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

danijar · Accepted Answer

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 @gobblepar, 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}

xapptocmdnormalsize{%
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}{ifvmodevspace*{-baselineskip}setlength{parskip}{0ex}fi}
newcommand{removeParAfter}{@ifnextcharpar@gobblerelax}
newcommand{eq}{begingroupremoveParBeforeendlinechar=32 eqinner}
newcommand{eqinner}[1]{endlinechar=32%
begin{equation}begin{aligned}#1end{aligned}end{equation}endgroupremoveParAfter}
makeatother

begin{document}

lipsum[2]

eq{

x &= y

y &= z

}

lipsum[2]

end{document}

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

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