for F#, Fantomas; for Rust, rustfmt; and for Zig, Zig fmt, which is included with the Zig toolchain. goimports is a free open-source command-line tool, while Prettier is a free open-source formatter with enterprise support and maintenance offered through Tidelift Subscription. Accessibility Test Framework for Android is another free alternative listed across Android, Linux, macOS and Windows.

For more options, see Code Formatting Tools.

Verdict

Choose Fourmolu if you work in Haskell and want configurable formatting that can draw on Cabal project settings and fit into editor and CI workflows at no software cost. Look elsewhere if your code depends on complex CPP conditionals or needs CRLF-preserving checks: its experimental CPP coverage and LF-only output are meaningful constraints.