Using multline or equation based on a boolean flag

Question

I am trying to define a new environment myeq which can either be an equation or a multline. My use case is that in the preamble, I will define myeq to be either replicate the functionaly of equation or multline and then the actual document will only contain myeq environments so that by changing the definition in the preamble, it is possible to get the spacing characteristics either of mutline or of equation.

In particular, this requires that I have to define myeq to simply replicate multline, as follows:

\newenvironment{myeq}{\begin{multline}}{\end{multline}}

However, this produces the following error (see MWE below)

\begin{multline} on input line 8 ended by \end{myeq}.

The error disappears if I replace the multline in the definition of myeq by equation.

MWE:

\documentclass{article}
\usepackage{amsmath}
\newenvironment{myeq}{\begin{multline}}{\end{multline}}

\begin{document}
\begin{myeq}
  t
\end{myeq}
\end{document}

Answer 1

The failure has a technical reason that's hinted at in texdoc technote, section 6.

I suggest something like

\iftrue
  \newenvironment{myeq}{\equation}{\endequation}
\else
  \newenvironment{\myeq}
    {\def\eqbreak{\\}\multline}
    {\endmultline}
\fi
\newcommand{\eqbreak}{}

with \eqbreak rather than \\ (that's not really ignored inside equation) inside myeq.

Change \iftrue into \iffalse if you want multline for two column documents.

A possibly better implementation:

\documentclass[
% twocolumn
]{article}
\usepackage{amsmath,amssymb}

\makeatletter
\newenvironment{multiequation}
 {%
  \if@twocolumn
    \def\eqbreak{\\}%
    \expandafter\multline
  \else
    \def\eqbreak{}%
    \expandafter\equation
  \fi
 }
 {%
  \if@twocolumn
    \expandafter\endmultline
  \else
    \expandafter\endequation
  \fi
 }
\makeatother
\begin{document}

Some text before the equation to see what happens
and embed the equation into a real paragraph
which should break nicely into a few lines
\begin{multiequation}
\biggl(\int_{-\infty}^{\infty}e^{-x^{2}}\,dx\biggr)^{\!2}
\eqbreak
=\biggl(\int_{-\infty}^{\infty}e^{-x^{2}}\,dx\biggr)
 \biggl(\int_{-\infty}^{\infty}e^{-y^{2}}\,dy\biggr) 
\eqbreak
=\int\limits_{\mathbb{R}^{2}}e^{-x^{2}-y^{2}}\,dx\,dy
\end{multiequation}
and some text after the equation to see what happens
and embed the equation into a real paragraph
which should break nicely into a few lines

\end{document}

Output with twocolumn commented out

Output with twocolumn active

---

Other than for this application, multline should never be used for single line displays. Maybe gather, with

\newenvironment{myeq}{\gather}{\endgather}

but you risk something like the following:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

A paragraph that ends with a short line preceding a display,
let's see if we can
\begin{equation}
(a+b)^2=a^2+2ab+b^2
\end{equation}

A paragraph that ends with a short line preceding a display,
let's see if we can
\begin{gather}
(a+b)^2=a^2+2ab+b^2
\end{gather}

A paragraph that ends with a short line preceding a display,
let's see if we can

\end{document}

No doubt the second one is ugly.

Answer 2

Here's a LuaLaTeX-based approach, which acts as a pre-processor: Before TeX's eyes get going, the Lua function switch_eq sweeps over all input lines and replaces any instances of \begin{myeq} with either \begin{equation} or \begin{gather} depending on whether the Boolean variable Gather is true or false. Ditto with all instances of \end{myeq}.

The code further provides two LaTeX macros, named \GatherTrue and \GatherFalse, which set the value of the Boolean variable Gather to either true or false.

In the following screenshot, note that equations (1) and (3) are spaced tightly, whereas (2) -- the one that's generated by gather rather than by equation -- is spaced profligately.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{amsmath} % for "gather" environment

%% Lua-side code
\usepackage{luacode}
\begin{luacode}
Gather = false -- default value of "Gather" is "false"
function switch_eq ( buff )
  if string.find ( buff , "\\begin{myeq}" ) then
    if Gather == true then
      return ( string.gsub ( buff, "\\begin{myeq}", "\\begin{gather}" ) )
    else
      return ( string.gsub ( buff, "\\begin{myeq}", "\\begin{equation}" ) )
    end
  elseif string.find ( buff , "\\end{myeq}" ) then
    if Gather == true then
      return ( string.gsub ( buff, "\\end{myeq}", "\\end{gather}" ) )
    else
      return ( string.gsub ( buff, "\\end{myeq}", "\\end{equation}" ) )
    end
  end
end
luatexbase.add_to_callback ( "process_input_buffer" , switch_eq , "switch_eq" )
\end{luacode}

%% TeX-side code
\newcommand\GatherTrue{\directlua{Gather = true}}
\newcommand\GatherFalse{\directlua{Gather = false}}

\setlength\textwidth{8cm} % just for this example
\begin{document}
----------------------------

\begin{myeq}
1+1=2
\end{myeq}

----------------------------

\GatherTrue
\begin{myeq}
1+1=2
\end{myeq}

----------------------------

\GatherFalse
\begin{myeq}
1+1=2
\end{myeq}

----------------------------
\end{document}