在 Pandoc 过滤器中嵌套额外的 Span 会使图像消失

Question

I am currently working on massaging the HTML output of a Pandoc filter due to some annoying restrictions in the CMS that is the eventual beneficiary of my hard work.

My working filter (now with the obvious declarations) is as follows:

local List = require 'pandoc.List'

local Emph = pandoc.Emph
local Quoted = pandoc.Quoted
local Span = pandoc.Span
local Str = pandoc.Str
local Strong = pandoc.Strong

local image_base = "http://my.website.example/images/"
local image_author = "Someone Not Stigma"

function process_images(el)
  el.src = el.src:gsub("^file:images/", image_base)
  el.caption = {
    Strong( Quoted( "DoubleQuote", el.caption ) ),
    Str(" by "),
    Emph(image_author)
  }
  return el
end

return {{Image = process_images}}

In the eventual HTML, this gives me a nice figure with img and figcaption element inside of it. Wonderful. Unfortunately, my CMS destroys the figcaption (like it tends to destroy other stuff), and as such I figured I'd wrap everything in an extra span so I can style that one instead.

function process_images(el)
  el.src = el.src:gsub("^file:images/", image_base)
  el.caption = {
    Span(
      {
        Strong( Quoted( "DoubleQuote", el.caption ) ),
        Str(" by "),
        Emph(image_author)
      },
      { class="img-caption" }
    )
  }
  return el
end

And yet somehow, this causes Pandoc to completely delete the image from the resulting HTML.

I have tried replacing the table syntaxes with List({}) syntaxes, but that just gives me upvalue complaints. I looked at the manual, but for as far I can tell I am doing everything right.

What am I missing here?

I call pandoc as follows:

pandoc --from=markdown-tex_math_dollars "Content.pure.txt" --lua-filter=".\pandoc-filter.lua" --to=html5 --template=".\pandoc-template.txt" -o "Content.txt"

Extensions are .txt (because these files are not browser ready). The template being used is rather lengthy (there's a fair bit of YAML variables and related markup), but be assured: $body$ can be found in there.

Answer 1

I am not a wise man. Always update to the latest version before posting questions, folks.

I was running an older version of Pandoc (v2.6), and upgrading to v2.9.1.1 suddenly made the output appear again. That's a lot of versions released in the span of about a year!

(In my defense, my Pandoc-filter-fu is not particularly strong, so it makes sense to assume user error rather than program bug. Why is it that every time you assume bug, it is user error, and every time you assume user error, it is an outright bug?)