LaTeX \newcommand default argument: is empty?

Question

I'm trying to write a simple example command that prints nothing without an argument, but with an argument it surrounds it with something.

I've read that the default value should be \@empty and the simple \ifx\@empty#1 condition should do the job:

\newcommand{\optarg}[1][\@empty]{%
\ifx\@empty#1  {}  \else  {(((#1)))}  \fi
}

\optarg % (((empty)))
\optarg{} % (((empty)))
\optarg{test} % (((empty))) test

The latter three commands all print the empty word for some reason, and I want the first two to print nothing and the last to print (((test))).

I'm using TeXLive/Ubuntu. An ideas?

Answer 1

In the underlying TeX engine with which LaTeX is written, the number of arguments a command can take is fixed. What you've done with the default [\@empty] is ask LaTeX to examine the next token to see if it is an open square bracket [. If so, LaTeX takes the contents of square brackets as the argument, if not, the next token is put back into the input stream and the default \@empty argument is used instead. So to get your idea to work, you have to use square brackets to delimit the optional argument when present:

\optarg
\optarg[]
\optarg[test]

You should have better luck with this notation.

It's annoying that you can't use the same brackets for an optional argument as you use for a required argument, but that's the way it is.

Answer 2

Try the following test:

\documentclass{article}

\usepackage{xifthen}% provides \isempty test

\newcommand{\optarg}[1][]{%
  \ifthenelse{\isempty{#1}}%
    {}% if #1 is empty
    {(((#1)))}% if #1 is not empty
}

\begin{document}

Testing \verb|\optarg|: \optarg% prints nothing

Testing \verb|\optarg[]|: \optarg[]% prints nothing

Testing \verb|\optarg[test]|: \optarg[test]% prints (((test)))

\end{document}

The xifthen package provides the \ifthenelse construct and the \isempty test.

Another option is to use the ifmtarg package (see the ifmtarg.sty file for the documentation).

Answer 3

Using the LaTeX3 xparse package:

\usepackage{xparse}
\NewDocumentCommand\optarg{g}{%
  \IfNoValueF{#1}{(((#1)))}%
}