Editor Setup

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

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:

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).