helix/book/src/guides/adding_languages.md

# Adding languages

## Submodules

To add a new language, you should first add a tree-sitter submodule. To do this,
you can run the command
```sh
git submodule add -f <repository> helix-syntax/languages/tree-sitter-<name>
```
For example, to add tree-sitter-ocaml you would run
```sh
git submodule add -f https://github.com/tree-sitter/tree-sitter-ocaml helix-syntax/languages/tree-sitter-ocaml
```
Make sure the submodule is shallow by doing
```sh
git config -f .gitmodules submodule.helix-syntax/languages/tree-sitter-<name>.shallow true
```

or you can manually add `shallow = true` to `.gitmodules`.

## languages.toml

Next, you need to add the language to the [`languages.toml`][languages.toml] found in the root of
the repository; this `languages.toml` file is included at compilation time, and
is distinct from the `language.toml` file in the user's [configuration
directory](../configuration.md).

These are the available keys and descriptions for the file.

| Key                 | Description                                                   |
| ----                | -----------                                                   |
| name                | The name of the language                                      |
| scope               | A string like `source.js` that identifies the language. Currently, we strive to match the scope names used by popular TextMate grammars and by the Linguist library. Usually `source.<name>` or `text.<name>` in case of markup languages |
| injection-regex     | regex pattern that will be tested against a language name in order to determine whether this language should be used for a potential [language injection][treesitter-language-injection] site. |
| file-types          | The filetypes of the language, for example `["yml", "yaml"]`  |
| shebangs            | The interpreters from the shebang line, for example `["sh", "bash"]` |
| roots               | A set of marker files to look for when trying to find the workspace root. For example `Cargo.lock`, `yarn.lock` |
| auto-format         | Whether to autoformat this language when saving               |
| diagnostic-severity | Minimal severity of diagnostic for it to be displayed. (Allowed values: `Error`, `Warning`, `Info`, `Hint`) |
| comment-token       | The token to use as a comment-token                           |
| indent              | The indent to use. Has sub keys `tab-width` and `unit`        |
| config              | Language server configuration                                 |

## Queries

For a language to have syntax-highlighting and indentation among
other things, you have to add queries. Add a directory for your
language with the path `runtime/queries/<name>/`. The tree-sitter
[website](https://tree-sitter.github.io/tree-sitter/syntax-highlighting#queries)
gives more info on how to write queries.

> NOTE: When evaluating queries, the first matching query takes
precedence, which is different from other editors like neovim where
the last matching query supercedes the ones before it. See
[this issue][neovim-query-precedence] for an example.

## Common Issues

- If you get errors when building after switching branches, you may have to remove or update tree-sitter submodules. You can update submodules by running
    ```sh
    git submodule sync; git submodule update --init
    ```
- Make sure to not use the `--remote` flag. To remove submodules look inside the `.gitmodules` and remove directories that are not present inside of it.

- If a parser is segfaulting or you want to remove the parser, make sure to remove the submodule *and* the compiled parser in `runtime/grammar/<name>.so`

- The indents query is `indents.toml`, *not* `indents.scm`. See [this](https://github.com/helix-editor/helix/issues/114) issue for more information.

[treesitter-language-injection]: https://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-injection
[languages.toml]: https://github.com/helix-editor/helix/blob/master/languages.toml
[neovim-query-precedence]: https://github.com/helix-editor/helix/pull/1170#issuecomment-997294090