Fatou

Fatou is a language server, formatter, and linter for Julia that never has to run Julia itself. It bundles three tools in one:

Formatter (fatou format): fast, deterministic, and opinionated
Linter (fatou lint): configurable, with auto-fix support for many rules
Language server (fatou lsp): both of the above, plus IDE features like completion, hover, go-to-definition, and more

Fatou is fast, safe, and easy to embed in editors and tooling. The architecture follows rust-analyzer: a lossless rowan syntax tree that reconstructs the input byte-for-byte, salsa for incremental recomputation, and lsp-server for the language-server transport. It is named after the French mathematician Pierre Fatou, whose Fatou set is the complement of the Julia set.

Installation

Fatou is available from several sources:

crates.io: cargo install fatou
Homebrew: brew install jolars/tap/fatou
npm: npm install -g fatou-cli (bundles a prebuilt binary)
PyPI: uv tool install fatou/pipx install fatou
AUR (Arch Linux): paru -S fatou-bin (or any other AUR helper)
mise/Aqua: see the installation guide
NixOS: the fatou package is available in the Nixpkgs repository
Prebuilt binaries: from the releases page
VS Code/Open VSX: the Fatou extension (Marketplace, Open VSX); also works in Positron

Install Script

If you prefer a one-liner installer that picks the right release artifact for your platform, you can use the installer scripts below, which download the latest matching Fatou release for your platform, installing to a user-local directory by default. If you prefer, download and inspect the script before running it.

For macOS and Linux:

curl --proto '=https' --tlsv1.2 -sSf https://fatou.dev/install | sh

For Windows PowerShell:

powershell -NoProfile -ExecutionPolicy Bypass -Command "irm https://fatou.dev/install.ps1 | iex"

Usage

fatou format file.jl
fatou lint file.jl

Formatting rewrites the file in place; linting reports findings and exits nonzero if it finds any. See Formatting for directory runs, checks, and preservation verification, and Linting for rule selection, fixes, and suppressions.

Put shared settings in fatou.toml; the configuration guide explains setup and discovery.

Editor Integration

The language server (fatou lsp) runs over stdio and provides a broad set of features:

formatting (whole-document and range),
linting (push and pull),
completion (including the REPL's LaTeX and emoji input sequences), hover,
go-to-definition, -find references,
document highlights,
rename (of symbols, and of files and folders),
document and workspace symbols,
call and type hierarchy,
signature help,
code actions,
folding and selection ranges,
document links, and
semantic tokens.

Static docstrings participate in those features: decoded local and indexed documentation renders as Markdown, headings appear in the outline and fold as sections, external and internal Markdown links navigate, explicit Documenter @ref targets complete and jump to definitions, and Julia-declared fences get completion, hover, signature help, definition, selection, folding, and semantic highlighting.

The Fatou extension for VS Code and Open VSX (Marketplace, Open VSX) bundles the binary and starts the server automatically; it also works in Positron. See editors/code for the extension, or the editor setup guide for Neovim and other editors.

Fatou is also available from Zed's extension registry. Install Julia, then Fatou, from Zed's extensions view. See the Zed setup guide for configuration.

Integrations

Format Julia files with dprint using dprint-plugin-fatou. The plugin bundles the formatter and uses the fatou settings in dprint.json. See the integrations guide for setup, configuration, and updates.

Run format and lint checks in GitHub Actions with fatou-action, which installs a prebuilt, checksum- and provenance-verified binary. See the GitHub Actions guide for a complete workflow and configuration options.

Run Fatou through pre-commit hooks with fatou-pre-commit. See the pre-commit guide for installation, autofixes, and updates.

Library Crates

The parser and formatter are published as standalone, wasm-compatible crates for embedding in other tools:

fatou-parser: lossless CST parser, typed AST wrappers, and incremental reparser.
fatou-formatter: the formatting engine, with optional serde/schema features.

Documentation

See https://fatou.dev/ for the full documentation.

Contributing

See CONTRIBUTING.md.

License

MIT, see LICENSE.