Wacky Behaviour for \csdef

Question

I am using \csdef{} to define a meaning to a given open/close bracket:

\csdef{Bracket Meaning \CsToStr{\lvert}}{Left |}
\csdef{Bracket Meaning \CsToStr{\rvert}}{Right |}
\csdef{Bracket Meaning \CsToStr{(}}{open (}
\csdef{Bracket Meaning \CsToStr{)}}{close )}

The first two and the last seem to work just fine, but the third one has an issue. When I attempt to access these via #1~\csuse{Bracket Meaning \CsToStr{#1}} (where #1 is one of the four given brackets \lvert, \rvert, ( and ), the output is:

I had expected the one highlighted in red to be ( open (.

So, what is going on here, and how should I restructure this to obtain the desired result for the ( bracket?

Code:

\documentclass{article}
\usepackage{etoolbox}
\usepackage{amsmath}
\usepackage{xcolor}
%% Following package not needed with newer binaries
\usepackage{expl3}%    \cs_to_str:N
\ExplSyntaxOn
    %% https://tex.stackexchange.com/a/100543/4301
    \newcommand{\CsToStr}[1]{\cs_to_str:N #1}%
\ExplSyntaxOff
\csdef{Bracket Meaning \CsToStr{\lvert}}{Left |}
\csdef{Bracket Meaning \CsToStr{\rvert}}{Right |}
\csdef{Bracket Meaning \CsToStr{(}}{open (}% FAILS !!
\csdef{Bracket Meaning \CsToStr{)}}{close )}
\newcommand{\ShowBracketMeaning}[1]{%
    #1~
    \ifcsdef{Bracket Meaning \CsToStr{#1}}{%
        \csuse{Bracket Meaning \CsToStr{#1}}%
     }{\text{No Meaning Defined}}%
}%
\begin{document}
$\ShowBracketMeaning{\lvert}$
$\ShowBracketMeaning{\rvert}$
{\color{red}%
    $\ShowBracketMeaning{(}$% <---- ????? This should show "open ("
}
$\ShowBracketMeaning{)}$
\end{document}

egreg · Accepted Answer · 2023-02-02T18:24:56.573

If you try

\edef\test{\CsToStr{(}}
\show\test

you'll see that the answer is

> \test=macro:
->.

The \cs_to_str:N function should only be applied to control sequences; otherwise its result is unpredictable (well, it is predictable, but not what you seem to want to get anyway).

Since both \CsToStr{(} and \CsToStr{)} return empty, the second definition involving them overrides the first.

Solution: if you really want that, don't use \CsToStr:

\csdef{Bracket Meaning \CsToStr{\lvert}}{Left |}
\csdef{Bracket Meaning \CsToStr{\rvert}}{Right |}
\csdef{Bracket Meaning (}{open (}
\csdef{Bracket Meaning )}{close )}

On the other hand, you don't need to remove the backslash when you form a control sequence, as long as the backslash is “other”.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor}
\ExplSyntaxOn
\NewExpandableDocumentCommand{\TokenToStr}{m}{\token_to_str:N #1}%
\NewExpandableDocumentCommand{\csdefinedTF}{mmm}
 {
  \cs_if_exist:cTF { #1 } { #2 } { #3 }
 }
\ExplSyntaxOff
\ExpandArgs{c}\newcommand{Bracket Meaning \TokenToStr{\lvert}}{Left |}
\ExpandArgs{c}\newcommand{Bracket Meaning \TokenToStr{\rvert}}{Right |}
\ExpandArgs{c}\newcommand{Bracket Meaning \TokenToStr{(}}{open (}
\ExpandArgs{c}\newcommand{Bracket Meaning \TokenToStr{)}}{close )}
\newcommand{\ShowBracketMeaning}[1]{%
    #1~
    \csdefinedTF{Bracket Meaning \TokenToStr{#1}}{%
        \UseName{Bracket Meaning \TokenToStr{#1}}%
     }{\text{No Meaning Defined}}%
}
\begin{document}
$\ShowBracketMeaning{\lvert}$
$\ShowBracketMeaning{\rvert}$
$\ShowBracketMeaning{(}$
$\ShowBracketMeaning{)}$
\end{document}

So, is there a \ifControlSequence{} equivalent macro that I can use in \ShowBracketMeaning to determine if I am going to use \csuse{Bracket Meaning \CsToStr{#1}} or \csuse{Bracket Meaning #1} — Peter Grill, Feb 02 '23 at 08:33
@PeterGrill I'm not sure what your purpose is, but there's the conditional \token_if_cs:NTF, so you can branch. And I'd avoid mixing expl3 with etoolbox. — egreg, Feb 02 '23 at 09:05
@PeterGrill I added a slightly different approach that requires no branching. — egreg, Feb 02 '23 at 18:25
Works great (not that there was any doubt). Thanks. I assume \UseName is just a \csuse{}? I also found using \detokenize{} yields the desired results, but wondering if there is a problem with that approach. — Peter Grill, Feb 02 '23 at 20:16
@PeterGrill Yes, \UseName is the kernel version of \csuse. About \detokenize, it's \tl_to_str:n in expl3, for multiple tokens, whereas \token_to_str:N is for a single token. — egreg, Feb 02 '23 at 22:42

score 2 · Answer 2 · answered Feb 02 '23 at 20:14

Befor egreg posted the detailed solution, I found that using \detokenize{} instead of relying on \CsToStr{} yields the desired results.

Am posting this to see in case there are any issues that I am not aware of with this approach.

Code:

\documentclass{article}
\usepackage{etoolbox}
\usepackage{amsmath}
\usepackage{xcolor}
\csdef{Bracket Meaning \detokenize{\lvert}}{Left |}
\csdef{Bracket Meaning \detokenize{\rvert}}{Right |}
\csdef{Bracket Meaning \detokenize{(}}{open (}
\csdef{Bracket Meaning \detokenize{)}}{close )}
\newcommand{\ShowBracketMeaning}[1]{%
    #1~
    \ifcsdef{Bracket Meaning \detokenize{#1}}{%
        \csuse{Bracket Meaning \detokenize{#1}}%
     }{\text{No Meaning Defined}}%
}%
\begin{document}
$\ShowBracketMeaning{\lvert}$
$\ShowBracketMeaning{\rvert}$
$\ShowBracketMeaning{(}$
$\ShowBracketMeaning{)}$
\end{document}

Wacky Behaviour for \csdef

Code:

2 Answers2

Code: