Overloading command parameters in LaTeX

Question

Wondering if you can accomplish all of these variations in LaTeX by defining a custom command:

\mycmd{tag}{color}{keyval}
\mycmd{tag}{keyval}
\mycmd{keyval}

So it could be used like this:

\mycmd{foo}{black}{size=1cm}
\mycmd{foo}{color=black,size=1cm}
\mycmd{tag=foo,color=black,size=1cm}

Either one would work. How to define the command so this sort of thing would work?

\newcommand\mycmd[3][...]

Answer 1

You can implement more or less any syntax in tex, but latex has clear guidelines that optional arguments should be [] delimited. \newcommand only defines commands with at most one option, so to have two you would need to define it in two steps but xparse allows it to be declared directly as having two options so for example the following (complete) example test document:

\documentclass{article}

\usepackage{xparse}
\NewDocumentCommand\mycmd{O{deftag}O{red}m}{\typeout{tag=#1,color=#2,#3}}
\begin{document}


\mycmd[foo][black]{size=1cm}
\mycmd[foo]{color=black,size=1cm}
\mycmd{tag=foo,color=black,size=1cm}

\end{document}

which produces

tag=foo,color=black,size=1cm
tag=foo,color=red,color=black,size=1cm
tag=deftag,color=red,tag=foo,color=black,size=1cm