Tree-sitter has taken the world of programming by a storm. Together with LSP, it’s probably the technology that has influenced the most programming editors and IDEs in the past several years. And now that Emacs 29+ comes with built-in Tree-sitter support I’ve been spending a lot of quality time with it, working on clojure-ts-mode and neocaml-mode. There’s a lot I’d like to share with you about using Tree-sitter effectively, but today I’ll focus on a different topic. When most people hear about Tree-sitter they think of font-locking (syntax highlighting) and indentation powered by the abstract syntax tree (AST), generated by a Tree-sitter grammar. For a while I’ve also been thinking that the AST data can also be used for simple, yet reasonably accurate, code completion. (within the context of a single code buffer, that is) That’s definitely not nearly as powerful of what you’d normally get from a dedicated tool (e.g. an LSP server), as those usually have project-wide completion capabilities, but it’s pretty sweet given that it’s trivial to implement and doesn’t require any external dependencies. Below, you’ll find a simple proof of concept for such a completion, in the context of clojure-ts-mode:1 (defvar clojure-ts--completion-query-globals (treesit-query-compile 'clojure `((source (list_lit ((sym_lit) @sym (:match ,clojure-ts--variable-definition-symbol-regexp @sym)) :anchor [(comment) (meta_lit) (old_meta_lit)] :* :anchor ((sym_lit) @var-candidate))) (source (list_lit ((sym_lit) @sym (:match ,clojure-ts--function-type-regexp @sym)) :anchor [(comment) (meta_lit) (old_meta_lit)] :* :anchor ((sym_lit) @fn-candidate)))))) (defconst clojure-ts--completion-annotations (list 'var-candidate " Global variable" 'fn-candidate " Function")) (defun clojure-ts--completion-annotation-function (candidate) (thread-last minibuffer-completion-table (alist-get candidate) (plist-get clojure-ts--completion-annotations))) (defun clojure-ts-completion-at-point-function () (when-let* ((bounds (bounds-of-thing-at-point 'symbol)) (source (treesit-buffer-root-node 'clojure)) (nodes (treesit-query-capture source clojure-ts--completion-query-globals))) (list (car bounds) (cdr bounds) (thread-last nodes (seq-filter (lambda (item) (not (equal (car item) 'sym)))) (seq-map (lambda (item) (cons (treesit-node-text (cdr item) t) (car item))))) :exclusive 'no :annotation-function #'clojure-ts--completion-annotation-function))) I hope you’ll agree that the code is both simple and easy to follow (especially if you know a bit about Tree-sitter queries and Emacs’s completion APIs). The meat of the example is clojure-ts--completion-annotation-function, the rest is just completion scaffolding. And the result looks like this: Not too shabby for 30 lines of code, right? With a bit more efforts this can be made smarter (e.g. to include local bindings as well), and potentially we can even be consulting all open buffers running clojure-ts-mode to fetch completion data from the as well. (although that’s probably an overkill) Still, I think that’s an interesting use of Tree-sitter that some of you might find useful. It seems that Nic Ferrier has been playing with this idea recently as well - check out his recent video on the subject here. In time Tree-sitter will redefine how we’re building Emacs major modes and what they can do.2 It’s still early days and sky is the limit. Exciting times ahead! That’s all I have for you today. Keep hacking! P.S. I plan to write more on the topic of Tree-sitter and how to use it in Emacs major modes, but in the mean time you might find some of my development notes useful: Neocaml notes clojure-ts-mode design notes Kudos to Roman Rudakov, who put this prototype together earlier today after a short discussion we had on the topic. ↩ I can easily imagine things like Tree-sitter based linters or complex refactoring commands. ↩