pandoc version 3 issue: Null contrstuctor removed

[zkamvar]

Windows tests were failing last week due to {rlang} not being available, but they failed yesterday's check, which flagged an error in the build_lesson() example using the nightly build for pandoc:

Pandoc details

pandoc.exe 3.0-nightly-2023-01-16
Features: +server +lua
Scripting engine: Lua 5.4

Error

Error running filter Error: 
  ! in callr subprocess.
  Caused by error: 
  ! pandoc document conversion failed with error 83
  ---
  Standard error:
  Error running filter D:\a\sandpaper\sandpaper\check\sandpaper.Rcheck\sandpaper\rmarkdown\lua\lesson.lua:
  ...heck\sandpaper.Rcheck\sandpaper\rmarkdown\lua\lesson.lua:343: attempt to call a nil value (field 'Null')
  stack traceback:
  ---
  Backtrace:
   1. base::(if (getRversion() >= "3.4") withAutoprint else force)({ …
   2. base::source(exprs = exprs, local = local, print.eval = print., echo = echo, …
   3. | base::withVisible(eval(ei, envir))
   4. | base::eval(ei, envir)
   5. | base::eval(ei, envir)
   6. sandpaper::build_lesson(tmp)
   7. sandpaper:::build_site(path = path, quiet = quiet, preview = preview, override = overr… at build_lesson.R:100:3
   8. sandpaper::build_episode_html(path_md = abs_md[i], path_src = abs_src[i], … at build_site.R:91:5
   9. sandpaper:::render_html(path_md, quiet = quiet) at build_episode.R:80:3
  10. callr::r(function(...) rmarkdown::pandoc_convert(...), args = args, … at render_html.R:59:3
  11. callr:::get_result(output = out, options)
  12. callr:::throw(callr_remote_error(remerr, output), parent = fix_msg(remerr[[3]]))
  ---
  Subprocess backtrace:
  1. rmarkdown::pandoc_convert(...)
  2. rmarkdown:::stop2("pandoc document conversion failed with error ", result)
  3. base::stop(..., call. = FALSE)
  4. | base::.handleSimpleError(function (e) …
  5. global h(simpleError(msg, call))
  Execution halted

Source

The specific message (Error running filter [path]\lesson.lua: [path]\lesson.lua:343: attempt to call a nil value (field 'Null')) points to an error in line 343 of lesson.lua, which returns a null object to replace a questions block so that questions and objectives are grouped into the same block:

sandpaper/inst/rmarkdown/lua/lesson.lua

Lines 335 to 345 in 58e8d06

	handle_our_divs = function(el)
	-- Questions and Objectives should be grouped
	v,i = el.classes:find("questions")
	if i ~= nil then
	questions = el
	if objectives ~= nil then
	return overview_card()
	else
	return pandoc.Null()
	end
	end

[zkamvar]

Doing some digging, it appears that the Null() block type has been removed in pandoc types 1.23: jgm/pandoc-types#91.

The author suggests this replacement/workaround:

Note also that one can always write a filter that is capable of removing Blocks by making the type of the underlying transformation [Block] -> [Block]; in fact I often use a Block -> [Block] and promote it to [Block] -> [Block] using concatMap.

--- jgm/pandoc-types#91 (comment)