update meta data using lua filter

[ThierryO]

Description

Can we use a Lua filter to update the meta data? I'm trying to set the value of book:body-footer. The Lua filter below is my failing attempt.

return {
  {
    Meta = function(meta)
      meta.book.body-footer = 'test'
      return meta
    end
  }
}

[cscheid]

Did you follow the error message you're probably getting? body-footer is not valid Lua syntax in that context. You need meta.book["body-footer"].

(In addition, this might not work depending on what you're trying to do with the metadata, because of the order in which documents are processed and filters are run matters.)

[mcanouil]

If it helps, here is a real life example:

• https://github.com/mcanouil/quarto-lua-env/blob/main/_extensions/lua-env/lua-env-filter.lua#L73-L88

[ThierryO]

Updating meta.book["body-footer"] did work. I can display the content with the shortcode {{< meta book.body-footer>}}. However it does not add the text as a footer to the pages. I worked around this by creating a shortcode and placing that in the _quarto_yml.

book:
  body-footer: '{{< footer >}}'

[mcanouil]

Why not use the variable shortcode? (https://quarto.org/docs/authoring/variables.html#var)

• _variables.yml

my-footer: Whatever

• _quarto.yml

book:
  body-footer: "{{< var my-footer >}}"

[ThierryO]

I'm in the process moving our corporate identity from bookdown to quarto. The shortcode combines several meta items into the footer. Ideally the extension should always set the footer without the user having to set book: body-footer.

function is_empty(s)
  return s == nil or s == ''
end

return {
  ['footer'] = function(args, kwargs, meta)
    z = '<a rel="license" href="http://creativecommons.org/licenses/by/4.0/">' ..
      '<img alt="Creative Commons-License" style="border-width:0" class="ccby"/>' ..
      '</a>' .. meta.shortauthor
    if is_empty(meta.flandersqmd.year) then
      z = z .. '<h1 class = "missing">!!! Missing flandersqmd.year !!!</h1>'
    else
      z = z .. ' (' .. pandoc.utils.stringify(meta.flandersqmd.year) .. ')'
    end
    if is_empty(meta.flandersqmd.doi) then
      z = z .. '<h1 class = "missing">!!! Missing flandersqmd.doi !!!</h1>'
    else
      local x = pandoc.utils.stringify(meta.flandersqmd.doi)
      z = z .. ' <a href="https://doi.org/' .. x .. '">' .. x .. '</a>'
    end

    return pandoc.RawInline("html", z)
  end
}

[mcanouil]

Nice, thanks for the explanation!