treesit-auto

Automatically install and use tree-sitter major modes in Emacs 29+. If the tree-sitter version can’t be used, fall back to the original major mode.

Features

Each of these behaviors are configurable and documented under the “Configuration” section. By activating global-treesit-auto-mode, Emacs will:

• Automatically switch to <name>-ts-mode when the grammar for <name> is installed
• Stick with <name>-mode if the grammar isn’t installed
• (Optional) automatically install a grammar before opening a compatible file

There is also a convenience function M-x treesit-auto-install-all, which will install all of the maintained and compatible grammars.

Installation

treesit-auto is available from MELPA. After following their setup, you can use your preferred package manager. If that’s the default package.el, simply M-x package-refresh-contents and then

M-x package-install RET treesit-auto

If you want a local clone of the repository, rather than just a copy of the source, you might instead use package-vc-install

M-x package-vc-install RET https://github.com/renzmann/treesit-auto.git

Then, in your Emacs configuration file (~/.emacs.d/init.el),

(use-package treesit-auto
  :config
  (global-treesit-auto-mode))

For most users, this will be enough. There are some nifty things you might want to enable, though, which are covered in the “Configuration” section below.

What this package does

Emacs 29, while featuring treesit.el and a convenient treesit-install-language-grammar, will not feature an intelligent way to choose between a default mode, such as python-mode, and its tree-sitter enhanced version, python-ts-mode. This package attempts to remedy that by adjusting the major-mode-remap-alist and treesit-language-source-alist variables in order to get the following behavior:

1. If the grammar is installed, then switch to the appropriate tree-sitter mode:

In this case, assuming we open a Python buffer, and the Python tree-sitter grammar is installed, then Emacs will use python-ts-mode instead of python-mode.

2. The grammar is NOT installed and treesit-auto-install is non-nil:

When the grammar is not installed and treesit-auto-install is t, then upon activating any major mode that has a corresponding tree-sitter mode, the grammar will be downloaded and compiled using treesit-install-language-grammar. Emacs will then activate the tree-sitter major mode for that buffer.

prompt is like t, except a message will be displayed in the echo area asking for a yes/no response before attempting the installation.

As an example for both cases: if I visit a Python file and didn’t already have the grammar installed, I wind up with an installed grammar and a buffer using python-ts-mode.

Otherwise, when treesit-auto-install is nil, it will try to fall back to another major mode as described in the following two rules.

3. If the grammar is NOT installed, and a fallback is specified

Most languages will have a fallback mode specified, such as python-ts-mode falling back to python-mode, if the grammar is not installed. If you ever need to double-check what that fallback will be, you can double check what’s in the recipe for that language like this:

(treesit-auto-recipe-remap (alist-get 'python treesit-auto-lang-recipe-alist))
    ⇒ python-mode

See “Configuration/Configuring behavior for a specific language” in case you would like to specify different fallback modes than the default.

4. All other cases…

This is the most general case, where the grammar is not installed, treesit-auto-install is nil, and no fallback mode is specified in the language recipe present on treesit-auto-recipe-list. In this case, we still gain the benefit of quickly installing grammars through treesit-install-language-grammar without having the build the recipe interactively, but treesit-auto will make no attempt to switch away from the tree-sitter mode.

Configuration

If you have modified treesit-language-source-alist through setq, then it is recommended to put any configuration of this package AFTER that setq.

Automatically install grammars if they are missing

The treesit-auto-install variable controls whether a grammar should be installed automatically when activating a major mode compatible with tree-sitter.

1. nil, the default, means treesit-auto won’t try to install anything, and will rely on the fallback logic outlined above
2. t means treesit-auto should always try to clone and install a grammar when missing
3. prompt will cause a yes/no prompt to appear in the minibuffer before attempting installation

(setq treesit-auto-install 'prompt)

Then, supposing I don’t have libtree-sitter-python.so (or its mac/Windows equivalent) under ~/.emacs.d/tree-sitter (or anywhere else in treesit-extra-load-path), visiting a Python file or calling M-x python-ts-mode will generate this prompt:

Tree-sitter grammar for python is missing.  Would you like to install it from https://github.com/tree-sitter/tree-sitter-python? (y or n)

Responding with “yes” will use treesit-install-language-grammar to go fetch and compile the missing grammar.

The other function that respects this variable is treesit-auto-install-all. When treesit-auto-install is t, using M-x treesit-auto-install-all will skip all prompts. Otherwise, it will ask before attempting the installation.

Configuring behavior for a specific language

The variable treesit-auto-recipe-list keeps track of all the language “recipes.” These control how treesit-auto decides which modes to upgrade/downgrade to/from, where the source code of the language grammar is hosted, and which C/C++ compiler to use. Each recipe can take these arguments:

:lang
:ts-mode
:remap
:url
:revision
:requires
:source-dir
:cc
:c++

To create a recipe, use make-treesit-auto-recipe:

 (setq my-js-tsauto-config
	(make-treesit-auto-recipe
	 :lang 'javascript
	 :ts-mode 'js-ts-mode
	 :remap '(js2-mode js-mode javascript-mode)
	 :url "https://github.com/tree-sitter/tree-sitter-javascript"
	 :revision "master"
	 :source-dir "src"))

 (add-to-list 'treesit-auto-recipe-list my-js-tsauto-config)

Here, we’ve specified that the tree-sitter compiler will be creating a file named libtree-sitter-javascript.so (or .dylib or .dll), based on the :lang field. The corresponding tree-sitter mode in Emacs is called js-ts-mode, and all of js2-mode, js-mode, and javascript-mode should attempt switching to the js-ts-mode, if possible.

Moreover, since js-2-mode is first under the :remap section, that is the “primary fallback.” Meaning that if the tree-sitter grammar is not available, it will be the first mode tried. If that doesn’t work, it will try js-mode, and javascript-mode, in that order, until one does work. If only one fallback needs to be specified, a single quoted symbol is also acceptable. For instance, python-ts-mode just uses :remap 'python in this argument position.

If a grammar mandates any other grammars be installed as a dependency, the :requires keyword can specify a language symbol or list of symbols that should be installed. One example of this is found in the TypeScript recipe, which specifies :requires 'tsx, since activating typescript-ts-mode on some Emacs builds will attempt to load the TSX grammar.

The :url, :revision, :source-dir, :cc, and :c++ arguments are all documented under treesit-language-source-alist, which is part of base Emacs, not this package.

Keep track of your hooks

This package does not modify any of your major mode hooks. That is, if you have functions in python-mode-hook, but not in python-ts-mode-hook, then your hook from python-mode will not be applied, assuming python-ts-mode is what gets loaded. For major modes in which this is a concern, the current recommendation is to address this as part of your configuration.

(setq rust-ts-mode-hook rust-mode-hook)

Some modes have a shared base, such as python-ts-mode and python-mode both deriving from python-base-mode. For these languages, you can opt to hook into python-base-mode-hook instead of explicitly setting the tree-sitter mode’s hook.

Full example

This is how I configure treesit-auto for my own personal use.

(use-package treesit-auto
  :demand t
  :config
  (setq treesit-auto-install 'prompt)
  (global-treesit-auto-mode))

Caveats

This package is, admittedly, a hack. treesit.el provides an excellent foundation to incremental source code parsing for Emacs 29, and over time that foundation will expand into an improved core editing experience. With that in mind, I fully expect this package to eventually be obsolesced by the default options in Emacs 30 and beyond. That does not preclude us from adding a few quality of life improvements to Emacs 29, though, and so it still seems prudent to have this plugin available in the meantime.

Contributing

Bug reports, feature requests, and contributions are most welcome. Even though this is a small project, there is always room for improvement. I also appreciate “nitpicky” contributions, such as formatting, conventions, variable naming, code simplification, and improvements to language in documentation.

Issues are tracked on GitHub, which is also where patches and pull requests should be submitted.

If you would like to submit a new language recipe to be distributed as part of this package, see CONTRIBUTING.md for a quick guide on how to write and submit the new recipe.