How can I ignore empty lines before, after, and inside an equation environment?

Question

I would like an equation environment that doesn't care about empty lines. Normally, empty lines before and after equation start a new paragraph, resulting in additional vertical spacing around the display equation. Empty lines inside the equation environment cause a compilation error:

Text above
                    % <- Empty line causes additional spacing
\begin{equation}
\begin{aligned}
                    % <- Empty line causes error
x &= y \\
                    % <- Empty line causes error
y &= z
                    % <- Empty line causes error
\end{aligned}
\end{equation}
                    % <- Empty line causes additional spacing
Text below

Answer 1

Solution:

• Empty lines before. Once LateX processed a \par it can't be removed. Instead, we check if we're at the beginning of a new paragraph with \ifvmode and then set parskip to zero and compensate for the empty line above with negative space.

• Empty lines after. We can prevent the \par after an environment from being written using \@gobble\par, which ignores both spaces and a line break that might follow the equation environment. For more details, see this answer.

• Empty lines inside. To hide empty lines from amsmath, we change \endlinechar to a space character. This should not affect the equation otherwise, because spaces are ignored in math mode. I'm unsure why a nested command is needed but it does work.

Example:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xpatch}
\usepackage{lipsum}

\xapptocmd\normalsize{%
\abovedisplayskip=.8em plus .2em minus .2em
\belowdisplayskip=.6em plus .1em minus .1em
\abovedisplayshortskip=.8em plus .2em minus .2em
\belowdisplayshortskip=.6em plus .1em minus .1em
}{}{}

\makeatletter
\newcommand{\removeParBefore}{\ifvmode\vspace*{-\baselineskip}\setlength{\parskip}{0ex}\fi}
\newcommand{\removeParAfter}{\@ifnextchar\par\@gobble\relax}
\newcommand{\eq}{\begingroup\removeParBefore\endlinechar=32 \eqinner}
\newcommand{\eqinner}[1]{\endlinechar=32%
\begin{equation}\begin{aligned}#1\end{aligned}\end{equation}\endgroup\removeParAfter}
\makeatother

\begin{document}

\lipsum[2]

\eq{

x &= y \\


y &= z

}


\lipsum[2]

\end{document}