Check if at begin of a line

Question

For a list of bullet-separated items, I want the separator to be swallowed if it is the first or last element of the line. That is, I want something like

\renewcommand{\and}{%
  \ifnolinebreak%    <=== HERE'S THE PROBLEM
    {\hfill\textbullet\hfill\xspace}%
  \else{}\fi
}
Aa Bb \and Cc Dd \and veryveryverylongE \and Ff Gg \and H

to end up like

Aa Bb   *   Cc Dd
veryveryverylongE
Ff Gg     *     H

I know that \discretionary can be exploited in the following way:

\renewcommand{\and}{\discretionary{}{}{\mbox{~~{\textbullet}~~}}}

This does the job, but unfortunately I can't find a way to insert a stretchable skip in the discretionary definition. Thus, it looks awkward in justified mode:

Aa   Bb * Cc   Dd
veryveryverylongE
Ff         Gg * H

Any ideas how to implement the \ifnolinebreak? If it matters, for this project I use LuaLaTeX...

Answer 1

Borrowing code by Marcin Woliński published as the TeX pearl "How to make a box disappear at a line break" one can do

\documentclass{article}
\newcommand{\AND}{\unskip
  \hskip 2pt plus 1pt minus 1pt
  \cleaders\copy\ANDbox\hskip\wd\ANDbox
  \hskip 2pt plus 1pt minus 1pt
  \ignorespaces}
\newsavebox\ANDbox
\sbox\ANDbox{\textbullet}

\begin{document}
\settowidth{\dimen0}{someveryverylongword}
\parbox{\dimen0}{
some text \AND some text \AND
someveryverylongword \AND
text \AND text}
\end{document}