Why does hanging indent with \everypar mess up table spacing?

Question

I'm trying to add a hanging indent to every paragraph of a particular column of a table:

\documentclass{article}
\usepackage{array}
\newcolumntype{R}{>{\everypar{\hangindent1em\hangafter1}\arraybackslash}m{3in}}
\usepackage[a4paper, total={7in, 9in}]{geometry}
\begin{document}
\begin{tabular}{|p{1.5in}|p{1.5in}|R|}\hline
first column & second column & first line of third column \newline This line is indented. \par This one is not, but if it goes on and on and eventually wraps, the next line is indented. \\\hline
etc. & etc. & etc. \\\hline
\end{tabular}
\end{document}

The problem is that by using \everypar, the space between the top of the cell and the text is reduced, only in cells that have hanging indents: even the third cell in the second row looks fine. How can I avoid this?

Answer 1

When a p (or m) cell is started, \everypar is not void, but it contains

\vrule\@height\ht\@arstrutbox\@width\z@\everypar{}

which is the code that provides the insertion of a strut at the beginning of the paragraph. With your code you're removing this.

\documentclass{article}
\usepackage{array}
\newcommand{\dohang}{\hangindent1em\hangafter1 }
\newcolumntype{R}{%
  >{\everypar\expandafter{\the\everypar\dohang\everypar{\dohang}}\arraybackslash}%
  m{3in}%
}
\usepackage[a4paper, total={7in, 9in}]{geometry}
\begin{document}
\begin{tabular}{|p{1.5in}|p{1.5in}|R|}\hline
first column & second column &
  first line of third column \newline
  This line is indented. \par
  This one is not, but if it goes on and on and eventually wraps, 
  the next line is indented. \\\hline
etc. & etc. & etc. \\\hline
\end{tabular}
\end{document}

A different solution that avoids messing with \hangindent, \hangafter and \everypar, using LaTeX lists. The output is the same.

\documentclass{article}
\usepackage{array}
\newcommand{\dohang}{\hangindent1em\hangafter1 }
\newcolumntype{R}{%
  >{\tablist}%
  m{3in}%
  <{\endtablist}%
}

\newenvironment{tablist}
 {\list{}{%
    \leftmargin=1em
    \itemindent=-1em
    \listparindent=-1em
    \topsep=0pt
    \parsep=0pt
    \partopsep=0pt}\item[\strut]}
 {\endlist\vspace{-\baselineskip}}

\usepackage[a4paper, total={7in, 9in}]{geometry}
\begin{document}
\begin{tabular}{|p{1.5in}|p{1.5in}|R|}\hline
first column & second column &
  first line of third column \newline
  This line is indented. \par
  This one is not, but if it goes on and on and eventually wraps,
  the next line is indented. \\\hline
etc. & etc. & etc. \\\hline
\end{tabular}
\end{document}