Claude Code: Setting up ast-grep with R support

When coding or using coding agents, one way to improve your workflow is by using CLI tools with a very clear focus. This is especially useful since coding agents primarily ingest text.

One new amazing tool I found was ast-grep, which allows us to search code by its syntax instead of using regular expressions. This lets us search for things that are otherwise really hard to get right in regex, like:

“All if-else statements with an return() in one of the branches”
“All functions with 5-6 arguments”
“All used of util_fun() used with debug argument”

ast-grep doens’t have native R support, but we are able to add it using custom language support.

Tip

A different way of approaching this is by installing the R package astgrepr and have Claude Code use the R functions instead of the CLI.

Installing things

First, you need to install ast-grep. I used brew with

brew install ast-grep

Next we need to install tree-sitter-cli which i did with npm.

npm install -g tree-sitter-cli

Lastly, we need to add the R grammar that tree-sitter understands.

Note

I’m on a Mac, and that is what I know to work with, the general spirit of what to do should easily translate to other OS.

First, we create a config directory for astp-grep.

mkdir -p ~/.config/ast-grep

Then we need to generate the dynamic library of tree-sitter-r such that ast-grep can use it.

# Clone and compile the R grammar
cd /tmp
git clone https://github.com/r-lib/tree-sitter-r.git
cd tree-sitter-r
tree-sitter build --output ~/.config/ast-grep/r.dylib

Adding R config

Now that we have everything we need, we can let ast-grep ingest and understand tree-sitter-r. Start by creating ~/.config/ast-grep/sgconfig.yml and add the following to it

customLanguages:
  r:
    libraryPath: /Users/YOUR-USER-NAME-HERE/.config/ast-grep/r.dylib
    extensions: [r, R]
    expandoChar: _

And as a quality-of-life improvement, we also add an alias so we don’t need to type out the config each time we use it.

Add to ~/.zshrc:

alias sg='ast-grep run -c ~/.config/ast-grep/sgconfig.yml'

Claude Code integration

I personally want Claude Code to know about this tool, no matter which project I’m on, So I am adding the following to ~/.claude/CLAUDE.md so future sessions know about this setup:

## ast-grep with R support

R language support is configured globally via `~/.config/ast-grep/sgconfig.yml`.

Usage:
\```bash
sg -l r -p 'pattern' .
\```

Note: Use `_VAR` for named metavariables and `___` for wildcards (not `$VAR`) because R uses `$` for column access.

Usage

We can also use the tool, the documentation has a wonderful guide, and the examples listed above can be done with:

# Find if-else statements with return() in a branch
sg -l r -p 'if (___) { return(___) } else ___' .
sg -l r -p 'if (___) ___ else { return(___) }' .

# Find functions with 5-6 arguments
sg -l r -p 'function(_A, _B, _C, _D, _E) _BODY' .
sg -l r -p 'function(_A, _B, _C, _D, _E, _F) _BODY' .

# Find uses of util_fun() with debug argument
sg -l r -p 'util_fun(debug = ___)' .