Dost thou feel that I reinvent the wheel? (Trying to create my own lightweight antilabe macro.)

Question

Here is an MWE representing my attempt to achieve antilabe in dramatic verse. (I felt the implementation in eledmac was more than I needed.) Ideally, I'd like the macro to set the spacing by just "looking" at the previous line of dialogue, but I think I may need to learn more TeX to do that. Do you think there is a simple way to do that?

\documentclass{article}
\usepackage{calc}
\makeatletter
\def\@hspace#1{\begingroup\setlength\dimen@{#1}\hskip\dimen@\endgroup}
\makeatother
\newcommand{\cn}[1]{\textsc{#1}}
\newcommand{\labe}[1]{\hspace{\widthof{#1}-\widthof{\cn{Xxx.}}}}
\begin{document}
\setlength{\parindent}{0in}
\cn{Sat}. But who would not lie down here in the meadow\
& long daydream on the blossom-stained glade’s\
Wide & warm & welcoming breast?
\cn{Fur}. \labe{Wide & warm & welcoming breast?} Please let’s go\
Into the Deep White Wood as the light fades\
For this bright pocked & dappled field abrades\
The eye!  & no one wants more songs about...
\end{document}

Update: the solutions that are working well appear to use the position on the page rather than the width of the text to be followed. egreg's solution, for example, handles multiple antilabes per metrical line:

Answer 1

You can use zref-savepos. Two runs of LaTeX are necessary if a new \antilabe macro appears.

The macro \antilabe will save the x-position of the last word in the line and in the next nonempty line (that should start with \cn) the initial x-position is saved too, so we can issue the wanted \hspace (the character's name is typeset in a zero width box, in this case).

\documentclass{article}
\usepackage{zref-savepos,zref-user}
\newif\ifantilabe
\newcounter{antilabe}
\newcommand{\cn}[1]{%
  \par
  \ifantilabe
    \makebox[0pt][l]{\textsc{#1}.}%
    \zsaveposx{antilabe-\theantilabe-b}%
    \hspace{\numexpr\zposx{antilabe-\theantilabe-a}-\zposx{antilabe-\theantilabe-b}\relax sp}%
    \space
  \else
    \textsc{#1.}%
  \fi
  \antilabefalse
}
\newcommand{\antilabe}{%
  \unskip
  \stepcounter{antilabe}%
  \antilabetrue
  \zsaveposx{antilabe-\theantilabe-a}%
}
\newenvironment{dialogue}
 {\par\setlength{\parindent}{0pt}}
 {\par}
\begin{document}
\begin{dialogue}
\cn{Sat} But who would not lie down here in the meadow\
& long daydream on the blossom-stained glade’s\
Wide & warm & welcoming breast?\antilabe
\cn{Fur} Please let’s go\
Into the Deep White Wood as the light fades\
For this bright pocked & dappled field abrades\
The eye!  & no one wants more songs about...
\cn{Sat} But who would not lie down here in the meadow\
& long daydream on the blossom-stained glade’s\
Wide & warm & welcoming breast?\antilabe
\end{dialogue}
\end{document}

Answer 2

Out-of-the box approach: Use \hphantom

You are reimplementing \hphantom. (And verse). The following produces the same results, except for some indentation before the verse environment. You can remove that by adding this to your preamble: \AtBeginEnvironment{verse}{\setlength{\leftmargini}{0pt}}.

\documentclass{article}
\newcommand{\cn}[1]{\textsc{#1.}}
\begin{document}
\begin{verse}
    \cn{Sat} But who would not lie down here in the meadow\
    & long daydream on the blossom-stained glade’s\
    Wide & warm & welcoming breast?
\cn{Fur} \hphantom{Wide \& warm \& welcoming breast?} Please let’s go\\
Into the Deep White Wood as the light fades\\
For this bright pocked \& dappled field abrades\\
The eye! \& no one wants more songs about\dots{}

\end{verse}
\end{document}

More automatic solution

Instead of repeating the text you want to offset in the argument to \hphantom you can create a command that will store that text the first time you use it and then recall it.

\documentclass{article}
\NewDocumentCommand{\cn}{ m }{\textsc{#1.}}
\makeatletter
% Define blank dummy text in private command
\NewDocumentCommand{@thisantilabe}{}{}
% Create a command to store dummy text, and print the text
\NewDocumentCommand{\antilabethis}{ m }{%
    \RenewDocumentCommand{@thisantilabe}{}{#1}%
    #1%
}
% Insert horizontal space, using either stored dummy text, or the optional
% argument if provided
\NewDocumentCommand{\antilabe}{ o }{%
    \IfValueTF{#1}
        {\hphantom{#1}}
        {\hphantom{@thisantilabe}}%
}
\makeatother
\begin{document}
\begin{verse}
    \cn{Sat} But who would not lie down here in the meadow \
    & long daydream on the blossom-stained glade’s \
    \antilabethis{Wide & warm & welcoming breast?} % mark the text
\cn{Fur} \antilabe{} Please let’s go \\
Into the Deep White Wood as the light fades \\
For this bright pocked \& dappled field abrades \\
The eye! \& no one wants more songs about\dots{}

\antilabethis{\cn{Knuth} expanding macros---} \\
\antilabe{} but still no one wants \\
to set the type by hand \\

\cn{Lamport}
\antilabe[to set the type by hand] so we keep using \\
software built in nineteen eighty-two.

\end{verse}
\end{document}

Answer 3

Just for fun. Don't forget to run it twice.

\documentclass{article}
\usepackage{calc}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\newcounter{markindex}
\newcommand{\eol}{\stepcounter{markindex}\tikzmark{eol\themarkindex}\par}
\newcommand{\cn}[1]{\tikz[remember picture, baseline=(origin)]{\coordinate (origin) at (0,0);
  \node[inner sep=0pt, anchor=base west]{\textsc #1.};
  \coordinate (eol) at (pic cs:eol\themarkindex);
  \path(origin -| eol);}}
\begin{document}
\setlength{\parindent}{0in}\rlap{\tikzmark{eol0}}%
\cn{Sat} But who would not lie down here in the meadow
& long daydream on the blossom-stained glade’s
Wide & warm & welcoming breast?\eol
\cn{Fur} Wide & warm & welcoming breast? Please let’s go
Into the Deep White Wood as the light fades
For this bright pocked & dappled field abrades
The eye!  & no one wants more songs about...\eol
\end{document}

Answer 4

Here is a solution using the tabbing environment from plain LaTeX: Remember the current position (\=), start a new line, typeset the label, and jump to the remembered position (\>).

\documentclass{article}
\newcommand\cn[1]{\textsc{#1}.}
\begin{document}
\begin{tabbing}
\cn{Sat}  But who would not lie down here in the meadow\\
\& long daydream on the blossom-stained glade’s\\
Wide \& warm \& welcoming breast? \=\\
\cn{Fur}\>Please let’s go\\
Into the Deep White Wood as the light fades\\
For this bright pocked \& dappled field abrades\\
The eye!  \& no one wants more songs about...
\end{tabbing}
\end{document}