LaTeX: optional arguments with square brackets

Question

LaTeX is trying to add a missing $ that I don't want to the following code. Minimal working example:

\documentclass[letter,12pt]{article}

\newcommand{\monkeyBeans}[2][]{
  \parbox{4in}{#1}
}

\begin{document}

\parbox{4in}{$[a, b]$} 

\monkeyBeans[$[a, b]$]{yo monkey}

\end{document}

If I try any of the following lines in my document instead of the call to monkeyBeans, there is no error:

\parbox{4in}{$[a, b]$} 
\monkeyBeans[yo monkey]{$[a, b]$}
\monkeyBeans[$(a, b)$]{yo monkey}
\monkeyBeans{$[a, b]$}{yo monkey}

What is the deal with square brackets in optional arguments?

Where is the rest of the tabular that is ended in \monkeyBeans? A minimal working example (MWE) would really help here. It should start with \documentclass{...} and end with \end{document}. — Werner, Nov 28 '12 at 03:15
You will need to enclose an optional argument with square brackets in braces (i.e. \monkeyBeans[{$[a, b]$}]{yo monkey} or [${[a, b]}$], …) otherwise the inner/first ] is used as the end of your optional argument, and then all hell breaks loose. — Qrrbrbirlbel, Nov 28 '12 at 05:04
Does this answer your question? Bug? LaTeX misparses nested optional arguments — user202729, Nov 07 '21 at 15:19

score 24 · Accepted Answer · answered Nov 28 '12 at 05:25

Note: Without a MWE I will remove \end{tabular} from the definition of your macro and ignore the fact that you declare two argments (one optional, one mandatory) but use only #1.

An optional argument behaves quite different from normal arguments and groups as it is catched by TeX with the help of the [/] delimiters.

With a macro is defined with

\newcommand{\monkeyBeans}[2][]{\parbox{4in}{#1}}

and called with

\monkeyBeans[$[a, b]$]{yo monkey}

#1 will be $[a, b. #2 will be $ and in this case discarded as #2 is not used in your macro definition. Therefore, \monkeyBeans[$[a, b]$]{yo monkey} is expanded to

\parbox{4in}{$[a, b}]{yo monkey}

No wonder there's a $ missing! I'd miss it, too.

With a macro \monkeyBean defined as

\newcommand{\monkeyBean}[1][]{\parbox{4in}{#1}}

and used with

\monkeyBean[$[a, b]$]yo monkey

It's even clearer what happens, as TeX complies twice about an missing $ because that line expands to

\parbox{4in}{$[a, b}$]yo monkey

and “]yo monkey” is typeset in math mode, too.

Solution: Enclose optional arguments with a ] in braces, so that they get grouped:

\monkeyBean[{$[a, b]$}]
\monkeyBeans[{$[a, b]$}]{yo monkey}

Qrrbrbirlbel: Thank you, this was extremely informative and clear. You even got around my copy-paste error in the original question! It makes some sense, that LaTeX wouldn't notice the entering/exiting of math mode inside the square brackets. I was a bit surprised that: \monkeyBeans[([a, b])]{yo monkey} also causes the error. — Kyle, Nov 28 '12 at 14:06

score 6 · Answer 2 · answered Nov 28 '12 at 13:36

Qrrbrbirlbel's solution is the standard LaTeX2e approach, and is certainly what you have to do for an arbitrary command. The LaTeX3 package xparse deals with the nesting in a cleaner way:

\documentclass[letter,12pt]{article}
\usepackage{xparse}

\NewDocumentCommand{\monkeyBeans}{O{}m}{%
  \parbox{4in}{#1}%
}

\begin{document}

\parbox{4in}{$[a, b]$} 

\monkeyBeans[$[a, b]$]{yo monkey}

\end{document}

which works of course only if you are defining the command where [] nesting is required.

LaTeX: optional arguments with square brackets

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