How do I convert Markdown a YAML key:value to Typst output, when the value is sometimes wrapped in quotes, sometimes not?

[fungai2000]

Hello. I am trying convert a key:value pair in my Makrdown YAML to Typst output, using a template.

I am trying to set a numbering pattern for headers through a key-value pair: numbering: X”, where X could be a string (none) or could be a alpha-numerical pattern (“1.1”, “I.1.A”, etc.). In the latter case, Typst wants the value to be wrapped in quotations marks. If the value is none, no quotations marks should be used.

I need to detect when the numbering value is specifically none (as a string) versus other values. However, Pandoc doesn't appear to have a built-in equals comparison operator in its templating language, as far as I am aware.

This is the code in my template to convert, via template, from Markdown to Typst:


$if(numbering)$
$if(numbering-is-none)$
  set heading(numbering: none)
$else$
  set heading(numbering: "$numbering$")
$endif$
$else$
  set heading(numbering: none)
$endif$

I want to achieve the following:

1. if the YAML has a numbering key-value pair, and the value is anything other than none (like 1.1, or 1.a, etc.), then output set heading(numbering: “X”) where X is the pattern, and ensure there are quotation marks around the number.

---
title: foo
numbering: "1.1"
---
# Heading
Hello world.

2. if the YAML a numbering key-value pair, and the value is none, then output set heading(numbering: none) without any quotations marks around the word none.

---
title: foo
numbering: none
---
# Heading
Hello world.

3. if the YAML has no numbering key-value pair at all, then output set heading(numbering: none), without any quotation marks around the word none.

---
title: foo
---
# Heading
Hello world.

With the assistance of Claude Code, the only solution I’ve been able to arrive at is to use a Lua filter:

-- Lua filter to handle Typst numbering values
-- Converts 'none' to unquoted none, keeps other values quoted

function Meta(meta)
  if meta.numbering then
    local numbering_value = pandoc.utils.stringify(meta.numbering)
    if numbering_value == "none" then
      -- Set a flag for template to use unquoted none
      meta['numbering-is-none'] = true
      meta.numbering = nil
    else
      -- Keep the original numbering value (will be quoted in template)
      meta['numbering-is-none'] = false
    end
  end
  return meta
end

However, I would rather not have to remember to depend on this lua filter, if at all possible. I’d be grateful for any guidance. Thank you.

[tarleb]

The LLM is right in this case, and I don't see any simpler way with the constraints you've given.

If possible, use false instead of none, that would allow you to keep everything in the template.

[fungai2000]

This works perfectly and indeed keeps it in the template without needing another lua filter. Thank you!

[iandol]

@tarleb -- an == operator would nevertheless be cool for templates! I have a request for this from 2017: #3697

@fungai2000 -- don't forget that command-line settings can be set up in a defaults file, so you can add the filter to a defaults file (store din your pandoc data dir) then just call pandoc -d typst and the filter and any other typst-specific options would be specified...

[fungai2000]

Thank you for this, @iandol. I agree that an == operator would have solved the original query, and gives flexibility for many other operations as well. But @tarleb's solution worked perfectly well for my immediate need.

And thank you for the tip on a defaults file!