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.

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.