General ⇒ Error for new Command with default Argument

[abakus]

Hi there,

I have an error I do not understand. I'm running TeX Live 2009 on a Linux Debian. The following is a minimal example. The log file is attached.

Combining the new command \aaa with \testa brings the error message below. What I do not understand is that it works with \bbb, \ccc and \testb where the only differences are
either the parenthesis (\ccc) or the missing default argument (\bbb, \testb).

I've read that this error sometimes is related to "robust" and "fragile" so I tested using \DeclareRobustCommand, \protect and \newrobustcmd (from the etoolbox package) but it did not changed a thing.

Code: Select all

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\newcommand{\aaa}[1][]{{#1}}
\newcommand{\bbb}[1]{{#1}}
\newcommand{\ccc}[1][]{#1}

\newcommand{\testa}[1][]{#1}
\newcommand{\testb}[1]{#1}

\begin{document}
%Don't work
$\aaa[ \testa[1] ]$
$\aaa[ \protect\testa[1] ]$

%Works
$\aaa[ \testa ]$
$\bbb{ \testa[1] }$
$\ccc[ \testa[1] ]$

$\aaa[ \testb{1} ]$
$\bbb{ \testb{1} }$
$\ccc[ \testb{1} ]$
\end{document}

In my use case I need the brackets because I have a subscript, something like \newcommand{\aaa}[1][]{A_{#1}}.

[Stefan Kottwitz]

Hi,

welcome to the board!

Nested square brackets can be a problem for the parsing of arguments. You can fix it by putting braces around the inner expression. This works:

Code: Select all

$\aaa[ {\testa[1]} ]$
$\aaa[ {\protect\testa[1]} ]$

Stefan

[abakus]

Hi Stefan,

many thanks for the solution.
Why does it make a difference if the new command has a default argument like \ccc?

[sommerfee]

Why does it make a difference if the new command has a default argument like \ccc?

It does not make a difference but this one needs an adaption as well:

Not working:

Code: Select all

\ccc[ \testa[1] ]

Working:

Code: Select all

\ccc[{ \testa[1] }]

The problem is that TeX pay attention to nested curly braces like {..{..}..} (and therefore handles them correctly), but not to other brace types. So actually [..[..]..] will be read as "[..[..]" (from the starting brace until the very first(!) closing brace, i.e. "..[.." inside square braces) plus "..]" (the rest), which will result in a parsing error.

To work around this problem one can use extra curly braces to "protect" the argument, i.e. [{..[..]..}], since this one will be read as "..[..].." inside square braces.

[cgnieder]

Just as an addendum: commands defined by the xparse packages are a “more intelligent” and handle nested square brackets:

Code: Select all

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{xparse}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\NewDocumentCommand{\aaa}{O{}}{{a:#1}}
\NewDocumentCommand{\bbb}{m}{{b:#1}}
\NewDocumentCommand{\ccc}{O{}}{c:#1}

\NewDocumentCommand{\testa}{O{}}{A:#1}
\NewDocumentCommand{\testb}{m}{B:#1}

\begin{document}
%Don't work
$\aaa[ \testa[1] ]$
$\aaa[ \protect\testa[1] ]$

%Works
$\aaa[ \testa ]$
$\bbb{ \testa[1] }$
$\ccc[ \testa[1] ]$

$\aaa[ \testb{1} ]$
$\bbb{ \testb{1} }$
$\ccc[ \testb{1} ]$
\end{document}

It's still probably not a fact one should always rely on.

Regards

[abakus]

Why does it make a difference if the new command has a default argument like \ccc?

It does not make a difference but this one needs an adaption as well:

Not working:

Code: Select all

\ccc[ \testa[1] ]

Its working for me, so I do not need the curly braces here. That's what confused me in the first place.

The problem is that TeX pay attention to nested curly braces like {..{..}..} (and therefore handles them correctly), but not to other brace types. So actually [..[..]..] will be read as "[..[..]" (from the starting brace until the very first(!) closing brace, i.e. "..[.." inside square braces) plus "..]" (the rest), which will result in a parsing error.

To work around this problem one can use extra curly braces to "protect" the argument, i.e. [{..[..]..}], since this one will be read as "..[..].." inside square braces.

Makes sense. "[" and "]" are not special characters like curly braces. Still I do not see why the \ccc example works (at least for me).

Just as an addendum: commands defined by the xparse packages are a “more intelligent” and handle nested square brackets:

thanks for the hint. After a look at the manual, it seem that xparse can help me with some other stuff as well .

Originally, I was using \newcommandx from the xargs package but is has the same problem as \newcommand.

ciao
abakus