Editor Setup

This guide covers how to get syntax highlighting, indentation, and code folding for .bnf files in Neovim, Helix, Emacs, and VS Code.

Neovim (nvim-treesitter)

This section targets the current (“main” branch) nvim-treesitter, which registers parsers inside a User TSUpdate autocmd rather than through the old get_parser_configs() table. If you’re on nvim-treesitter’s legacy “master” branch, adapt accordingly or use the plugin-free alternative below instead.

1 — Register the parser

Add the following to your Neovim config (e.g. init.lua) before the nvim-treesitter setup call:

vim.api.nvim_create_autocmd('User', { pattern = 'TSUpdate',
  callback = function()
    require('nvim-treesitter.parsers').bnf = {
      install_info = {
        url = 'https://github.com/ambs/tree-sitter-bnf-tools',
        location = 'tree-sitter-bnf', -- repo is a monorepo; parser lives in this subdir
        queries = 'queries',          -- symlinks queries/ automatically, see step 4
      },
    }
  end })

For a local checkout instead of cloning from GitHub, replace url / location with path:

      install_info = {
        path = '<path-to-repo>/tree-sitter-bnf',
        queries = 'queries',
      },

2 — Install the parser

Inside Neovim, run:

:TSInstall bnf

3 — Register the filetype

Neovim does not associate .bnf files with the bnf filetype automatically. Add this to your config:

vim.filetype.add({ extension = { bnf = "bnf" } })

4 — Enable highlighting

install_info.queries from step 1 already symlinks the query directory for you, so no manual copying is needed. The current nvim-treesitter does not start the highlighter automatically — you must call vim.treesitter.start() yourself, e.g.:

vim.api.nvim_create_autocmd('FileType', {
  pattern = 'bnf',
  callback = function() vim.treesitter.start() end,
})

Without this step, the parser and queries are installed but nothing will visibly highlight.

5 — Enable folding (optional)

To use tree-sitter-based folding, add this to your config or a ftplugin/bnf.lua file:

vim.opt_local.foldmethod = "expr"
vim.opt_local.foldexpr   = "nvim_treesitter#foldexpr()"
vim.opt_local.foldenable  = false   -- open all folds by default

Neovim (plugin-free alternative)

Neovim’s built-in vim.treesitter doesn’t require the nvim-treesitter plugin at all — it can load a compiled parser and query files directly, which avoids the nvim-treesitter API/branch concerns above entirely.

1 — Build the parser

cd tree-sitter-bnf
tree-sitter build -o bnf.so

2 — Install the parser and queries

Drop the result into any directory on Neovim’s &runtimepath, e.g. ~/.config/nvim:

RUNTIME_DEST="$HOME/.config/nvim"

mkdir -p "$RUNTIME_DEST/parser" "$RUNTIME_DEST/queries/bnf"
cp bnf.so "$RUNTIME_DEST/parser/"
cp queries/*.scm "$RUNTIME_DEST/queries/bnf/"

3 — Register the filetype and enable highlighting

Same as steps 3 and 4 above:

vim.filetype.add({ extension = { bnf = "bnf" } })

vim.api.nvim_create_autocmd('FileType', {
  pattern = 'bnf',
  callback = function() vim.treesitter.start() end,
})

Helix

1 — Build the parser

Clone the repository and compile the parser:

git clone https://github.com/ambs/tree-sitter-bnf-tools
cd tree-sitter-bnf-tools/tree-sitter-bnf
tree-sitter generate   # only needed if grammar.js changed
gcc -shared -o bnf.so -fPIC src/parser.c

2 — Install the parser

Place the compiled shared library where Helix expects it:

mkdir -p ~/.config/helix/runtime/grammars
cp bnf.so ~/.config/helix/runtime/grammars/

3 — Install the query files

QUERIES_DEST="$HOME/.config/helix/runtime/queries/bnf"
QUERIES_SRC="<path-to-repo>/tree-sitter-bnf/queries"

mkdir -p "$QUERIES_DEST"
cp "$QUERIES_SRC/highlights.scm" "$QUERIES_DEST/"
cp "$QUERIES_SRC/indents.scm"    "$QUERIES_DEST/"

4 — Register the language

Add the following to ~/.config/helix/languages.toml:

[[language]]
name        = "bnf"
scope       = "source.bnf"
file-types  = ["bnf"]
roots       = []
comment-token = "#"

[[grammar]]
name   = "bnf"
source = { path = "<path-to-repo>/tree-sitter-bnf" }

Open a .bnf file and run :lang-support to confirm the language is active.

Emacs (treesit)

This targets Emacs 29+, which has treesit built in. treesit-install-language-grammar cannot be used here to clone-and-compile the grammar automatically: this repo’s generated parser sources (tree-sitter-bnf/src/) are gitignored, so a fresh clone has no src/parser.c until tree-sitter generate creates it — build the grammar by hand instead.

1 — Build the parser

git clone https://github.com/ambs/tree-sitter-bnf-tools
cd tree-sitter-bnf-tools/tree-sitter-bnf
tree-sitter generate

2 — Install the parser

Emacs loads compiled grammars from ~/.emacs.d/tree-sitter/, and expects the filename to match libtree-sitter-<language> exactly:

mkdir -p ~/.emacs.d/tree-sitter

# Linux
gcc -shared -fPIC -o ~/.emacs.d/tree-sitter/libtree-sitter-bnf.so \
    -I./src src/parser.c

# macOS
gcc -shared -fPIC -o ~/.emacs.d/tree-sitter/libtree-sitter-bnf.dylib \
    -I./src src/parser.c

Verify Emacs can load it:

M-: (treesit-language-available-p 'bnf)

Should return t. If it returns nil, the .so/.dylib file is missing or misnamed.

3 — Install the major mode

This repository ships a ready-made major mode at editors/emacs/bnf-ts-mode.el — copy it somewhere on your load-path and require it:

mkdir -p ~/.emacs.d/lisp
cp editors/emacs/bnf-ts-mode.el ~/.emacs.d/lisp/

(add-to-list 'load-path "~/.emacs.d/lisp")
(require 'bnf-ts-mode)

bnf-ts-mode provides:

Syntax highlighting, translated from tree-sitter-bnf/queries/highlights.scm into Emacs font-lock faces
Structural navigation (C-M-a / C-M-e jump between rule definitions)
Imenu / consult-imenu integration — all rule names as jumpable entries
Indentation — TAB on a | or ; line aligns it under the > of the enclosing ->
.bnf files are associated with the mode automatically

4 — Try it

Open (or create) a .bnf file — the mode name in the modeline should show BNF. Run M-x treesit-explore-mode to see the live syntax tree alongside your file, useful if you want to extend the mode’s font-lock rules.

Updating the grammar

When the grammar changes upstream, regenerate and recompile:

cd tree-sitter-bnf-tools/tree-sitter-bnf
git pull
tree-sitter generate
gcc -shared -fPIC -o ~/.emacs.d/tree-sitter/libtree-sitter-bnf.so \
    -I./src src/parser.c

Then restart Emacs (or run M-x treesit-parser-delete on the current buffer and reopen the file).

VS Code

VS Code has no built-in tree-sitter support, so highlighting comes from a TextMate grammar instead. This repository ships one — a minimal local extension at editors/vscode/ — approximating the BNF dialect: rule names, ->/=>, literals, patterns, directives, and comments. It doesn’t provide indentation or folding (those need a real tree-sitter-backed extension — out of scope here).

1 — Get the extension folder

git clone https://github.com/ambs/tree-sitter-bnf-tools

The extension lives entirely in tree-sitter-bnf-tools/editors/vscode/.

2 — Install it as an unpacked extension

Copy (or symlink) that folder into VS Code’s extensions directory:

# Linux / macOS
mkdir -p ~/.vscode/extensions
cp -r tree-sitter-bnf-tools/editors/vscode ~/.vscode/extensions/bnf-syntax

# Windows (PowerShell)
Copy-Item -Recurse tree-sitter-bnf-tools\editors\vscode "$env:USERPROFILE\.vscode\extensions\bnf-syntax"

3 — Reload VS Code

Reload the window (Developer: Reload Window from the Command Palette, or just restart VS Code). Open a .bnf file — the language mode in the bottom status bar should read BNF, and highlighting should be active.

Trying it without installing

To try the grammar without copying anything into your extensions folder, launch VS Code in Extension Development Host mode against the folder directly:

code --extensionDevelopmentPath=tree-sitter-bnf-tools/editors/vscode <some-file>.bnf

Updating the grammar

The TextMate grammar (editors/vscode/syntaxes/bnf.tmLanguage.json) is maintained by hand and only approximates the dialect — it does not track tree-sitter-bnf/grammar.js automatically. If the grammar changes in ways that affect highlighting (new directives, changed terminal syntax), update it manually and re-copy the folder, or git pull if you installed via symlink.