4

Is there a way to create a qr code in LaTeX with koma variables as input?

am using the package qrcode and everything works fine and the package is easy to use. However, when I am trying to use a koma variable as the input for the text LaTeX crashes.

This works:

\documentclass{scrlttr2}
\usepackage[final]{qrcode}

\setkomavar{fromemail}{me@it.is}


\newcommand{\texttoconvert}{mailto:me@it.is}
\begin{document}
    \begin{letter}{}
        \qrcode[height=1.5cm]{\texttoconvert}
    \end{letter}
\end{document}

But this does not:

\documentclass{scrlttr2}
\usepackage[final]{qrcode}

\setkomavar{fromemail}{me@it.is}


\newcommand{\texttoconvert}{mailto:\usekomavar{fromemail}}
\begin{document}
    \begin{letter}{}
        \qrcode[height=1.5cm]{\texttoconvert}
    \end{letter}
\end{document}

Why is that? It also does not work to use koma variables directly in qrcode...

muzimuzhi Z
  • 26,474
mrCarnivore
  • 1,505

3 Answers3

5

Unfortunately \usekomavar is not fully expandable, see its doc in Koma-Script manual (2022-10-12), sec. 4.5. Variables.

But \usekomavar accepts a "styling" command as its optional argument, which will act on the value of variable. That is, after \setkomavar{<name>}{<content>},

\usekomavar[\mycmd]{<name>}
% is equivalent to
\mycmd{<content>}

Apply this trick to OP's use case:

\documentclass{scrlttr2}
\usepackage[final]{qrcode}

\setkomavar{fromemail}{me@it.is}


\newcommand{\QRmailto}[2][]{\qrcode[#1]{mailto:#2}}


\begin{document}
    \begin{letter}{}
      \usekomavar[{\QRmailto[height=1.5cm]}]{fromemail}
    \end{letter}
\end{document}

enter image description here

Since now the text to be encoded in qr code is tokenized before passed to \qrcode, special characters (#$&^_~%␣\{}) may need to be escaped (\#\$\&\^\_\~\%\␣\\\{\}), see qrcode package doc (2015/01/08 v1.51), sec. 2.3 Special characters.

\documentclass{scrlttr2}
\usepackage[final]{qrcode}

\setkomavar{fromemail}{me@it.is}


%    \myQrcode[<opts>]{<pre>}{<post>}{<text>}
% => \qrcode  [<opts>]{<pre><text><post>}
\newcommand{\myQrcode}[4][]{\qrcode[#1]{#2#4#3}}


\begin{document}
    \begin{letter}{}
      \usekomavar
        [{\myQrcode[height=1.5cm]
                   {mailto:}
                   {?subject=ABC/123&body=Hello\ and\ welcome}}]
        {fromemail}
      % <QR code requested for "mailto:me@it.is?subject=ABC/123&body=Hello and welcome" in version 0-M.>
    \end{letter}
\end{document}
muzimuzhi Z
  • 26,474
  • Thank you. This does work and is very simple. However, it does no longer work when I want to add subject and text as in \newcommand{\QRmailto}[2][]{\qrcode[#1]{mailto:#2?subject=ABC/123&body=Hello and welcome,}}. Then the spaces in the body are omitted. Why? – mrCarnivore Mar 12 '23 at 14:01
  • 1
    @mrCarnivore According to qrcode doc, using \ does the trick. – muzimuzhi Z Mar 12 '23 at 14:59
  • Thank you for your answer and the help you have given me! escaping the spaces does work. I was confused since it worked when using plain text in qrcode without needing to escape the spaces. – mrCarnivore Mar 12 '23 at 15:18
5

The \usekomavar command is not expandable. You can define an expandable version (that won't check whether the variable is actually defined, beware).

\documentclass{scrlttr2}
\usepackage[final]{qrcode}

\setkomavar{fromemail}{me@it.is}


\newcommand{\texttoconvert}{mailto:\expkomavar{fromemail}}


\makeatletter
\NewExpandableDocumentCommand{\expkomavar}{sO{@firstofone}m}{%
  \IfBooleanTF{#1}{% \expkomavar*
    \expkomavar@name{#2}{#3}%
  }{% \expkomavar
    \expkomavar@var{#2}{#3}%
  }%
}
\newcommand{\expkomavar@name}[2]{#1{@nameuse{scr@#2@name}}}
\newcommand{\expkomavar@var}[2]{#1{@nameuse{scr@#2@var}}}
\makeatother


\begin{document}


\begin{letter}{}


\qrcode[height=1.5cm]{\texttoconvert}


\bigskip


\qrcode[height=1.5cm]{mailto:me@it.is}


\end{letter}


\end{document}

enter image description here

The two QR-codes are the same, as witnessed by the log file where we find

<Reading QR code for "mailto:me@it.is" at level 2-Q from aux file.>

for the second instance, meaning that the package recognizes it's the same as a previously built one.

egreg
  • 1,121,712
1

I have talked to Markus Kohm (author of koma-script) and he told me that the intended solution with koma-script is to use the ability of \usekomavar to expand the koma-variable into the first parameter of the optional variable \usekomavar[<command>]{<komavariable>}.

Like this:

\newcommand*{\qrcodewithprefix}[2][mailto:]{\qrcode{#1#2}}
\usekomavar[\qrcodewithprefix]{fromemail}

or even shorter:

\usekomavar[\def\fromemail]{fromemail}
\qrcode{mailto:\fromemail}

His main reason for this are that this solution (compared to the more low level solutions) enables helpful error messages.

If you try a typo in the koma-variable and write \usekomavar[\def\fromemail]{frommail} instead you get the following error message:

Class scrlttr2 Error: KOMA-Script variable not defined. \usekomavar[\def\fromemail]{frommail}

This does not happen if you instead used:

\def\fromemail{example-\csname scr@fromemail@var\endcsname}

When you introduce a typo then you only get a very confusing:

Undefined control sequence. \qrcode{mailto:\fromemail}

To sum it up the easy and clean solution is to define \fromemail and with \usekomavar expand the contents of the koma-variable fromemail into it and from then on use \fromemail instead of the koma-variable when needing the expanded variant:

\documentclass{scrlttr2}

\setkomavar{fromemail}{john@doe.com}
\usepackage[final]{qrcode}
\usekomavar[\def\fromemail]{fromemail}
\begin{document}
    \qrcode{mailto:\fromemail}
\end{document}
mrCarnivore
  • 1,505