Comparing strings; genitiv version of \cref command

Question

I am working with cleveref in a German document and i need in some cases the genitive of the word given by \cref{}. The problem is, that the German genitive is not equally build for all words, so i need some kind of switch-case. Therefore i want a command \crefgen{}, which works similar to \cref{}, but returns the genitive. I think my code shows pretty well what I am trying to achieve. I am sorry that this example is manly in German, but i did not find a useful English example in reasonable time. Nevertheless, I emphasised the important parts I want to add, so it will be clear, even without understanding German.

\documentclass[ngerman,11pt,a4paper]{scrreprt}

\usepackage{cleveref}
\usepackage{ulem}

\newtheorem{problem}{Problem}
\newtheorem{satz}[problem]{Satz}
\newtheorem{proposition}[problem]{Proposition}

\crefname{problem}{Problem}{Probleme}
\crefname{satz}{Satz}{S{\"a}tze}
\crefname{proposition}{Proposition}{Propositionen}

\newcommand{\crefgen}[1]{
    \def\tempStyle{\namecref{#1}}
    \def\tempProblem{Problem}
    \def\tempSatz{Satz} 
    \ifx\tempStyle\tempProblem{
        \namecref{#1}s \labelcref{#1}       % genitive for Problem
    }\else{
        \ifx\tempStyle\tempSatz{
            \namecref{#1}es \labelcref{#1}  % genitive for Satz
        }\else{
            No genitiv defined              % error if no genitive defined
        }\fi
    }\fi
}

\begin{document}

\begin{problem}\label{prob}
Some Text
\end{problem}
\begin{satz}\label{sat}
Some Text
\end{satz}
\begin{proposition}\label{propo}
Some Text
\end{proposition}

\paragraph{What i want:}
Die L{\"o}sung des Problem\uline{s} \ref{prob}.\\
Die Aussage des Satz\uline{es} \ref{sat}.\\
\uline{No genitiv defined} f{\"u}r Propositionen.

\paragraph{How i want to achieve that:}
Die L{\"o}sung des \verb!\crefgen{prob}!.\\
Die Aussage des \verb!\crefgen{sat}!.\\
\verb!\crefgen{propo}! f{\"u}r Propositionen.

\paragraph{How my command looks like:}
Die L{\"o}sung des \crefgen{prob}.\\
Die Aussage des \crefgen{sat}.\\
\crefgen{propo} f{\"u}r Propositionen.

\end{document}

Answer 1

Welcome to TeX.SE!

You can do as follows using cleveref's \cref@gettype, some \csname, \edef and a tiny bit of expl3 code for the convenient switch-case statement (\str_case:onF). If a gerund form is requested but hasn't been defined, you'll get a real error message that you can't miss, thanks to \errmessage. Of course, one could insert a fallback string as in your example, but I believe \errmessage is better here.

\documentclass[ngerman]{scrreprt}
\usepackage{babel}
% Without amsthm, your 'satz' and 'proposition' are considered to be of type
% 'problem'... which is indeed a problem!
\usepackage{amsthm}
\usepackage{cleveref}
\usepackage{ulem}
\usepackage{expl3}

\newtheorem{problem}{Problem}
\newtheorem{satz}[problem]{Satz}
\newtheorem{proposition}[problem]{Proposition}

\crefname{problem}{Problem}{Probleme}
\crefname{satz}{Satz}{S{\"a}tze}
\crefname{proposition}{Proposition}{Propositionen}

\makeatletter

% Let's borrow \str_case:onF from expl3 (\cs_set_eq:NN is \let)
\ExplSyntaxOn
\cs_set_eq:NN \mycref@str@oswitch \str_case:onF
\ExplSyntaxOff

\newcommand*{\crefgen}[1]{%
    \cref@gettype{#1}{\mycref@type}% type: problem, satz, proposition...
    \edef\mycref@name{%
      \unexpanded\expandafter\expandafter\expandafter{%
        \csname cref@\mycref@type @name\endcsname}}%
    %
    \mycref@str@oswitch
      {\mycref@name}%
      {%
        {Problem}{Problems}%
        {Satz}{Satzes}%
      }%
      {\errmessage{Gerund form undefined for '\mycref@name'}}%
    ~\labelcref{#1}%
}

\makeatother

\begin{document}

\begin{problem}\label{prob}
Some Text
\end{problem}
\begin{satz}\label{sat}
Some Text
\end{satz}
\begin{proposition}\label{propo}
Some Text
\end{proposition}

\paragraph{What I want:}
Die L{\"o}sung des Problem\uline{s} \ref{prob}.\\
Die Aussage des Satz\uline{es} \ref{sat}.\\
\uline{No genitiv defined} f{\"u}r Propositionen (or rather, throw an error).

\paragraph{How I want to achieve that:}
Die L{\"o}sung des \verb!\crefgen{prob}!.\\
Die Aussage des \verb!\crefgen{sat}!.\\
\verb!\crefgen{propo}! f{\"u}r Propositionen.

\paragraph{How my command looks like:}
Die L{\"o}sung des \crefgen{prob}.\\
Die Aussage des \crefgen{sat}.\\
An error is thrown with \verb!\errmessage! if one uncomments
\verb!\crefgen{propo}!.
%\crefgen{propo} f{\"u}r Propositionen.

\end{document}