I’m trying to write a lua filter for Quarto/pandoc that removes all code blocks that do not match the target languages as defined in the yaml header of the document. This is the filter I got from another answer:

function Pandoc (doc)
    if doc.meta.target_lang then
        local target_lang = pandoc.utils.stringify(doc.meta.target_lang)
        print("target lang is " .. target_lang)
        return doc:walk {
            CodeBlock = function (cb)
              if cb.classes[1] ~= target_lang then
                return {}
              end
            end
          }
    end  
    return doc
  end

However, this one returns an empty codeblock, which is annoying when rendering to ipynbb:

{
      "cell_type": "code",
      "execution_count": null,
      "metadata": {},
      "outputs": [],
      "source": [],
      "id": "c0eaebc8-2af1-42bc-840a-dda2e54fddbb"
}

This is the original document:

---
title: Some title
author: Some author
date: last-modified
format:
  ipynb: 
    toc: false
    filters: 
      - langsplit.lua
taget_lang: "python"
---

Here is some text.

```{python test-py}
print("some python code")
```

```{r test-r}
print("some R code")
```