[luatex] How to pass a box from TeX to lua

[Andreas Matthias]

\directlua {%
  function out(t)
     x = node.vpack(t)
     node.write(x)
   end
}

\def\foo{%
  \directlua {%
    s = node.new(node.id('glyph'))
    s.char = 97
    s.font = 1
    out(node.hpack(s))
  }}
\foo


This code creates an hbox that is passed to function `out' which
then packs everything into a vbox. Fine. But now I would like
to do the first part (creation of the hbox) in TeX and the second
part (creation of the vbox) in lua. The following code doesn't
work but it might show what I'm trying to do.


\def\bar#1{%
  \directlua {
    out(#1)
  }}
\bar{\hbox{a}}


How can this be done?


Ciao
Andreas