Why does \everypar not work?

Question

I tried to put a \P (¶ sign) after each paragraph. Therefore, I chose the

\everypar={\P}

command. First of all, I figured out that LaTeX redefines it internally after

\begin{document}

Hence, I placed my code inside the document environment.

However, I did not have the wanted effect. Please find the minimal code:

\documentclass{article}
\begin{document}
  \everypar={\P}
  \section{Intro} some text \par next paragraph \par\noindent even more
  \section{Outro} the end of my document
\end{document}

My output shows the \P before the section numbering which totally confused me.

I use MikTeX 2.9.4813 (x64) and TeXnicCenter to build my documents.

Answer 1

LaTeX uses \everypar itself all over the place, to manage indentation (or not) after section headings, to preserve the paragraph shape of indented list structures, it is also reset in minipages and parboxes and tabular p columns so setting \everypar is tricky.

Apart from the fact that the value can get over-written at any time section headings (as most text) are internally a paragraph and so trigger \everypar as you observe.

You can (with care) do this to avoid your definition being lost after the first heading but it would require a local test inserting to avoid adding the \P before section heads

\documentclass{article}
\begin{document}
  \let\oldep\everypar  \newtoks\everypar  \oldep{\the\everypar\P}

  \section{Intro} some text \par next paragraph \par\noindent even more
  \section{Outro} the end of my document
\end{document}

So here is a version with the heading code defined to remove the mark:

\documentclass{article}
\begin{document}
  \let\oldep\everypar  \newtoks\everypar  \oldep{\the\everypar\hbox{\P}}

\makeatletter
\def\@sect#1#2#3#4#5#6[#7]#8{%
  \ifnum #2>\c@secnumdepth
    \let\@svsec\@empty
  \else
    \refstepcounter{#1}%
    \protected@edef\@svsec{\@seccntformat{#1}\relax}%
  \fi
  \@tempskipa #5\relax
  \ifdim \@tempskipa>\z@
    \begingroup
      #6{%
        \@hangfrom{\hskip #3\relax\@svsec}%
          \interlinepenalty \@M 
% remove indentation box, remove \P box put indentation box back
\leavevmode \setbox\z@\lastbox\setbox\tw@\lastbox\box\z@
#8\@@par}%
    \endgroup
    \csname #1mark\endcsname{#7}%
    \addcontentsline{toc}{#1}{%
      \ifnum #2>\c@secnumdepth \else
        \protect\numberline{\csname the#1\endcsname}%
      \fi
      #7}%
  \else
    \def\@svsechd{%
      #6{\hskip #3\relax
      \@svsec #8}%
      \csname #1mark\endcsname{#7}%
      \addcontentsline{toc}{#1}{%
        \ifnum #2>\c@secnumdepth \else
          \protect\numberline{\csname the#1\endcsname}%
        \fi
        #7}}%
  \fi
  \@xsect{#5}}

\makeatother

  \section{Intro} some text \par next paragraph \par\noindent even more
  \section{Outro} the end of my document
\end{document}

Answer 2

In addition to what was said in other answers and comments, \everypar is used at the beginning of each paragraph (or more precisely, when TeX changes from vertical to horizontal mode), so it is useless for the purpose you tried to achieve.

For this purpose it is better to redefine \par, as in:

\documentclass{article}

\begin{document}
\let\oldpar=\par
\def\par{\P\oldpar}
  \section{Intro} some text \par next paragraph \par\noindent even more
  \section{Outro} the end of my document
\end{document}

But even this produces unwanted results, since section headers use also internally \par in a unexpected way. See the result: