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09/07 — 2022
29.17 cm   2.6 min

Incremental select with treesitter

treesitter is a tool that can build a concrete syntax tree for a source file and incrementally update the tree as the file is modified.

Recently, I’ve been experimenting more and more with treesitter’s query DSL and an abstraction layer in the form of a neovim plugin, and found a configuration option to be particularly useful when editing code: incremental selection.

When you write a treesitter parser, you must write a grammar that treesitter can read, which it then uses to generate the parser. This grammar specifies the named syntax nodes in the tree and how they relate to other nodes.

For instance, if we head over to the syntax tree playground and parse rust source code that looks like this:

fn main() {
  println!("Hello, world!");
}

we get the following syntax tree output:

source_file [0, 0] - [3, 0]
  function_item [0, 0] - [2, 1]
    name: identifier [0, 3] - [0, 7]
    parameters: parameters [0, 7] - [0, 9]
    body: block [0, 10] - [2, 1]
      macro_invocation [1, 2] - [1, 27]
        macro: identifier [1, 2] - [1, 9]
        token_tree [1, 10] - [1, 27]
          string_literal [1, 11] - [1, 26]

The neovim plugin written as an abstraction layer for treesitter usage within neovim is able to parse your source file as you type. That is, it’s able to compute a syntax tree that resembles the one shown above, on the fly.

As you hover over a certain part of a file, it is able to detect which node you’re currently at in the tree.

The plugin includes a configuration option which allows for the setting of keybindings for initializing a node selection and incrementing a selected node range – configured like so:

incremental_selection = {
  enable = true,
  keymaps = {
    init_selection = "<cr>",
    node_incremental = "grn",
    scope_incremental = "grc",
    node_decremental = "grm",
  }
}

Incremental selection has improved the rapidity in which I can yank or replace blocks of text in treesitter parsed source files, as expanding a node selection with a keybinding is super fast and usually results in the exact block of text I want to modify.

Here’s a quick demo of it in action:

asciicast

A reference of my full neovim configuration can be found on github, in my dotfiles repo. Be sure to check it out!

Hi.

I'm Liam.

I am compsci undergrad with a minor in mathematics @ mcgill university, programmer and rationalist. For fun I enjoy working on open-source projects, reading, and lifting weights.

Send me a message by email: liam [at] scalzulli.com or matrix: worse:matrix.org

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