Sstrhsrtj

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?

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%

    longdef#1##1{%

        @ifnextchar[{@flip{#1}{##1}}{csuse{flip@string#1}{##1}}%

    }%

}

longdef@flip#1#2[#3]{%

    csuse{flip@string#1}[#3]{#2}%

}

makeatother

For example, given the macro

newcommandfoo[2][none]{%

    par

    Required argument: #2\

    Optional argument: #1%

}

you can write flipfoo 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%

    longdef#1##1{%

        @ifnextchar[{@flip{#1}{##1}}{csuse{flip@string#1}{##1}}%

    }%

}

longdef@flip#1#2[#3]{%

    csuse{flip@string#1}[#3]{#2}%

}

makeatother



newcommandfoo[2][none]{%

    par

    Required argument: #2\

    Optional argument: #1%

}





begin{document}



Normal use:

foo{blah}

foo[option]{asdf}



Now we can flip the arguments:

flipfoo

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 flipfoo{required}[optional] expanded to foo[optional]{required}. Something like (untested)

longdefflip#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.