Make inline maths break before binary operator

Question

By default, inline maths breaks long expressions in such a way that the line break happens after binary operators, e.g.

a + b +
c + d

Is there any easy way to convince TeX to break before the binary operator, to get

a + b
+ c + d

I would prefer if I do not have to insert special instructions at every binary operator by hand.

Answer 1

EDIT to make + active only in math mode, using egreg's answer from Make Characters Active via Macro in Math Mode

When + is active, I use a \discretionary to allow the breakpoint where desired.

The MWE shows

1. it making a break at the right point when it hits the margin. Further, the terms are properly nestled to the margin boundary,

2. it gives proper math spacing when not making a break

3. the math-active + does not screw up the use of + for dimensioning.

Here is the MWE.

\documentclass{article}

\usepackage{amsmath}
\usepackage{xcolor,etoolbox}

\makeatletter
\newcommand{\DeclareMathActive}[2]{%
  % #1 is the character, #2 is the definition
  \expandafter\edef\csname keep@#1@code\endcsname{\mathchar\the\mathcode`#1 }
  \begingroup\lccode`~=`#1\relax
  \lowercase{\endgroup\def~}{#2}%
  \AtBeginDocument{\mathcode`#1="8000 }%
}

\DeclareMathActive{+}{\mathbreak\std{+}}

\newcommand{\std}[1]{\csname keep@#1@code\endcsname}
\patchcmd{\newmcodes@}{\mathcode`\-\relax}{\std@minuscode\relax}{}{\ddt}
\AtBeginDocument{\edef\std@minuscode{\the\mathcode`-}}
\makeatother

\textwidth 1.75cm
\parindent 0pt
\parskip 1ex
\def\mathbreak{\discretionary{}{}{}}
\begin{document}
\hrulefill

\rule{1cm}{1pt}$ a + b + c + d$ \rlap{(broken spacing aligns to margins)}

$+ c$ \rlap{(for left-margin comparison)}

\mbox{\rule{1cm}{1pt}$ a + b + c + d$ \rlap{(unbroken spacing is proper)}}

Did + revert to original definition?\parskip=+30pt\relax

Yes it did
\end{document}

Now that I have tested the approach to my satisfaction, here it is in a more general setting:

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}
\makeatletter
\newcommand{\DeclareMathActive}[2]{%
  % #1 is the character, #2 is the definition
  \expandafter\edef\csname keep@#1@code\endcsname{\mathchar\the\mathcode`#1 }
  \begingroup\lccode`~=`#1\relax
  \lowercase{\endgroup\def~}{#2}%
  \AtBeginDocument{\mathcode`#1="8000 }%
}
\DeclareMathActive{+}{\mathbreak\std{+}}
\newcommand{\std}[1]{\csname keep@#1@code\endcsname}
\patchcmd{\newmcodes@}{\mathcode`\-\relax}{\std@minuscode\relax}{}{\ddt}
\AtBeginDocument{\edef\std@minuscode{\the\mathcode`-}}
\makeatother
\def\mathbreak{\discretionary{}{}{}}
\usepackage[nopar]{lipsum}
\begin{document}
\lipsum[4]. And now we will demonstrate the breaking ability
$a + b + c + d$ of the method. And here, $a + b + c + d$ when not broken.
\end{document}