eappto trigger undefined control sequence error when used with hyperlink

Question

You can jump to the question in bold at the bottom if you want

I want to add examples of theorem at the end of my documents (with link after theorem to acess examples) but while having the exemple written under the theorem in my code (easier to manage).

I made this code :

\documentclass{article}
\usepackage{etoolbox}
\usepackage{hyperref}

\newcounter{exemplei}

\def\exemples{}

\newcommand{\exemple}[1]{
\hypertarget{exemple\arabic{exemplei}}{\theexemplei}
    \appto\exemples{\hyperlink{exemple\arabic{exemplei}}{Back} \theexemplei #1

     }
    \stepcounter{exemplei}
}
\begin{document}

Additions
\exemple{1+1=2}

\newpage

Multiplications
\exemple{1x1=1}

\newpage

\exemples

\end{document}

Unfortunatly back link didn't work. I discovered it was because \arabic{exemplei} was returning the last value and not the current value (probably because \appto was executing the code when showing the variable and not when adding text to it). I found that I had to replace \appto by \eappto:

\documentclass{article}
\usepackage{etoolbox}
\usepackage{hyperref}

\newcounter{exemplei}

\def\exemples{}

\newcommand{\exemple}[1]{
\hypertarget{exemple\arabic{exemplei}}{\theexemplei}
    \eappto\exemples{\hyperlink{exemple\arabic{exemplei}}{Back} \theexemplei #1

     }
    \stepcounter{exemplei}
}
\begin{document}

Additions
\exemple{1+1=2}

\newpage

Multiplications
\exemple{1x1=1}

\newpage

\exemples

\end{document}

But now this code trigger undefined control sequence error. Why ? How to fix it ?

I also discovered that I don't have error when not having hyperlink in content I add to the variable.

Answer 1

While the counter values need to be expanded, \hyperlink should not, so you should use \noexpand\hyperlink as part of the \exemple macro:

\newcommand{\exemple}[1]{%
  \hypertarget{exemple\arabic{exemplei}}{\theexemplei}%
  \eappto\exemples{\noexpand\hyperlink{exemple\arabic{exemplei}}{Back}
    \theexemplei{} #1\noexpand\par}%
  \stepcounter{exemplei}%
}

Spurious spaces have been removed (using % at the line-ends) and I inserted a forced \par if you want to separate each element.