LyX | change macro argument order

Question

In LyX, is it possible to change the order of required and option arguments of a macro?

I want to make the option argument come after the required argument in this macro.

If it's possible, how can I do it?

Answer 1

I don't recommend doing this because it'll be confusing to anyone else who reads your document. It will also not work for all macros, particularly ones that play around with category codes prior to reading their arguments. That said it is possible.

Here's some LaTeX code that defines a macro \flip that will flip the required and optional arguments of a macro that's defined to have exactly one required argument and one optional argument.

\usepackage{etoolbox}

\makeatletter
\newcommand*\flip[1]{%
    \cslet{flip@\string#1}#1%
    \long\def#1##1{%
        \@ifnextchar[{\@flip{#1}{##1}}{\csuse{flip@\string#1}{##1}}%
    }%
}
\long\def\@flip#1#2[#3]{%
    \csuse{flip@\string#1}[#3]{#2}%
}
\makeatother

For example, given the macro

\newcommand\foo[2][none]{%
    \par
    Required argument: #2\\
    Optional argument: #1%
}

you can write \flip\foo one time and it'll redefine \foo to take a required argument followed by an optional argument. Here's a complete document illustrating this.

\documentclass{article}
\usepackage{parskip}
\usepackage{etoolbox}

\makeatletter
\newcommand*\flip[1]{%
    \cslet{flip@\string#1}#1%
    \long\def#1##1{%
        \@ifnextchar[{\@flip{#1}{##1}}{\csuse{flip@\string#1}{##1}}%
    }%
}
\long\def\@flip#1#2[#3]{%
    \csuse{flip@\string#1}[#3]{#2}%
}
\makeatother

\newcommand\foo[2][none]{%
    \par
    Required argument: #2\\
    Optional argument: #1%
}


\begin{document}

Normal use:
\foo{blah}
\foo[option]{asdf}

Now we can flip the arguments:
\flip\foo
\foo{blah}
\foo{asdf}[option]
\end{document}

I've never used LyX before now, but I was able to insert my code into the preamble and achieve the same effect.

Note that \flip doesn't respect the lack of \long on a macro (i.e., macros defined via the \def family without \long or defined via \newcommand*). This is likely to lead to confusing errors in some cases. It's possible to deal with this similar to how etoolbox patches macros, but it didn't seem worth it to do so for this answer.

It would also be possible to define \flip such that it didn't redefine its argument but instead such that \flip\foo{required}[optional] expanded to \foo[optional]{required}. Something like (untested)

\long\def\flip#1#2[#3]{#1[#3]{#2}}

which would turn the optional argument into a required one, but if you don't have the optional argument, there's no reason to use the \flip.

In conclusion, it's possible to do, but you probably shouldn't do it.