tree-sitter-fsharp

tree-sitter grammar for F# (still WIP) Based on the 4.1 F# language specification (Mostly, Appendix A) and the F# compiler parser

Using the grammar

If you just want to use the F# parsers rather than work on the grammar, install the package for your ecosystem — each has its own usage guide:

The rest of this README covers building and developing the grammar itself.

Getting started

First, run npm install to install the tree-sitter CLI as a local devDependency (it lands at node_modules/.bin/tree-sitter, not on your global $PATH).

Run the CLI with npx tree-sitter <command> — this resolves the local binary automatically. To regenerate both parsers use npm run generate.

To parse a file: npx tree-sitter parse <file> (see the Development section for more).

Project structure

This project defines two parsers:

• ./fsharp/grammar.js — grammar for .fs/.fsx (source files).
• ./fsharp_signature/grammar.js — grammar for .fsi (signature files). This is not a standalone grammar: it starts with grammar(require("../fsharp/grammar"), { ... }) and overrides/adds a few rules on top of the base. Any change to fsharp/grammar.js also changes the derived signature grammar, so both parsers must be regenerated together (npm run generate).

Both parsers share the external scanner at ./common/scanner.h. The per-parser src/scanner.c files are thin shims that #include the header — real scanner logic (newlines, comments, indent/dedent bookkeeping) lives in the header. Each grammar starts with the file node.

The generated src/grammar.json and src/parser.c files are checked into git and validated by CI. Do not hand-edit them; merge conflicts are resolved by re-running npm run generate. See AGENTS.md for the full development workflow.

Adding to neovim

tree-sitter-fsharp is supported through the usual installation methods of nvim-treesitter.

Installing the lastest grammar from this repo involves the following three steps:

1. Update your Neovim config for nvim-treesitter to refer to tree-sitter-fsharp.
2. Run :TSInstall fsharp inside Neovim.
3. Copy the files from ./queries/ to the neovim config directory at$XDG_CONFIG_HOME/nvim/queries/fsharp/ - see the Adding queries section of the nvim-treesitter README.

The config you need is this (you can use a local path for url if you prefer):

local parser_config = require('nvim-treesitter.parsers').get_parser_configs()
parser_config.fsharp = {
  install_info = {
    url = 'https://github.com/ionide/tree-sitter-fsharp',
    branch = 'main',
    files = { 'src/scanner.c', 'src/parser.c' },
    location = "fsharp"
  },
  requires_generate_from_grammar = false,
  filetype = 'fsharp',
}

Development

The package.json defines some helpful targets for developing the grammar:

• Run commands from the repository root so tree-sitter picks up both grammars from tree-sitter.json.
• npm run generate rebuilds both parsers.
• npx tree-sitter test runs all tests for both parsers.
• npx tree-sitter parse $file runs the fsharp parser on $file and outputs the parse tree.
• npx tree-sitter parse -d $file runs the fsharp parser on $file and prints debug information.
• For .fsi files, use npx tree-sitter parse -p fsharp_signature $file because tree-sitter parse does not reliably select the signature parser automatically.

How to contribute

Clone the project and start playing around with it. If you find a code example which fails to parse, please reduce it to a minimal example and added to the corpus (fsharp/test/corpus/*.txt) as a test case.

For an introduction to developing tree-sitter parsers the official documentation is a good reference point.

PRs fleshing out the grammar or fixing bugs are welcome!