lua filter to act like we've hit EOF possible?

[jidanni]

From https://pandoc.org/filters.html#a-simple-example
we find we can stop early with

perl -pe 'exit if /QUIT/' source.txt

How might I do that with a lua filter?

Reading https://pandoc.org/lua-filters.html
the best I could come up with is

function Div(el) if el.identifier =="QUIT" then return {} end return el end

But that still requires me to wrap all the stuff I want to exclude into
a DIV, whereas I just want a single landmine, that when hit, derails the
train.

Is it possible to have a lua filter send a message to pandoc's
brain saying "party over", despite there possibly being other lua filters
waiting in line?

Note I am just trying to simulate hitting EOF (End Of input File), not
trying to say "kill -1" to the entire pandoc process.

Or could I use

function Div(el) if el.identifier =="QUIT" then KABLAAM end end

thus my Div need only be a couple lines long, no need to enclose the
whole rest of the file?

[MaxFreedomPollard]

Yes. Use a whole-document Pandoc(doc) filter instead of matching individual elements. It receives the entire block list, so you can walk it, stop at your marker, and drop everything from there on. No Div wrapping needed.

-- truncate.lua: stop at the first top-level block that is exactly "QUIT"
function Pandoc(doc)
  local kept = {}
  for _, block in ipairs(doc.blocks) do
    if pandoc.utils.stringify(block) == "QUIT" then
      break                     -- drop the marker and everything after it
    end
    kept[#kept + 1] = block
  end
  return pandoc.Pandoc(kept, doc.meta)
end

So a plain line on its own:

Keep this.

QUIT

Drop everything from here.

run through pandoc doc.md --lua-filter truncate.lua keeps "Keep this." and nothing after the QUIT line. That is your single landmine, no Div required.

On the "other filters waiting in line" question: pandoc runs --lua-filter filters left to right, each on the output of the previous one. So put this one first, and every filter after it sees the already-truncated document. I checked that by chaining a second filter that just prints #doc.blocks, and after truncation it reported only the surviving blocks.

A couple of notes:

• This works at the top-level block level, which matches the line-based exit if /QUIT/ example. The marker has to be its own top-level block (a paragraph on its own line), not buried inside another element.
• pandoc.utils.stringify ignores formatting, so == "QUIT" matches a clean marker. If you want the looser perl-regex behaviour (any block that contains QUIT), swap the test for pandoc.utils.stringify(block):find("QUIT").

Tested on pandoc 3.9.