How to expand a newcommand where it is called?

Question

I need to gather things that will be presented at the end of the document, as shown bellow

\documentclass{article}

\usepackage{collect}

\definecollection{somethings}

\newcommand{\collectthing}[1]{
    There is a #1 here!
    \begin{collect}{somethings}{}{}{}{} 
        \noindent #1 collected in section \arabic{section}
    \end{collect}
}

\begin{document}

\section{Section A}

\collectthing{ThingA}

\section{Section B}

\collectthing{ThingB}

\section*{Things collected}

\includecollection{somethings}

\end{document}

However, since the command \collectthing is expanded only when I call the command \includecollection, all sections are shown with the same value they share when they are expanded, as it can see in the result bellow

How do I fix it, so it shows the proper section number at the end?

Answer 1

You're storing \arabic{section}, not the current value.

Solution:

\documentclass{article}

\usepackage{collect}

\definecollection{somethings}

\newcommand{\collectthing}[1]{%
  There is a #1 here!%
  \expandandcollect{somethings}{\noindent #1 collected in section~\arabic{section}}%
}

\ExplSyntaxOn
\NewDocumentCommand{\expandandcollect}{mm}
 {
  \text_expand:n { \begin{collect}{#1}{}{}#2\end{collect} }
 }
\ExplSyntaxOff

\begin{document}

\section{Section A}

\collectthing{ThingA}

\section{Section B}

\collectthing{ThingB}

\section*{Things collected}

\includecollection{somethings}

\end{document}

Beware that this is not safe with fragile commands, so you'll need \protect in front of them. The created file will contain

\noindent ThingA collected in section~1

\noindent ThingB collected in section~2

Without expl3, but not as satisfying:

\makeatletter
\newcommand{\expandandcollect}[2]{%
  \begingroup\protected@edef\x{\endgroup
    \noexpand\begin{collect}{#1}{}{}#2\noexpand\end{collect}%
  }\x
}
\makeatother

Answer 2

If you don't mind a little expl3:

\documentclass{article}

\usepackage{xparse} % In case you are using LaTeX version older than 2021
\usepackage{collect}

\definecollection{somethings}

\ExplSyntaxOn

\newcommand{\collectthing}[1]{
    There ~ is ~ a ~ #1 ~ here!
    \exp_args:Nx \bmello_collectthing_internal:nn { \arabic{section} } { #1 }
}
\cs_new_protected:Nn \bmello_collectthing_internal:nn
  {
    \begin{collect}{somethings}{}{}{}{}
        \noindent #2 ~ collected ~ in ~ section ~ #1
    \end{collect}
  }

\ExplSyntaxOff

\begin{document}

\section{Section A}

\collectthing{ThingA}

\section{Section B}

\collectthing{ThingB}

\section*{Things collected}

\includecollection{somethings}

\end{document}

An internal command \bmello_collectthing_internal:nn is defined, and by using the x-type argument (\exp_args:Nx changes \bmello_collectthing_internal:nn to \bmello_collectthing_internal:xn), \arabic{section} is expanded before passing to this command.

Answer 3

If you don't wish the tokens forming #1 to be expanded, you can do something like this:

\documentclass{article}

\usepackage{collect}

\definecollection{somethings}

\newcommand\exchange[2]{#2#1}

\newcommand{\collectthing}[1]{%
  There is a #1 here!
  \expandafter\exchange\expandafter{%
    \number\arabic{section}%
  }{%
    \begin{collect}{somethings}{}{}{}{}%
    \noindent #1 collected in section~%
  }%
  \end{collect}%
}

\begin{document}

\section{Section A}

\collectthing{ThingA}

\section{Section B}

\collectthing{ThingB}

\section*{Things collected}

\includecollection{somethings}

\end{document}