Ormolu

Ormolu is a Haskell source formatter built for predictable, low-noise changes. It is a strong fit for developers and teams standardising one style across local editors and CI, but less suitable when a project needs configurable layout rules or extensive C preprocessor support.

Overview

Ormolu uses GHC’s parser rather than haskell-src-exts, reducing a common source of parsing trouble for Haskell code. Its formatter deliberately follows one consistent style and aims for minimal diffs. It also targets idempotence: code that already conforms should not be reformatted on each run. In practice, that makes pull-request changes easier to review and helps a team enforce a shared baseline. The trade-off is fundamental: there are no style switches for teams with established preferences that differ from Ormolu’s rules.

Key features

Project context comes from the matching Cabal file, which Ormolu uses to read default language extensions and dependencies. An .ormolu file can add fixity overrides and module re-export instructions. These mechanisms help the formatter understand a project without turning its core style into a configurable system. A library API is also available for integrating formatting into Haskell tooling.

Installation is flexible. Release binaries cover Linux, macOS, and Windows, while cabal and stack provide package-manager installation paths. Ormolu Live supplies a browser version for trying the formatter or checking a snippet without a local setup. Editor routes include Emacs, VS Code, and Vim through neoformat or vim-ormolu. Haskell Language Server has built-in support for using Ormolu, so formatting can sit in an existing language-server workflow rather than requiring a separate manual command.

For automation, the project recommends the run-ormolu GitHub Action to keep a repository formatted in CI. That combination of editor, language-server, browser, and CI entry points is valuable when a team wants one formatter applied consistently at different stages.

CPP support remains experimental and is limited to simple cases in which conditionals surround top-level declarations. Codebases with more involved preprocessor structure should treat this as a material compatibility constraint, not a minor edge case.

Pricing

Ormolu costs nothing under its open-source plan. The plan is an open-source Haskell formatter with no paid tier, seat charge, usage quota, trial period, or renewal term stated. That makes it suitable for individuals, commercial teams, and CI environments that want to deploy the same tool without a software subscription. Its license permits redistribution and use in source and binary forms subject to stated conditions, and the software is provided “as is,” so adopters should account for those license terms and their own support responsibilities.

Platforms

Supported platforms are Linux, macOS, Windows, and web. Local binaries cover the three desktop operating systems, cabal and stack offer alternative installation methods, and Ormolu Live covers browser use. Editor integrations are available for Emacs, VS Code, and Vim; Haskell Language Server provides another route for editor-based formatting. The project is focused on Haskell rather than a multi-language workspace.

Who it's for

Ormolu suits Haskell developers who want a single enforced style, small formatting diffs, and output that remains stable on repeated runs. Teams can apply it while editing, through Haskell Language Server, and in GitHub CI, reducing the chance that local and automated checks disagree. It is also a sensible choice for open-source projects that want to redistribute a formatter without licensing fees.

Look elsewhere if contributors need configurable indentation or layout rules, or if the code relies heavily on CPP patterns beyond top-level declaration guards. Ormolu’s project-aware Cabal and .ormolu support adds context, but it does not change the underlying style.

Pros and cons

Pros

• GHC-based parsing: Uses the compiler’s parser to avoid haskell-src-exts parsing problems.
• Predictable diffs: One style, minimal-change goals, and idempotence keep reviews focused.
• Broad workflow coverage: Binaries, cabal, stack, browser use, editor integrations, Haskell Language Server, and a GitHub Action support different working habits.
• Project awareness: Cabal discovery plus .ormolu fixity and re-export instructions help formatting reflect project context.
• No software fee: The open-source plan is free to redistribute and use under its license conditions.

Cons

• No configurable style: Teams cannot tune the formatter to match a preferred house style.
• Limited CPP: Experimental support handles only simple top-level declaration conditionals.
• Haskell-only focus: It does not serve mixed-language formatting needs.
• “As is” licensing posture: Users receive no stated paid support or warranty through the free plan.

Alternatives

Code Formatting Tools is the broader directory when you want to compare formatters across languages and workflows.

Choose Fourmolu instead when you want another free, open-source Haskell formatter, particularly if its Linux, web, and Windows coverage fits your deployment needs. Black is the relevant free alternative for Python projects, while Fantomas targets F# and .NET. Pick Prettier for a free, open-source formatter spanning a wider set of development environments and extension/API routes. goimports is the free open-source command-line choice for Go. Ruff fits Python teams seeking Astral’s open-source developer tooling. rustfmt is the straightforward free option for Rust. Accessibility Test Framework for Android is an Android-focused free tool rather than a Haskell formatter.

Verdict

Choose Ormolu when a Haskell project benefits from one uncompromising style, stable minimal diffs, and the ability to enforce it from editor through CI at no software cost. Choose another formatter when style configuration is a requirement or your code depends on complex CPP; Ormolu’s consistency is its chief strength, and its fixed rules and limited preprocessor handling are the boundaries.