Fantomas

Fantomas is an opinionated formatter for F# code, best suited to developers who want a shared style across editors and repositories. Microsoft-aligned defaults and a CI check make it a practical fit for F# projects; it cannot format source that the F# compiler cannot parse.

Overview

Fantomas applies configured formatting to F# source files and folders. Its defaults follow Microsoft's F# style guide, and it offers settings for G-Research conventions. That gives teams a defined starting point without requiring them to design a style from scratch, though the focus on F# makes it the wrong choice for a repository that needs one formatter for several languages.

Fantomas is installed as a .NET tool. A local installation is recommended so a repository can pin its version, which helps collaborators and automated checks use the same formatter. The command line accepts files ending in .fs, .fsi, .fsx, .ml or .mli, or folders containing them. Source files are expected to parse with the F# compiler before formatting, so Fantomas does not resolve syntax errors as part of the formatting process.

Key features

• Style defaults and settings: Microsoft-style defaults provide a useful baseline, with settings for G-Research conventions. A limited set of options can be kept in .editorconfig and read automatically by the command line tool, so teams wanting broad control over formatting rules may find the configuration surface restrictive.
• Formatting checks: dotnet fantomas check reports formatting drift and exits with a failure code when files need formatting. That makes it suitable for CI enforcement, where a failing check can prompt contributors to format before merging.
• Editor setup: Setup guides cover JetBrains Rider, Visual Studio Code and Visual Studio. The editor integrations and online tool for trying Fantomas and its settings make it easier to evaluate in common F# workflows.
• Library and issue reporting: The tool and library use the Apache 2.0 license, and a library API supports use from other software. The package page directs users to an online bug reporting tool that opens an issue with reproduction details. Fantomas is an fsprojects incubation project.

Pricing

Fantomas is free and open source. Its sole Fantomas plan costs nothing and includes F# source code formatting, the .NET tool and an Apache 2.0 license. There is no paid tier or free trial to weigh; the free plan is the product.

Platforms

Fantomas is associated with web and self-hosted platforms, and its supported platforms also include Linux, macOS and Windows. The online tool offers a way to try formatting in a browser, while the .NET tool supports repository-based use. Editor integration is available.

Who it's for

Fantomas suits F# developers and teams who want consistent formatting in editors and a command-line check that can flag style drift in CI. Repository-level version pinning is useful when a team wants predictable formatting across contributors. It is less suitable for projects whose files do not parse or for teams that need extensive configuration or formatting across multiple languages.

Pros and cons

• Pro: Microsoft-aligned defaults and G-Research settings give F# teams a ready-made style baseline.
• Pro: The CI check returns a failure code for formatting drift, so automated workflows can enforce consistency.
• Pro: The Apache 2.0 license, free plan and library API support both direct use and integration.
• Con: Source must already parse with the F# compiler, which limits its use while code is syntactically invalid.
• Con: Only a limited set of settings are read from .editorconfig, which may not suit teams that want fine-grained style control.
• Con: Its F# focus does not cover unrelated languages in a mixed-language codebase.

Alternatives

For a different language or ecosystem, consider Black, a free MIT-licensed Python code formatter; Fourmolu or Ormolu, both free Haskell formatters; or goimports, a free open source command-line tool. Prettier is a free open-source code formatter, with enterprise support and maintenance offered through Tidelift Subscription. rustfmt is another free formatter, with no paid plans stated. Ruff is made by Astral, which builds open-source, permissively licensed developer tools for the Python ecosystem. Accessibility Test Framework for Android is also listed as an alternative.

For more options, browse Code Formatting Tools.

Verdict

Choose Fantomas for an F# project that needs a free, licensed formatter with editor guides and a CI check for style drift. Its repository-friendly version pinning and Microsoft-based defaults make it a straightforward standardization choice. Look elsewhere if your source cannot yet parse, your team needs a wider configuration surface, or your project needs formatting beyond F#.