Question

I’m trying to create a LaTeX command with two arguments, one of them optional. Normally I’d do this as

\newcommand{\whatever}[2][default]{first #1 second #2} 

where default is the default value for the first argument. But for this command I want the value of the second argument to be used as the default value for the first argument – that is, I want

\whatever{blah} \whatever{foo} \whatever[lol]{rofl} 

to be equivalent to

\whatever[blah]{blah} \whatever[foo]{foo} \whatever[lol]{rofl} 

Does anyone know how to do this? I can drop down to plain TeX if necessary.

Answer 1

The LaTeX kernel has an inbuilt method of doing this, although it’s not widely used. Here’s an example:

\makeatletter \newcommand\@foo[2][]{[1: #1, 2: #2.] } \newcommand\foo{\@dblarg\@foo} \makeatother  \foo{same} \foo[hello]{world} 

Obviously, the \makeatletter commands can be dropped if you’re doing this inside a .sty or .cls file.