Defining inside an \edef

Question

The following throws an "Undefined control sequence" error:

\edef\testa{\edef\testb{foobar}\testb}

when it gets to expanding \testb. This problem is also mentioned in "TEX by Topic" 12.6.2. But no solution is given.

EDIT2, this is a minimal not-working example:

\documentclass{article}

\newenvironment{definition}[1]{\def\currentdef{#1}}{}

\makeatletter

\newcommand\contains[2]{%
    \edef\listarg{#1}%
    \xdef\needle{#2}%
    \expandafter\contains@sub\listarg,\relax\noexpand\@eolst%
}
\def\contains@sub#1,#2\@eolst{%
    \edef\match{#1}%
    \ifx\needle\match%
        true
    \else
        \ifx\relax#2\relax\else
            \contains@sub#2\@eolst%
        \fi
    \fi}

\newcommand\cond[1]{%
    \edef\test{\contains{\currentdef}{#1}}%
    \ifx\relax\test\relax\def\ommitthis\relax\fi%
    \@ifundefined{ommitthis}{#1}{}%
}

\makeatother

\def\test{test2}

\begin{document}

\begin{definition}{test2,test3}
    \cond{test3}
    \cond{\test}
    \cond{test1}
\end{definition}

\end{document}

\cond should check whether the given argument was listed in the surrounding definition. So test2 and test3 must be ommited and only test1 should be printed.

Answer 1

You just need to cycle through the list given as argument:

\documentclass{article}

\newenvironment{definition}[1]
  {\def\currentdef{#1}}
  {}

\makeatletter
\def\cond#1{\edef\@tempa{#1}%
  \@tempswafalse
  \@for\@tempb:=\currentdef\do
    {\ifx\@tempa\@tempb\@tempswatrue\fi}%
  \if@tempswa\else\@tempa\fi
}
\makeatother

\begin{document}

\def\test{test2}

\begin{definition}{test2,test3}
    \cond{test3}
    \cond{\test}
    \cond{test1}
\end{definition}

\end{document}

The definition environment stores the argument in the macro \currentdef (as you do). The macro \cond expands completely its argument and stores the result in \@tempa. Then sets the scratch conditional \@tempswa to false and, with the help of \@for compares \@tempa with each part of \currentdef; the block

\@for\@tempb=\LIST\do{<code>}

where \LIST expands to a,b,c (any comma separated list of tokens) executes <code> with \@tempb expanding to a, b and c in turn.

The <code>, in our case, is "check if \@tempa and \@tempb have the same meaning (expansion, here) and, if so, set \if@tempswa to true.

Finally \cond tests \@iftempswa: if it's true nothing else is done, otherwise \@tempa is expanded.

---

The same effect, but with even some improvements can be obtained with expl3:

\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentEnvironment{definition}{m}
 { \canaaerus_store:n { #1 } } { }
\NewDocumentCommand{\cond}{m}
 { \canaaerus_check:x { #1 } }

\cs_new_protected:Npn \canaaerus_store:n #1
 {
  \seq_set_split:Nnn \l_canaaerus_list_seq { , } { #1 }
 }

\cs_new_protected:Npn \canaaerus_check:n #1
 {
  \seq_if_in:NnF \l_canaaerus_list_seq { #1 } { #1 }
 }
\cs_generate_variant:Nn \canaaerus_check:n { x }

\seq_new:N \l_canaaerus_list_seq
\ExplSyntaxOff

The improvement is that spaces around the list are stripped off, so

\begin{definition}{test2 , test3 }

is just as good as the input without spaces.

The strategy is the same, but the argument to definition is stored in a sequence (as a bonus, spaces around the list argument are stripped off). The cycle works similarly as before: we check whether the argument to \canaaerus_check:x (completely expanded) appears in the sequence; if not, the argument is output.

Notice how we get the complete expansion: the macro defined is \canaaerus_check:n, but we create also a variant so that using

\canaaerus_check:x {<tokens>}

is thus pretty much equivalent to saying

\edef\@tempa{<tokens>}\expandafter\canaaerus_check:n\expandafter{\@tempa}

(I apologize for the horrible mixture of LaTeX3 and primitive syntax, but it's just by way of example). The convenience of the "variant" method should be self-evident.