conflict between scrbook and dynkin-diagrams

Question

I find a strange conflict between scrbook and dynkin-diagrams, here is the code

\documentclass{scrbook} 
\usepackage{dynkin-diagrams} 
\begin{document} 
\chapter{One}
Here is one \dynkin A2
\end{document}

An error message "Command \do undefined." is showing.

However, if I use

\documentclass{scrbook} 
\usepackage{dynkin-diagrams} 
\begin{document} 
Here is one \dynkin A2
\end{document}

Then it works.

I wonder what cause it and how to fix this bug?

Answer 1

The style file dynkin-diagrams.sty uses LaTeX's \renewcommand when it defines a temporary macro \do because it supposes that \do was defined already by previous macros. This is almost always a correct assumption but not when the \chapter macro from scrbook.cls is used. This macro re-defines \do and finally does \let\do=\relax. Then the following \renewcommand\do invoked from \dynkin fails.

It will be more correct if dynkin-diagrams.sty does \def\do and no \renewcommand\do, but we are not author of this macro file.

One obscure workaround should be as follows:

\documentclass{scrbook} 
\usepackage{dynkin-diagrams}
{\catcode`\@=11 \expandafter\gdef\expandafter\dynkin@save
   \expandafter{\dynkin@save \let\do=\LaTeX}} % :)
\begin{document} 
\chapter{One}
Here is one \dynkin A2
\end{document}

Answer 2

UPDATE 2023-05-20

The new release of dynkin-diagrams released on 2023-05-18 has fixed the issue, by prefixing every usage of \renewcommand{\do}[1]{...} by \providecommand{\do}{}.

ORIGINAL ANSWER

For historical reasons, at the start of a document (that is, after \begin{document} is processed), \do has the meaning \noexpand.

There have been discussions about removing this “hidden feature”, but it was thought that preserving this is important, because it has been so for about 30 years and packages might use it.

And indeed do, as you discovered. If you look in the documentation of etoolbox, you'll find several examples of

\renewcommand{\do}[1]{...}

and dynkin-diagrams makes extensive usage of etoolbox for list processing. It could dispense with it, by choosing expl3 instead of mixing a few of it with etoolbox and xstrings.

As a consequence, no class or package should ever issue \let\do\relax, because, as far as \renewcommand is concerned, something equivalent to \relax is seen as undefined. Unfortunately, scrbook does it.

You can use a workaround. When starting a diagram, the package issues \dynkin@save and you can hook in it to provide a definition for \do.

\documentclass{scrbook}
\usepackage{dynkin-diagrams}

\AddToHook{cmd/dynkin@save/before}{\providecommand{\do}{}}

\begin{document}
\chapter{One}
Here is one \dynkin A2
\end{document}