mathtools brackets

Question

Using DeclarePairedDelimiter from mathtools I have defined a command to enclose expressions with parenthesis.

\DeclarePairedDelimiter\p{(}{)}

Now, I want to defined a command for a function

\newcommand{\myfunc}[1]{f\p{#1}}

Is there an easy way to pass the optional parameter from \myfunc to \p so that I can write \myfunc[\Big]{x}. I know I can use \DeclarePairedDelimiterXPP to declare \myfunc from scratch but I was wondering if there was a way to make my definitions more modular.

MWE

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\p{(}{)}
\newcommand{\myfunc}[1]{f\p{#1}}

\begin{document}
\[\myfunc{x}\]
% How to make these work? Without breaking \myfunc{x}
% \[\myfunc[\Big]{x}\] 
% \[\myfunc*{x}\] 
\end{document}

Answer 1

Just define \myfunc without arguments.

\documentclass{article}
\usepackage{mathtools}

\DeclarePairedDelimiter\p{(}{)}
\newcommand{\myfunc}{f\p}

\begin{document}

\begin{gather*}
\myfunc{x} \\
\myfunc[\Big]{x} \\
\myfunc*{\frac{x}{2}}
\end{gather*}

\end{document}

Answer 2

It is a lot easier to simply use \DeclarePairedDelimiterXPP, which is build for this kind of constructions. Just use

\DeclarePairedDelimiterXPP\myfunc[1]{f}{(}{)}{}{#1}

On the other hand, you you really want to use \p, then you'll need to manually parse the arguments using the xparse package, and then call \p in the appropriate manner using the detected options (you need to look for starred, optional in [] and mandatory, so in xparse you need to make a macro with an s o m 'signature').

Answer 3

I defined a macro that inserts the optional argument before the ( and ). The definition of the macro is made inside a local group so that once it exits, the effect of the \DeclarePairedDelimiter is erased so it can be used again in the document.

I also added the solution that @campa and @daleif provided in the comments, which is much cleaner and less prone to errors.

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\p{(}{)}

\newcommand{\myfunc}[1]{f\p{#1}}

% Keeping my previous solution
\newcommand{\myotherfunc}[2][]{%
\begingroup% < Make the definition of \pp local to this group (thanks @campa)
\DeclarePairedDelimiter\pp{#1(}{#1)}
f\pp{#2}%
\endgroup%
}

% Easier, faster, cleaner solution by @campa and @daleif
\DeclarePairedDelimiterXPP\myanotherfunc[1]{f}{(}{)}{}{#1}

\begin{document}
\[\myfunc{x}\]
% How to make these work? Without breaking \myfunc{x}
\[\myotherfunc[\Big]{x} \text{ and } \myotherfunc[\Big]{y}\]
\[\myanotherfunc[\Big]{x} \text{ and } \myanotherfunc[\Big]{y}\] 
% \[\myfunc*{x}\] 
\end{document}

I just couldn't understand the purpose of the starred version.