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How (Not) To Use Treesitter


Tree-sitter is a parser generator tool and an incremental parsing library. It can build a concrete syntax tree for a source file and efficiently update the syntax tree as the source file is edited.

Treesitter is a efficient and fast parser for languages. It has quite a bit of uses, the most popular one being for syntax highlighting. Since Treesitter gives you a syntax tree, you can work with that to accurately highlight code, even based on what is where without you having to mess with regex that is longer than this post.

So seeing as my editor of choice, Lite XL, does not have a plugin for Treesitter highlighting, I decided to make it.

Starting Off

Lite XL plugins can be made in either Lua or C. I don’t like C, or even write it, so I had to go with Lua and find a Treesitter Lua binding. Luckily, ltreesitter exists.

So after a while of reading both the ltreesitter and Treesitter documentation, and also figuring out how Lite XL’s highlighting internals worked, I made the initial commit. One thing you will notice here is that there is a function Highlight:each_token. What this does is basically ask for highlight tokens for idx, that being the line. That means that on every line to highlight, it queries the entire tree and loops over it to find the right tokens based on the line.

It was slow as hell and it was still broken.

I realized a few things:

Improvements

I made it add spacing according to the amount of columns between the adjacent nodes, and also added spaces if the first node of a line doesn’t start at 0. That improved basic files like this:

But still had a problem here:

It turns out that Treesitter gives the same node multiple times based on what I am querying, so the j there is both a parameter and a variable. A solution to this is to just filter if the node is the same except for the group. A simple filter function would replace subsequent nodes instead of the last one, because the one that is returned after is the correct one (according to my queries).

And with that, I have made it correct:

Even More Problems

You might have possibly thought that I was done and that I now have rich syntax highlighting in Lite XL and while that is technically true, it doesn’t 100% work. There were a lot more problems, one of them being that with the way I made it retrieve the nodes for the line, any multiline nodes (like a comment block) would just not be shown and the file would be displayed incorrectly.

Another issue is when actually editing text. Evergreen (this Lite XL Treesitter project) asks ltreesitter to reparse the file on every key stroke, which (surprisingly, for some reason) does not line up text properly.

The solution to the first issue is just to check if a node takes up more than one line. My good friend Takase helped with that. The solution for the other is to make an edit to the tree.

Treesitter has a special way to highlight changes to the tree. You still have to reparse the file, but before doing that you make an edit to the tree which specifies a byte offset and row/column difference. If the user inserts at 0,1 then you would make an edit to the tree that specifies the old positions as 0,0 and the new positions as 0,1. You also have to set a byte offset, so inserting an a would maybe be 0 -> 1 for the byte count.

This is a problem for me as my stupid brain can’t figure out how to calculate based on the position of the cursor and also check the amount of bytes for what the user inserted. Reparsing alone technically works.

Finishing Off

You can check out the “final” thing right here. It would be nice for someone to make some contributions so this can be more than a proof of concept. I feel that with my code is definitely not the best and most efficient way of using Treesitter like this, but it does work.


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