How can I ensure that an environment always starts on a new line? (a new paragraph / horizontal box)?

Question

Sometimes when creating environments, you want to ensure that they always

• start new paragraph when two environments appear immediately after one another
• eat up spaces
• generally idiot-proof (which, dare I say, might also be called "fully LaTeX compatible")

I have seen a few techniques. Are there any recommended ways of doing this to avoid trouble?

\documentclass{article}
\usepackage{fontspec}
\usepackage{lipsum}

\newenvironment{test}
  {\par\noindent}% <-- before env hook
  {}% <-- after env hook

\begin{document}
\begin{test}
\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}
\begin{test}
\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}

\begin{test}
\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}
\end{document}

Ideal Environment

This one can handle \begin{test}\end{test}\begin{test}\end{test} back to back. I used \par\noindent in the before environment hook. Is this the right way to do this? Based on this answer https://tex.stackexchange.com/a/22853/13552, it seems that I would also need \leavevmode. Could somebody clarify this and any other tips that are "good practice".

Without Auto-new paragraphs (\par in before hook), without \noindent

Without Auto-new paragraphs (\par in before hook), with \noindent

Related

• Ignore spaces and \par's after an environment
• Looking for an \ignorespacesandpars

Answer 1

If you want that the environment starts a new paragraph with no indentation and also ignores spaces after it,

\newenvironment{test}
 {\par\noindent\ignorespaces}
 {\ignorespacesafterend}

The missing \ignorespaces is the cause for the small spaces in the third example. This happens because \noindent starts a new paragraph (note that \par only ends a paragraph) and so the space after \begin{test} (generated by the end-of-line) is not ignored.

Answer 2

It is almost always best to avoid starting horizontal mode in the begin code of the environment, so do not use \noindent or \leavevmode etc.

The usual way is as a trivlist, the exact parameters of which can be customised, and this is how center quote verbatim etc work.

Note your test file makes overfull lines as the fbox adds space around a parbox that is already full width.

\documentclass{article}
%\usepackage{fontspec}
\usepackage{lipsum}

\newenvironment{test}
  {\trivlist{}\item\relax}% <-- before env hook
  {\endtrivlist}% <-- after env hook

\begin{document}

\noindent X\dotfill X

\begin{test}
\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}
\begin{test}
\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}

\begin{test}
\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}
\end{document}

---

To see one (of many) disadvantages to starting hmode in the environment definition try a test such as

\begin{test}

\fbox{\parbox{\linewidth}{\lipsum[1]}}
\end{test}

with the definition here and compare with other definitions. Environments that start a paragraph ought to silently accept the environment content starting with a paragraph break.