LaTeX: How to define command depending on existence of another command (e.g. `\nameref`)?

Question

I'd like to use \nameref with hyperref, but I want be able to render my document without hyperref, too.

So I'd like to define a wrapper command around nameref (not defining a "dummy" \nameref myself), depending on whether \nameref is defined.

The answer preferably works with any defined command, not just \nameref.

From similar questions I tried to derive some solution that did not work:

\newcommand{\checkAndDef}[1]{%
\ifcsname#1\endcsname%
\newcommand{\refn}[1]{\nameref{#1}}%
\else%
\newcommand{\refn}[1]{[#1]}% no \nameref available
\fi%
}
\checkAndDef{nameref}%

Note the \checkAndDef looks ugly, and I added it after I failed to do a literal check for \nameref. I guess I would need to know more details about TeX and LaTeX to fix that than I do.

First solution modeled after werner's proposals (does not work):

\newcommand{\namerefcheck}{%
\ifcsname{}nameref\endcsname%
\newcommand{\refn}[1]{\nameref{##1}}%
\else%
\newcommand{\refn}[1]{[\detokenize{##1}]}% no \nameref available
\fi%
}
\AtBeginDocument{\namerefcheck}

This "solution" defines \refn as if hyperref weren't loaded, regardless whether is is or not.

Note: If you replace {}nameref with \nameref, then it works (it seems to me).

Answer 1

A couple of things are going on here:

1. When defining a command within another command, you need to double up on the use of your parameter notation. So, #1 should become ##1, #2 should become ##2, and so forth. In that sense, the following definition of \checkAndDef is probably what you're after:

   \newcommand{\checkAndDef}[1]{%
     \ifcsname#1\endcsname%
       \newcommand{\refn}[1]{\nameref{##1}}%
     \else%
       \newcommand{\refn}[1]{[\detokenize{##1}]}% no \nameref available
     \fi%
   }
   

   I've added a \detokenize to the "no \nameref available" option, since labels can contain some odd characters (like underscore _) by default.

2. hyperref should usually be loaded last of all the packages, since it interfaces with many document elements, all of which could be changed by other included packages. As such, \checkAndDef may be required after \begin{document} in some cases. This is specifically required for \nameref.

\documentclass{article}

\usepackage{hyperref}% Comment out or not

\newcommand{\checkAndDef}[1]{%
  \ifcsname#1\endcsname
    \newcommand{\refn}[1]{\nameref{##1}}%
  \else
    \newcommand{\refn}[1]{[\detokenize{##1}]}% no \nameref available
  \fi
}

\AtBeginDocument{\checkAndDef{nameref}}

\begin{document}

\section{A section}\label{sec:section}

\refn{sec:section}

\end{document}

The above code results in

when hyperref is loaded, and

when hyperref is not loaded.

Answer 2

I propose

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\conditionaldef}{mmO{0}omm}
 {% #1 = command to define
  % #2 = command to test
  % #3 = number of args,
  % #4 = default optional
  % #5 = replacement text if #2 exists
  % #6 = replacement text if #2 doesn't exist
  \AtBeginDocument
   {
    \cs_if_exist:NTF #2
     {
      \IfNoValueTF{#4}
       {
        \newcommand{#1}[#3]{#5}
       }
       {
        \newcommand{#1}[#3][#4]{#5}
       }
     }
     {
      \IfNoValueTF{#4}
       {
        \newcommand{#1}[#3]{#6}
       }
       {
        \newcommand{#1}[#3][#4]{#6}
       }
     }
   }
 }
\ExplSyntaxOff

and you call it like

\conditionaldef{\refn}{\nameref}[1]{\nameref{#1}}{[\detokenize{#1}]}

Full example.

\documentclass{article}
\usepackage{xparse}
\usepackage{hyperref}

\ExplSyntaxOn
\NewDocumentCommand{\conditionaldef}{mmO{0}omm}
 {% #1 = command to define
  % #2 = command to test
  % #3 = number of args,
  % #4 = default optional
  % #5 = replacement text if #2 exists
  % #6 = replacement text if #2 doesn't exist
  \AtBeginDocument
   {
    \cs_if_exist:NTF #2
     {
      \IfNoValueTF{#4}{\newcommand{#1}[#3]{#5}}}{\newcommand{#1}[#3][#4]{#5}}
     }
     {
      \IfNoValueTF{#4}{\newcommand{#1}[#3]{#6}}{\newcommand{#1}[#3][#4]{#6}}
     }
   }
 }
\ExplSyntaxOff

\conditionaldef{\refn}{\nameref}[1]{\nameref{#1}}{[\detokenize{#1}]}

\begin{document}

\section{Test}\label{test}

\refn{test}

\end{document}

Output if hyperref is loaded

Output if hyperref is not loaded

Note

You essentially use the same syntax as for \newcommand, so also commands with a single optional argument are allowed. No need of doubling # when calling \conditionaldef.

Without xparse

\documentclass{article}
%\usepackage{hyperref}

%%% syntax
%% \conditionalref{<marg>}[<oarg>][<oarg>]{<marg>}{<marg>}
\makeatletter
\newcommand{\conditionaldef}[2]{%
  \@ifnextchar[{\cond@def@opt{#1}{#2}}{\cond@def@noopt{#1}{#2}}%
}
\newcommand{\cond@def@noopt}[4]{%
  \AtBeginDocument{%
    \ifdefined#2%
      \newcommand{#1}{#3}%
    \else
      \newcommand{#1}{#4}%
    \fi
  }%
}
\def\cond@def@opt#1#2[#3]{%
  \@ifnextchar[{\cond@def@twoopt{#1}{#2}{#3}}{\cond@def@oneopt{#1}{#2}{#3}}%
}
\def\cond@def@oneopt#1#2#3#4#5{%
  \AtBeginDocument{%
    \ifdefined#2%
      \newcommand{#1}[#3]{#4}%
    \else
      \newcommand{#1}[#3]{#5}%
    \fi
  }%
}
\def\cond@def@twoopt#1#2#3[#4]#5#6{%
  \AtBeginDocument{%
    \ifdefined#2%
      \newcommand{#1}[#3][#4]{#5}%
    \else
      \newcommand{#1}[#3][#4]{#6}%
    \fi
  }%
}
\makeatother

\conditionaldef{\refn}{\nameref}[1]{\nameref{#1}}{[\detokenize{#1}]}

\begin{document}

\section{Test}\label{test}

\refn{test}

\end{document}

Simpler version

Of course, the above is a waste of time if you just want to define one command: a simple

\AtBeginDocument{%
  \ifdefined\nameref
    \newcommand{\refn}[1]{\nameref{#1}}%
  \else
    \newcommand{\refn}[1]{[\detokenize{#1}]}%
  \fi
}

will suffice. The infrastructure above allows to define many commands without code duplication.

Answer 3

Assuming that the token \BizArReUnDEFiNeD is always undefined, you can do:

\documentclass{article}

\makeatletter
% \checkAndDef{<other command>}%
%             {<definition-command><new command><parameter text>}%
%             {<new command's definition-text in case other command is undefined>}%
%             {<new command's definition-text in case other command is defined>}
%
% <parameter text> can be omitted.
%
\newcommand\checkAndDef[4]{%
  \AtBeginDocument{%
    \ifx\BizArReUnDEFiNeD#1%
      \expandafter\@firstoftwo
    \else
      \expandafter\@secondoftwo
    \fi
    {#2{#3}}{#2{#4}}%
  }%
}%
\makeatother

\usepackage{hyperref}
%\csname @ifpackageloaded\endcsname{hyperref}{}{\usepackage{nameref}}

\checkAndDef{\nameref}{\newcommand*\refn[1]}{\detokenize{[#1]}}{\nameref{#1}}%

\begin{document}
\section{Test-Section}\label{test}%
Some text. Reference: \refn{test}
\end{document}