Generate \charge arguments with a macro

Question

I'm trying to use a macro to generate arguments that should be passed to \charge. I'm trying this:

\documentclass{minimal}
\usepackage{chemfig}

\begin{document}

\charge{-135:1pt=5}{C}

\NewDocumentCommand{\myxyz}{}{-135:1pt=5}
\charge{\myxyz}{C}

\end{document}

The first \charge command works, but the second does not.

I get these error messages:

line 10: Paragraph ended before \charge_g was complete.
line 11: Improper \prevdepth. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing } inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing \endgroup inserted. \end{document}
line 11: Missing } inserted. \end{document}

I have also tried

\NewDocumentCommand{\myxyz}{}{-135:1pt=5}
\expandafter\charge{\myxyz}{C}

and

\NewDocumentCommand{\myxyz}{}{{-135:1pt=5}{C}}
\expandafter\charge\myxyz

What is the problem here and how can I make it work?

Answer 1

The problem is that \myxyz needs more than one expansion step to deliver the replacement text, due to how \NewDocumentCommand works.

Using \NewExpandableDocumentCommand doesn't solve the issue. But you can force full expansion.

\documentclass{article}
\usepackage{chemfig}

\NewExpandableDocumentCommand{\myxyz}{}{-135:1pt=5}

\begin{document}

\charge{-135:1pt=5}{C}

\bigskip

\ExpandArgs{e}\charge{\myxyz}{C}

\end{document}

Please, don't use the minimal class for examples: it's not meant for such jobs.

It would be possible to redefine \charge in such a way that it fully expands its argument. But I can't recommend doing it, as it's necessary to redefine internal commands of chemfig.

\documentclass{article}
\usepackage{chemfig}

\NewExpandableDocumentCommand{\myxyz}{}{-135:1pt=5}

\catcode`_=11
\def\charge_a#1#2{% #1=TF #2=liste emplacements
  \expanded{\unexpanded{\CF_testopt{\charge_b{#1}}{}}#2}\_nil
}
\catcode`_=8

\begin{document}

\charge{-135:1pt=5}{C}

\bigskip

\charge{\myxyz}{C}

\end{document}