No More Fragile Commands

In standard , commands such as \section have moving arguments, that is, arguments that migrate to an auxiliary file jobname.aux to be used later in typesetting the table of contents and so on [#!Lamport!#, 33–34]. In this process, macros in the body of the argument are expanded, so that when you type, say,

\section{Annoying \TeX nicalities}

what goes into the auxiliary file is something like

\@writefile{toc}{\string\contentsline\space {chapter}{\string\numberline\space 
{1}Annoying T\kern -.1667em\lower .5ex\hbox {E}\kern -.125emXnicalities}{1}}

where the second line contains the expansion of the macro \TeX.

There are several problems with this. First, the expansion, and consequently the line where it occurs in the auxiliary file, can be very long, which in some implementations means that TEX cannot read the line back in. Secondly, the expansion might not make sense, or might do the wrong thing, at the time when it is read in again. Even worse, the expansion of \section{\small foo}, for example, actually causes an infinite loop.

to *Experienced users of TEX who want to know why this happens can try typing the following lines to plain TEX:

\def\foo{\let\bar\foo}
\foo                    %Nothing much happens
\immediate\write0{\foo} %Disaster strikes

The problem is that at the time of the \write no assignment is taking place, so the \foo at the end of the expansion is not handled as a literal.

Standard 's solution to these problems is for the user to precede ``fragile'' macros with the \protect command [#!Lamport!#, 33–34]. By contrast, in the geom style, you don't have to worry about ``fragile'' commands in the following situations:

in the arguments of the sectioning commands \chapter, \section, and so on;
in the optional argument to the theorem environment and its relatives (Theorems and Their Friends);
in the arguments to \caption, \fig and its relatives (PostScript Figures);
in the arguments to \markright and \markboth;
in the argument to \cite.

This covers almost all important situations where fragile commands need to be \protected [#!Lamport!#, 151]. In future versions I hope to extend this permissiveness to the remaining situations.

Notice, however, that the \verb|...| construction continues to be forbidden inside arguments to most commands. This is due to reasons deeply embedded in TEX itself, and is unlikely ever to change.

to *In some circumstances you might want the old behavior of macro expansion inside the argument of \chapter, say. For instance, you might have a locally defined macro whose definition will be unknown at the time the table of contents is read. In this case you can use the following construction:

\edef\mystring{...} % where ... stands for the argument to be expanded
\expandafter\chapter\expandafter{\mystring}