Reliable code for automatic \noindent after specific environments?

Question

This question led to a new package:
noindentafter

There are a number of environments which I never want to follow up with an indented paragraph. itemize, enumerate, theorem, definition, etc. I know there are people who think this is bad style. But it's a personal preference. Those environments basically always end a paragraph for me, so the visual clue is not needed in my case.

I've found several similar questions, but none of them do what I need:

• Automatic \noindent on the first paragraph of each theorem environment
• Automatically adding \noindent after \section{*}
• Automatic noindent after quote environment

That third one was actually most useful, but appears to be unreliable for patching existing environments with. It doesn't seem to be working in my memoir document.

To be clear, I don't want to remove the actual paragraph break. That's a semantic concept. I just want to patch the document style so that paragraphs following certain environments remain unindented.

I've tried the following two 'solutions' (which use etoolbox for patching):

\newcommand*{\noindentnext}{\everypar{{\setbox\z@\lastbox}\everypar{}}}
\newcommand*{\NoIndentAfterEnv}[1]{\AfterEndEnvironment{#1}{\noindentnext}}
\newcommand*{\NoIndentAfterCmd}[1]{\apptocmd{#1}{\noindentnext}{}{}}

\NoIndentAfterEnv{itemize}
\NoIndentAfterEnv{theorem}
...

This one is loosely based on the third post from the list above. It sometimes works. It sometimes does nothing. I'm unable to predict when it will fail. The second:

\let\old@par\par
\newcommand*{\noindentnextpar}{\def\par{\let\par\old@par\par\noindent}}
\newcommand*{\NoParIndentAfterEnv}[1]{\AfterEndEnvironment{#1}{\noindentnextpar}}
\newcommand*{\NoParIndentAfterCmd}[1]{\apptocmd{#1}{\noindentnextpar}{}{}}

I invented this one myself. It breaks when the patched environment is not followed by a paragraph. It has also shown itself to inexplicably fail in other situations.

Once and for all, I'd like to have a reliable way to do this.

Let me give an example of what I need, just to be clear:

\NoIndentAfterEnv{itemize}

\begin{itemize}
    \item ...
\end{itemize}

This starts a new paragraph. Note the empty line. But it should not be indented.

But this paragraph \emph{should} be indented.

Answer 1

You could make the following paragraph believe that it follows a heading where there's also no indent by appending \@afterindentfalse\@afterheading after the end of the environment. Package etoolbox provides the handy \AfterEndEnvironment which makes this an easy task. \@afterheading also temporarily sets \clubpenalty to \@M (10000), though. If you don't want this after the patched environment one could insert an adapted version of \@afterheading instead.

\documentclass{article}
% environment hooks and patching:
\usepackage{etoolbox}

\makeatletter
% uncomment the following if you don't want \clubpenalty\@M ...
% \let\nearly@afterheading\@afterheading
% \patchcmd\nearly@afterheading
%   {\@M}% original temporary setting for \clubpenalty replaced by ...
%   {\@clubpenalty}% ... or whichever value you deem right
%   {}{}
% ... and use \nearly@afterheading instead of \@afterheading here:
\newcommand*\NoIndentAfterEnv[1]{%
  \AfterEndEnvironment{#1}{\par\@afterindentfalse\@afterheading}}
\makeatother

\NoIndentAfterEnv{itemize}
% treat other environments you want to patch accordingly

% dummy text:
\usepackage{lipsum}
\begin{document}
\lipsum[1]

\begin{itemize}
  \item foo
  \item bar
\end{itemize}

\lipsum[1-2]
\end{document}

Answer 2

The correct way to unconditionally remove indentation after list environments (note that quotes and theorems are also based on list) seems to be to change the definition of \@doendpe, specifically, to remove the code where "\par is redefined to restore \everypar" (source2e). See What exactly does \@doendpe do? for details.

\documentclass{article}

\newtheorem{theorem}{Theorem}

\newcommand*{\sometext}{Hello, here is some text without a meaning. This
    text should show, how a printed text will look like at this place.
    If you read this text, you will get no information.}

\makeatletter
\def\@doendpe{%
  \@endpetrue
%  \def\par{% REMOVED
%    \@restorepar% REMOVED
%    \everypar{}% REMOVED
%    \par% REMOVED
%    \@endpefalse% REMOVED
%  }% REMOVED
  \everypar{%
    {\setbox\z@\lastbox}%
    \everypar{}%
    \@endpefalse
  }%
}
\makeatother

\begin{document}

\sometext

\begin{itemize}
\item \sometext
\end{itemize}

\sometext

\begin{quote}
\sometext
\end{quote}

\sometext

\begin{theorem}
\sometext
\end{theorem}

\sometext

\end{document}

Should you want to remove indentation only after specific list-based (!) environments, you can incorporate a new \noindentafter switch into \@doendpe and use etoolbox' \BeforeBeginEnvironment to set the switch to true for the respective environments.

\documentclass{article}

\newcommand*{\sometext}{Hello, here is some text without a meaning. This
    text should show, how a printed text will look like at this place.
    If you read this text, you will get no information.}

\newif\ifnoindentafter
\noindentafterfalse

\makeatletter
\def\@doendpe{%
  \@endpetrue
  \ifnoindentafter% ADDED
  \else% ADDED
    \def\par{%
      \@restorepar
      \everypar{}
      \par
      \@endpefalse
      \noindentafterfalse% ADDED
    }%
  \fi% ADDED
  \everypar{%
    {\setbox\z@\lastbox}%
    \everypar{}%
    \@endpefalse
    \noindentafterfalse% ADDED
  }%
}
\makeatother

\usepackage{etoolbox}
\BeforeBeginEnvironment{itemize}{\noindentaftertrue}

\begin{document}

\sometext

\begin{itemize}
\item \sometext
\end{itemize}

\sometext

\begin{enumerate}
\item \sometext
\end{enumerate}

\sometext

\end{document}