Accessing pointers in LuaTeX

Question

This code writes the subtype of disc nodes to the log file.

\documentclass{minimal}
\usepackage{fontspec}
\directlua{
show_hyph = function(head)
  while head do
    if head.id == 0 or head.id == 1 then % hlist, vlist
      show_hyph(head.head)
    elseif head.id == 7 then             % disc
      texio.write_nl("Subtype " .. head.subtype)
    end
  head = head.next
  end
  return true
end
luatexbase.add_to_callback("post_linebreak_filter",show_hyph,"show_hyph")

}

\begin{document}
\begin{minipage}{1pt}
  discretionary
\end{minipage}
\end{document}

The LuaTeX reference manual says in section 8.1.2.7 “disc nodes”

pre <node> pointer to the pre-break text

How do I access this?

Answer 1

How do I access this?

This is very simple and straightforward: use head.pre to access the list pointing to the pre part of the discretionary.

Now the question you might ask next is "why is the pre-part nil?" This is because you call your function show_hyph() on the post_linebreak_filter. At that point the pre part is already inserted before that disc node and thus the pre part is empty. The same is with post and replace. What I can't tell you is why the disc node stays in the node list.