Custom section numbering

[pwseo]

Is there an easy way to customize how section numbering is rendered in the documents? In a certain type of documents, I'd like for the level 3 headings to be rendered as "Article X" followed by a linebreak and then the actual section title (X being the counter). In the sidebar it would be ideal to keep the default rendering, though.
Does one have to create a new template in order to do this?

[jjallaire]

This isn't super straightforward right now but it's likely possible if you are willing to do some hacking. If you are only targeting HTML you could use CSS counters (https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Counter_Styles/Using_CSS_counters)

Another approach would be to use a pandoc filter (https://quarto.org/docs/authoring/filters.html). Note that you can arrange for your filter to run before or after quarto (where quarto will actually be doing the numbering). A key thing to know is that if you write a function to intercept headers you can actually return a list of elements from the filter (so the Header is e.g. replaced with Header,Paragraph)

[pwseo]

Thanks for the quick reply, @jjallaire
I think I will try to build a pandoc filter, then :) I'm sure in the future this will be easier to accomplish, once other, more important aspects of Quarto are finished.

[jjallaire]

Okay, great! To get you started here's a simple example. This document adds a filter that will be run after the quarto filters:

---
title: "pandoc-header-filter"
number-sections: true
filters: 
  - quarto
  - article-headers.lua
---

## Header

### Other Header

Here's the filter source code:

function Header(el)
  if el.level == 3 then
    return pandoc.List({
      el, 
      pandoc.Para(pandoc.Str("More content"))
    })
  end
end

Since this filter is run after quarto you'll see that if you look inside el.content there is a span with the heading number (you can dump an element for debugging in Lua with print(el)). If you instead run your filter before quarto there will no numbering (which you might want if you are implementing a counter). You can prevent automatic numbering by quarto by either setting number-depth or by having your filter add the unnumbered class to the heading.

[ratnanil]

@pwseo: Did you ever implement your own lua filter based numbering? I would be interested in this as well!

[pwseo]

Again, thank you very much for the quick reply and example code.