Fantomas

Fantomas is a free formatter for F# code, aimed at developers who want one shared style across editors and repositories. Its .NET tooling and CI check suit teams that already work in the F# toolchain; projects that need formatting for several languages will need another tool.

Overview

Fantomas applies configured formatting to F# source, using Microsoft’s F# style guide as its default and offering settings for G-Research conventions. This gives teams a clear baseline without requiring them to invent a style. An online tool linked from the official site lets developers try the formatter and its settings before adding it to a project.

It runs as a .NET tool, and a local installation is recommended so a repository can pin the version used by contributors and automation. The command line accepts folders and files ending in .fs, .fsi, .fsx, .ml or .mli. Source is expected to parse with the F# compiler before formatting, so Fantomas will not fix syntax errors. The project is an fsprojects incubation project, with releases published on GitHub.

Key features

• Style and configuration: Microsoft-aligned defaults and settings for G-Research conventions cover common team choices. Only a limited set of options can be stored in .editorconfig and read automatically by the command line tool, so teams seeking extensive per-project style control may find the configuration too narrow.
• Formatting checks: dotnet fantomas check reports formatting drift and exits with a failure code when files need formatting. That makes it useful in CI, where a repository can enforce consistent output without relying on reviewers to spot style differences.
• Editor setup: The project provides setup guides for JetBrains Rider, Visual Studio Code and Visual Studio. These help bring formatting into familiar editing workflows, although the guides are setup support rather than a promise of identical behavior across every editor.
• Library and reporting: The tool and library are available under the Apache 2.0 license, and the package page links to a bug-reporting tool that opens an issue with reproduction details. The library API also gives projects a way to use Fantomas programmatically.

Pricing

Fantomas is free and open source. Its sole plan costs nothing and includes F# source code formatting, the .NET tool and the Apache 2.0 license. There is no paid tier or trial to weigh, making it a straightforward choice for individual developers and teams that can manage the tool within their existing development workflow.

Platforms

Fantomas is listed for Linux, macOS, Windows and the web. The core formatter is a .NET tool, while the official site links to an online tool for trying formatting and settings. It also supports self-hosted use, which suits teams that want formatting within their own repository and build environment.

Who it's for

Fantomas is best suited to F# developers and teams who want consistent formatting in local editing and CI, especially when pinning a tool version per repository matters. It is less suitable for teams that need a formatter spanning multiple programming languages, need extensive configuration through .editorconfig, or expect formatting to work on code that currently fails to parse.

Pros and cons

• Pro: Microsoft-style defaults give F# projects a ready-made baseline, with additional settings for G-Research conventions.
• Pro: A repository can pin the locally installed .NET tool version, while the check command makes formatting drift fail CI.
• Pro: Editor setup guides, an online trial tool and a library API support use beyond a single command-line workflow.
• Con: The tool is focused on F# and accepts only its supported extensions, so it cannot serve as a repository-wide formatter for unrelated languages.
• Con: Input must already parse with the F# compiler, and only a limited set of settings can be read from .editorconfig.

Alternatives

For other language ecosystems, Black is a free Python formatter, while Fourmolu and Ormolu are free Haskell formatters. rustfmt is a free choice for Rust, and Zig fmt comes with the Zig toolchain. goimports is a free open-source command-line tool for Go. These are better fits when the repository’s language matches their focus; Fantomas remains the relevant option for F#.

Prettier supports a broader set of platforms, including API and extension use, and offers enterprise support and maintenance through Tidelift Subscription, making it a candidate for teams whose tooling needs match that broader offering. Accessibility Test Framework for Android is another free alternative.

Browse more options in Code Formatting Tools.

Verdict

F# developers who want a shared, enforceable style should choose Fantomas for its opinionated Microsoft-based defaults, repository-pinnable .NET tool and CI check, all at no cost. Look elsewhere if you need one formatter for multiple languages, broad configuration, or support for source that does not yet parse.