Differentiate reference based on type (problem with argument expansion)

Question

Since I use unnumbered sections, I'd like to have a variation of \autoref that uses names instead of numbers in the case of sectioning, but keeps its usual behaviour for other environments. I've come up with this naive piece of code, but I don't get it to work.

\documentclass{article}
\usepackage{xstring, hyperref, nameref}
\setcounter{secnumdepth}{-1}
\makeatletter
\newcommand{\KSautoref}[1]{
    \edef\KS@RefType{\expandafter\@fourthoffive\csname r@#1\endcsname}
    \newif\ifKSSectioning\KSSectioningfalse
    \IfBeginWith{\KS@RefType}{chapter}{\KSSectioningtrue}{}
    \IfBeginWith{\KS@RefType}{section}{\KSSectioningtrue}{}
    \IfBeginWith{\KS@RefType}{subsection}{\KSSectioningtrue}{}
    \IfBeginWith{\KS@RefType}{subsubsection}{\KSSectioningtrue}{}
    \IfBeginWith{\KS@RefType}{paragraph}{\KSSectioningtrue}{}
    \IfBeginWith{\KS@RefType}{subparagraph}{\KSSectioningtrue}{}
    \ifKSSectioning\chaptername\nameref{#1}\else\autoref{#1}\fi
}
\makeatother
\begin{document}
\section{Test} \label{test}
See \KSautoref{test}.
\end{document}

When I try to compile this, I get the following error:

! Argument of \@fourthoffive has an extra }.
<inserted text>
                \par
l.19 See \KSautoref{test}
                         .
?

Using \tracingall I get this additional information:

{\edef}
{\expandafter}
{\csname}

\@fourthoffive #1#2#3#4#5->#4
#1<-\r@test

How can I get \r@test to expand as well, so that it's not used as just the first argument?

If there is a package that provides the functionality, I'd rather use that, but still I'd like to know the bit about expansion...

Answer 1

You're doing only one expansion, but you need two. With

\expandafter\@fourthoffive\csname r@#1\endcsname

and #1=test as in the example, you get

\expandafter\@fourthoffive\csname r@test\endcsname

which becomes

\@fourthoffive\r@test

So you need \expandafter\expandafter\expandafter.

Here's a better version:

\documentclass{book}
\usepackage{xstring, hyperref, nameref}
\setcounter{secnumdepth}{-1}
\makeatletter
\newif\ifKSSectioning
\newcommand{\KSautoref}[1]{%
  \KSSectioningfalse
  \expandafter\ifx\csname r@#1\endcsname\relax
    \ref{#1}%
  \else
    \protected@edef\KS@RefType{\expandafter\expandafter\expandafter\@fourthoffive\csname r@#1\endcsname}%
    \IfBeginWith{\KS@RefType}{chapter}{\KSSectioningtrue}{}%
    \IfBeginWith{\KS@RefType}{section}{\KSSectioningtrue}{}%
    \IfBeginWith{\KS@RefType}{subsection}{\KSSectioningtrue}{}%
    \IfBeginWith{\KS@RefType}{subsubsection}{\KSSectioningtrue}{}%
    \IfBeginWith{\KS@RefType}{paragraph}{\KSSectioningtrue}{}%
    \IfBeginWith{\KS@RefType}{subparagraph}{\KSSectioningtrue}{}%
    \ifKSSectioning\chaptername~\nameref{#1}\else\autoref{#1}\fi
  \fi
}
\makeatother
\begin{document}
\section{Test} \label{test}
See \KSautoref{test}.
\end{document}

The added conditional is to cope with the case the reference is not already set; I moved the \newif outside the definition, as it's useless constantly creating a new conditional, all it suffices is to set it to false at the beginning of the macro.

I also used \protected@edef in case the title contains macros such as \emph that don't survive \edef.

---

Here's an implementation using expl3 and regular expressions.

\documentclass{book}
\usepackage{xparse,l3regex, hyperref, nameref}
\setcounter{secnumdepth}{-1}

\ExplSyntaxOn
% Document level commands
\NewDocumentCommand \KSautoref { m }
 {
  \pazathoth_ks_autoref:n { #1 }
 }

% variables
\tl_new:N \l__pazathoth_type_tl
\regex_const:Nn \c_pazathoth_class_regex
 {
  \A (chapter|section|subsection|subsubsection|paragraph|subparagraph) .*
 }

% internal functions
\cs_new_protected:Npn \pazathoth_ks_autoref:n #1
 {
  \cs_if_eq:cNTF { r@#1 } \scan_stop:
   { % if the reference is undefined go to the standard method
    \ref { #1 }
   }
   { % else scan for a named reference
    \__pazathoth_ks_autoref:cn { r@#1 } { #1 }
   }
 }

\cs_new_protected:Npn \__pazathoth_ks_autoref:Nn #1 #2
 {
  % get the fourth item in `r@#1`
  \tl_set:Nx \l__pazathoth_type_tl { \tl_item:Vn #1 { 4 } }
  % check if the fourth item starts with a name
  \regex_replace_all:NnNTF \c_pazathoth_class_regex { \1 } \l__pazathoth_type_tl
   { % the replacement was successful; use the section name
    \use:c { \l__pazathoth_type_tl name } \nobreakspace \nameref { #2 }
   }
   { % no name, use \autoref
    \autoref { #2 }
   }
 }

% function variants
\cs_generate_variant:Nn \cs_if_eq:NNTF { c }
\cs_generate_variant:Nn \tl_item:nn { V }
\cs_generate_variant:Nn \__pazathoth_ks_autoref:Nn { c }

\ExplSyntaxOff

\providecommand{\sectionname}{Section}

\begin{document}
\section{Test \emph{x}} \label{test}
See \KSautoref{test}.

\begin{enumerate}
\item\label{item} x
\end{enumerate}

See \KSautoref{item}.

\end{document}