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)
• 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

# Format in place
fatou format file.jl

# Verify formatting without writing
fatou format --check src/

# Lint (exits non-zero if there are any findings)
fatou lint src/

# Fix lint findings in place
fatou lint --fix file.jl

Configuration lives in fatou.toml: [format] sets line and indent widths, [lint] selects or ignores rules. Fatou walks up from the file's directory to find one, stopping at the repository root, then falls back to $FATOU_CONFIG and to a global user config at ~/.config/fatou/fatou.toml.

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.

Integrations

Run format and lint checks in GitHub Actions with fatou-action, which installs a prebuilt, checksum- and provenance-verified binary:

- uses: jolars/fatou-action@v1

Or as pre-commit hooks with fatou-pre-commit:

repos:
  - repo: https://github.com/jolars/fatou-pre-commit
    # fatou version
    rev: v0.7.0
    hooks:
      - id: fatou-lint
      - id: fatou-format

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.