First box in current vertical list?

Question

Is is possible to get first box in current vertical list? If it is impossible, is it possible to define a macro that gets an argument and return first line (first box of vertical list) of that argument. Excuse me for my bad English. Thanks

Edit

For calrify my quesion look at this wme and ouput of it.

\documentclass{article}
\usepackage{lipsum}
\def\gettfirstline#1{I want this macro return\par Nam dui ligula, fringilla a, euismod sodales, sollicitudin vel, wisi. Morbi auctor
}
\setlength\parindent{0pt}
\begin{document}
\lipsum[2]
\vspace{1.5cm}
\gettfirstline{\lipsum[2]}
\end{document}

Output:

Answer 1

You can assign your data to a \vbox and then extract the first box with \vsplit. It is quite hacky and probably fails if your box does not start with text and it only gives you the box. So in contrast to the solutions in Changing the style of the first *typeset* line of a paragraph you can't change the formatting anymore, but the spacing is preserved:

\documentclass{article}
\usepackage{lipsum}
\newbox\mylocalbox
\newbox\myglobalbox
\def\Ggetfirstline{%
  \vfuzz\maxdimen
  \setbox\mylocalbox=\vsplit0to1sp
  \setbox\mylocalbox\vbox{%
    \unvbox\mylocalbox
    \global\setbox\myglobalbox\lastbox
  }%
  \endgroup
  \box\myglobalbox
}
\def\Getfirstline{\aftergroup\Ggetfirstline}
\def\getfirstline{\begingroup\afterassignment\Getfirstline\setbox0\vbox}
\setlength\parindent{0pt}
\begin{document}
\lipsum[2]
\vspace{1.5cm}
\getfirstline{\lipsum[2]}
\end{document}

This code inserts the first line as the same type of box as it originally was, normally a \hbox. You can also keed the box wrapped in a vbox to keep horizontal displacement in place:

\documentclass{article}
\usepackage{lipsum}
\newbox\mylocalbox
\newbox\myglobalbox
\def\Ggetfirstline{%
  \vfuzz\maxdimen
  \setbox\mylocalbox=\vsplit0to1sp
  \global\setbox\myglobalbox\vbox{\unvbox\mylocalbox}
  \endgroup
  \box\myglobalbox
}
\def\Getfirstline{\aftergroup\Ggetfirstline}
\def\getfirstline{\begingroup\afterassignment\Getfirstline\setbox0\vbox}
\setlength\parindent{0pt}
\begin{document}
\lipsum[2]
\vspace{1.5cm}
\getfirstline{\lipsum[2]}
\end{document}

If you are open to use LuaTeX, you can use Lua to create a more general and robust macro which always gives you the first box in the current vertical list, even if you are in the main vertical list: (This is in plain LaTeX, just add \documentclass, \begin{document}, \end{document} if you prefer LuaLaTeX)

\def\setboxtovhead{\directlua{
  local vlistid, hlistid = node.id'vlist', node.id'hlist';
  local vlistlevel = tex.nest.ptr;
  while math.abs(tex.nest[vlistlevel].mode) \csstring~= 1 do
    vlistlevel = vlistlevel - 1
  end
  local nest = tex.nest[vlistlevel]
  local head = nest.mode == 1 and tex.lists.page_head or nest.head.next;
  while head and head.id \csstring~= vlistid and head.id \csstring~= hlistid do
    head = head.next
  end
  tex.box[token.scan_int()] = node.copy(head)
}}
\parindent0pt\parskip1em
abc\hfil\break def\hfil\break ghi

\setboxtovhead0
The first line was:\par
\box0
\end

Answer 2

A trick can be to set infinite \clubpenalty, so a \vsplit operation will catch that penalty and the box will be split there:

\documentclass{article}
\usepackage{lipsum}

\newsavebox\mainbox
\newsavebox\firstbox

\newcommand{\getfirstline}[1]{%
  \begingroup
  \setbox\mainbox=\vbox{%
    \clubpenalty=-10000
    \predisplaypenalty=-10000
    #1%
  }%
  \vbadness=10000 % don't be bothered with underfull boxes
  \setbox\firstbox=\vsplit\mainbox to \maxdimen
  \setbox\firstbox=\vbox{\unvbox\firstbox}
  \box\firstbox
  \endgroup
}

\begin{document}

\lipsum[2]
\getfirstline{\lipsum[2]}
\lipsum[2]

\end{document}