How do I create an automatic break in long math expressions

Question

Is there a way to allow long math expressions to break automatically, whenever (and only) in situations where they extend beyond the body text of a document?

For example, I would like something like the following to break over two lines if (and only if) it finds itself in a narrow column of text (say, if <class> is IEEEtran), but otherwise remain on a single line (if, for example <class> is article), as long as it fits within a single (wider) column.

\documentclass{article}
\usepackage{lipsum}

\begin{document}

\lipsum[1]
\[ 
\hat{S}=\sum_{l\in\left\{ 1,\ldots L\right\} }\left[\bigl[1\!-\! f_{p}(l\!+\!1)\bigr]\;\overline{w}(l-1)\;\biggl[\,\prod_{i=0}^{l-1}\; f_{p}(i)\;\,\overline{\! N}(i)\biggr]\;\hat{H}_{S}(l-1)\right]
\]   
\lipsum[2]

\end{document}

Answer 1

To combine some of the comments to the question, this answer uses the breqn package and redefines the \[ and \] commands as \begin{dmath} and \end{dmath}, respectively.

Code:

\documentclass[12pt]{book}
\usepackage{lipsum}
\usepackage[]{breqn}

\renewcommand\[{\begin{dmath}}
\renewcommand\]{\end{dmath}}

\begin{document}

\lipsum[1]
\[
\hat{S}=\sum_{l\in\left\{ 1,\ldots L\right\} }\left[\bigl[1\!-\! f_{p}(l\!+\!1)\bigr]\;\overline{w}(l-1)\;\biggl[\,\prod_{i=0}^{l-1}\; f_{p}(i)\;\,\overline{\! N}(i)\biggr]\;\hat{H}_{S}(l-1)\right]
\]
\lipsum[2]

\end{document}