http://tex.stackexchange.com/q/146168

mvl.lua

local traverse_nodes      = node.traverse
local iowrite             = io.write
local tableconcat         = table.concat
local utfchar             = unicode.utf8.char
local stringformat        = string.format
local stringrep           = string.rep
local nodelength          = node.length
local count_nodes         = node.count

local nodecodes           = table.mirrored (node.types())
local glyph_t             = nodecodes.glyph
local glue_t              = nodecodes.glue
local hlist_t             = nodecodes.hlist
local vlist_t             = nodecodes.vlist

local cbk = function (hd, group, size, pack)
  --[[--
    Demo function collecting text from glyph nodes in the list starting
    with ``hd``, printing them to stdout.
  --]]--
  local text = {
    "\n["
    .. (group == "" and "par" or group)
    --- the line below shows more info from the hpack_filter context
    --.. (size and (":" .. tostring (size) .. ":" .. pack) or "")
    .. "] "
  }
  for n in traverse_nodes (hd) do
    local ntype = n.id
    if ntype == glyph_t then
      text [#text + 1] = utfchar (n.char)
    elseif ntype == glue_t then
      text [#text + 1] = " "
    else -- print node type
      text [#text + 1] = "<" .. nodecodes [ntype] .. ":" .. tostring (n.subtype) .. ">"
    end
  end

  text [#text + 1] = "\n"
  iowrite (tableconcat (text))
  return true
end

local rec_cbk rec_cbk = function (hd, _, depth, where)
  --[[--
    Recursively walk a paragraph node list.
  --]]--
  if not depth then
    iowrite "\n"
    depth = 1
    where = "par"
  end
  iowrite (stringrep (" ", depth) ..
           stringformat ("[%s:%d] %d nodes, %d glyphs\n",
                         where, depth,
                         nodelength (hd),
                         count_nodes (glyph_t, hd)))
  for n in traverse_nodes (hd) do
    local ntype = n.id
    if ntype == vlist_t then
      rec_cbk (n.head, nil, depth + 1, "vbox")
    elseif ntype == hlist_t then
      rec_cbk (n.head, nil, depth + 1, "hbox")
      --- else pass
    end
  end
  return true
end

luatexbase.add_to_callback ("pre_linebreak_filter", cbk, "document.nodeprinter")
luatexbase.add_to_callback ("hpack_filter",         cbk, "document.nodeprinter")
--luatexbase.add_to_callback ("pre_linebreak_filter", rec_cbk, "document.recursenodes")

mvl.tex

\documentclass    {scrartcl}
\usepackage       {luatexbase}
\RequireLuaModule {lualibs}

\directlua {dofile "\jobname.lua"}

\begin {document}

  \hbox {Horizontal list only, without line break.}\par
  \vbox {Vertical list, triggers the line break mechanism.}\par

  \hbox {\hsize 1cm This text won’t fit but isn’t line broken either.}\par

  This is some \hbox {ordinary} paragraph material.

  \begin {figure}
    \input ward
    \caption {Some brief caption.} %% same as with the hbox above
  \end {figure}

  This is some \vbox {\hbox {significantly}
                      \hbox {less}
                      \hbox {ordinary}} paragraph material.

\end {document}