How to build a conditional based on a resize paramater

Question

I would like to define my own macro that accepts an optional parameter such as \big, \Big, etc that apply that if it is specified, otherwise apply the auto resizing \left, \right pair.

So, how do I get the following to work for the case where the optional parameter is specified.

I see that if I change the syntax to accept only \bracc[big]{1, 2} instead of \bracc[\big]{1, 2} (i.e., remove the \), then I can use

\csname#1\endcsname\{#2\csname#1\endcsname\}

which works fine but I would rather be able to specify \big (i.e, with the \) instead.

Code:

\documentclass{article}
\usepackage{xstring}
\usepackage{xparse}

\NewDocumentCommand{\bracc}{O{} m}{%
    \IfEq{#1}{}{%
        \left\{#2\right\}%
    }{%
        #1\{#2#1\}%
    }%
}%


\begin{document}
    $\bracc{1, 2}$  
    %$\bracc[\big]{1, 2}$% How to get this case to work?
\end{document} 

Answer 1

You don't need any packages for this, also you should (effectively) use \bigl and \bigr if \big is specified:

\documentclass{article}


\newcommand\bracc[2][\left]{%
  \ifx\left#1\else\mathopen\fi#1\{%
  #2%
  \ifx\left#1\right\else\mathclose\expandafter#1\fi\}%
}%


\begin{document}
    $\bracc{1, 2}$  

    $\bracc[\big]{1, 2}$% How to get this case to work?
\end{document}