Conditional based on displaystyle

Question

I'm trying to write a macro that returns different things depending on it if is used in a displayed equation or if it is used in an inline equation. Something like this, I assume:

\newcommand{\foo}{%
    \ifdisplaystyle
        This equation is displayed!
    \else
        This equation is inline!
    \fi}

I assume there's a way to do this, as $\sum$ and $$\sum$$ look different. How is it done?

Answer 1

You can use \mathchoice in the following way to condition:

\mathchoice
  {<display style>}
  {<text style>}
  {<script style>}
  {<script-script style>}

to discern between what is displayed within the respective styles. Here's an example:

\documentclass{article}

\newcommand{\foo}{{%
  \mathchoice
    {D} % \displaystyle
    {T} % \textstyle
    {S} % \scriptstyle
    {s} % \scriptscriptstyle
}}

\begin{document}

See $\foo^{\foo^{\foo}}$. Also see
\[
  \foo^{\foo^{\foo}}
  \quad \mbox{and} \quad
  \textstyle \foo_{\foo_{\foo}}
\]

\end{document}