Misplaced \omit. \multispan with \newcommand

Question

I'm trying to build my own cookbook-style, which has an ingredient makro. Sometimes the ingredients should be separated and have a headline, so I tried with an optional argument. A minimum example looks like this:

\documentclass{scrartcl}
\newcommand{\ingredient}[3][]{
    #1 #2 & #3 \\
}
\begin{document}
    \begin{tabular}{r|l}        
        \ingredient[\multicolumn{2}{l}{Head} \\]{test1}{test2}
        \ingredient[test3]{test4}{test5}
        \ingredient{test6}{test7}
    \end{tabular}
\end{document}

What's wrong with this code? I get the following error message:

! Misplaced \omit.
\multispan ->\omit 
                   \@multispan 
l.7 ...[\multicolumn{2}{l}{Head} \\]{test1}{test2}

Answer 1

Tabular material is really picky about what is allowed to go "before" a line. In particular, it has to be fully expandable.

Hence, even the assignment which \ingredient needs to do to check for its optional argument is enough to start the line, and then \multicolumn is no longer allowed to appear.

The easiest way to remedy this is to make the first argument of \ingredient mandatory:

\documentclass{scrartcl}
\newcommand{\ingredient}[3]{
    #1 #2 & #3 \\
}
\begin{document}
    \begin{tabular}{r|l}        
        \ingredient{\multicolumn{2}{l}{Head} \\}{test1}{test2}
        \ingredient{test3}{test4}{test5}
        \ingredient{}{test6}{test7}
    \end{tabular}
\end{document}

Answer 2

You can retain your original syntax and have a single macro with an optional parameter if you use \DeclareExpandableDocumentCommand from the the xparse package.

Code:

\documentclass{scrartcl}
\usepackage{xparse}

\DeclareExpandableDocumentCommand{\ingredient}{O{} m m}{%
    #1 #2 & #3 \\%
}%

\begin{document}
    \begin{tabular}{r|l}        
        \ingredient[\multicolumn{2}{l}{Head} \\]{test1}{test2}
        \ingredient[test3]{test4}{test5}
        \ingredient{test6}{test7}
    \end{tabular}
\end{document}