# What is the difference between passing optional and mandatory arguments?

I wanted to define a macro which makes a program listing, and finds out a program language from an optional argument. The output I receive from the MWE below is

Language set = 'Verilog'

Passed argument is 'Verilog'

So, is looks like that in both calls to '\lstset' the same argument is passed. However, if the argument is produced from optional, rather than a mandatory argument, I am presented with the error message: "language Verilog undefined". What do I wrong?

\documentclass{article}
\usepackage{listingsutf8}
\usepackage{xkeyval}

\makeatletter
\define@key{MESourceFile}{language}{\def\pb@language{#1}}
\setkeys{MESourceFile}{language={[ANSI]C}}%
\newcommand\MESourceFile[4][]{
\begingroup%
\setkeys{MESourceFile}{#1}
Passed argument is '\pb@language'\\
\lstset{language={\pb@language}}
\endgroup
}
\makeatother

\newcommand\MESetListingFormat[2][]
{
Language set = '#2'
\lstset{language={#2}}  e
}

\begin{document}
\MESetListingFormat{Verilog}
\MESourceFile[language=Verilog]{lst/TestHello.c}{A hello világ program}{lst:TestHello.c}
\end{document}

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• Try, instead of \lstset{language={\pb@language}}, the code \begingroup\edef\x{\endgroup\noexpand\lstset{language=\pb@language}}\x – egreg Aug 20 '14 at 15:45
• @ Martin Schröder Sorry for being unpolite, but: I could not find out how to register, or change user name. From your homepage I learned "No registration required". In the Help center, I took the "2-minute tour", which takes back to the Help center. Even Google takes to the "No registration required" page. When asking, I do not see an obvious possibility to register or provide username. Some hint on the homepage would be great. @egreg Many thanks, it works. Is there a more elegant method, than checking for empty string, for absence of an argument? A link to your info source would be great. – katang Aug 20 '14 at 17:32
• @user11708 It's true that “no registration is required”, but a registered user can better enjoy the site. The \begingroup\edef\x trick is very common; you need nothing special if the argument is empty, as \setkeys happily accepts an empty argument. – egreg Aug 20 '14 at 17:40

It's an expansion problem: when the key is set, no expansion is performed, so you have to pass the expanded \pb@language macro:

\documentclass{article}
\usepackage{listingsutf8}
\usepackage{xkeyval}

\makeatletter
\define@key{MESourceFile}{language}{\def\pb@language{#1}}
\setkeys{MESourceFile}{language={[ANSI]C}}
\newcommand\MESourceFile[4][]{%
\begingroup
\setkeys{MESourceFile}{#1}
%    Passed argument is '\pb@language'\\% uncomment for debugging
\begingroup\edef\x{\endgroup
\noexpand\lstset{language={\pb@language}}}\x
\lstinputlisting{#2}%
\endgroup
}
\makeatother

\begin{document}

\MESourceFile[language=Verilog]{helloworld.v}{A hello világ program}{helloworld.v}

\end{document}


However, it would be easier to do it using \lstset directly:

\documentclass{article}
\usepackage{listingsutf8}

\makeatletter
\newcommand{\MESourceFile}[4][]{%
\begingroup
\lstset{#1}%
\lstinputlisting{#2}%
\endgroup
}
\makeatother

\begin{document}

\MESourceFile[language=Verilog]{helloworld.v}{A hello világ program}{helloworld.v}

\end{document}


Source of the listing: http://www.asic-world.com/verilog/first1.html

See Explanations about \begingroup\edef\x{\endgroup for an explanation of the trick.

• I do use \lstset in \MESetListingFormat directly. I do work on multi-language document, and I want to do a uniform look for the listings (the MWE is oversimplified). This default argument looks to be a better idea than adding an extra mandatory parameter. – katang Aug 20 '14 at 17:50