Error with expansions

Question

I am getting an error while trying to compile this minimal example:

min.cls:

\ProvidesClass{min}

\LoadClass{minimal}
\NeedsTeXFormat{LaTeX2e}

\newenvironment{myenv}{}{
\def\aaa{test }
\expandafter\def\csname bbb\endcsname{$4\sqrt{2}$}
\bbb
\edef\aaa{\aaa \bbb}
\aaa
}

\endinput

mintest.tex:

\documentclass{min}

\begin{document}

\begin{myenv}
Test
\end{myenv}

\end{document}

Running latex over mintest.tex produces this error:

ERROR: Use of \@sqrt doesn't match its definition.

--- TeX said ---
\@ifnextchar ...eserved@d =#1\def \reserved@a {#2}
                                                  \def \reserved@b {#3}\futu...
l.7 \end{myenv}

Peculiar. I want to expand the definition of \aaa by concatenation. Any thoughts?

LaTeX doesn't like that commands with optional arguments are passed through an \edef. It's difficult to say more from your example, but \protected@edef\aaa{\aaa\bbb} should work. — egreg, Oct 29 '12 at 10:07
@egreg WOW, thanks, it worked. Can you post as an answer so I can mark it? — osolmaz, Oct 29 '12 at 10:09

score 7 · Accepted Answer · edited Apr 13 '17 at 12:34

TeX doesn't like to find commands with optional arguments or defined with \DeclareRobustCommand inside an \edef; \sqrt fall in the first type, \bfseries in the second one.

In your case

\protected@edef\aaa{\aaa\bbb}

will work.

Be careful, though: fragile commands such as \linebreak can't go in \protected@edef directly and must be preceded by \protect. Some knowledge of how dubious commands are defined is necessary.

See also Minimal \protected@edef example and Problem with nested \noexpand and \edef

Error with expansions

1 Answers1