Choice keys and exception handling

Question

Recently, I switched a self-written "bundle package" from using xkeyval to keyreader. One reason for this, beside the backward compatibility, was that the definition of keys is more convenient in keyreader, especially for choicekeys. One thing that is missing tough is the variant \define@choicekey+, where faulty user input can be caught by a self-defined macro.
My first, and rather obvious, question is if there is a solution for this in keyreader.

However, the more I thought about the problem, the more I wished there was an exception mechanism in LaTeX. As far as I understand it (which admittedly is not so much), in pgfkeys (and maybe ltxkeys?) there exists something like that when you define handlers for keys in the /error/ tree.

So apart from shoehorning code to fit pgfkeys paradigm, do approaches to exceptions in LaTeX exist?

Answer 1

The plus form of choice key has the syntax

\define@choicekey+[<pref>]{<fam>}{<key>}[<bin>]{<nominations>}[<default>]{<f1>}{<f2>}

When the user input is found in <nominations>, <f1> will be executed, otherwise <f2> will be called. The command \krddefinekeys of keyreader package hardwires <f2> to \krd@keyvalerr because in most cases the user would usually enter an error message in <f2>. pgfkeys will call /errors/unknown choice value={<key>}{<value>} in this case, which is equivalent to \krd@keyvalerr.

<f1> and <f2> are called "functions" in the xkeyval package, and "callbacks" in the keyreader package.

When you specify the key type choice, the command \krddefinekeys automatically and always uses the star (*) and plus (+) variant of \define@choicekey internally. So the choice key align in

\krddefinekeys*[KV]{ebox}[mp@]{% 
  choice/align/center/ 
    center.do=\let\mp@alignright\hfil\let\mp@alignleft\hfil, 
    right.do=\let\mp@alignright\hfill\let\mp@alignleft\relax, 
    left.do=\let\mp@alignright\relax\let\mp@alignleft\hfill, 
    justified.do=\let\mp@alignright\relax\let\mp@alignleft\relax
} 

is equivalent to the following code. This is what is executed internally in defining the key align.

\define@choicekey*+[KV]{ebox}[mp@]{align}[\krduserinput\krdorder]{%
  center,right,left,justified
}[{center}]{%
  \krd@executealt{#1}{<f1>}%
}{%
  \krd@keyvalerr
}

The macro \krd@keyvalerr is defined as

\protected\def\krd@keyvalerr{%
  \krd@getinnoval
  \@latex@error{Erroneous value '\krd@ival' \MessageBreak for key or 
    option '\XKV@tkey'}{Invalid value for key '\XKV@tkey'.}%
}

where \krd@ival is the "innocent value" of the key. It is computed by \krd@getinnoval and is normally of length 20 "innocent" characters at most. \krd@keyvalerr is perhaps the "exception handler" that you refer to. If you like, you can redefine it, but be careful.

The example file of keyreader package:

\documentclass{standalone}
\usepackage{keyreader}
\usepackage{xcolor}
\makeatletter
\newdimen\shadowsize
\krddefinekeys*[KV]{ebox}[mp@]{%
  bool/frame/true;
  bool/shadow/true;
  cmd/framecolor/black;
  cmd/shadecolor/white;
  cmd/shadowcolor/gray;
  cmd/framesize/.4pt;
  cmd/boxsize/.1\columnwidth;
  cmd/shadowsize/1pt;
  choice/align/center/
    center.do=\let\mp@alignright\hfil\let\mp@alignleft\hfil,
    right.do=\let\mp@alignright\hfill\let\mp@alignleft\relax,
    left.do=\let\mp@alignright\relax\let\mp@alignleft\hfill,
    justified.do=\let\mp@alignright\relax\let\mp@alignleft\relax
  ;
}
\savevaluekeys[KV]{ebox}{frame,framecolor,framesize}
\krdpresetkeys[KV]{ebox}{%
  frame,framecolor=black,shadecolor=white,framesize=0.5pt,boxsize,align
}
\krdpostsetkeys[KV]{ebox}{%
  shadow=\usevalue{frame},shadowcolor=\usevalue{framecolor}!40,
  shadowsize=\usevalue{framesize}*4
}
\newcommand*\ebox[2][]{%
  \krdsetkeys[KV]{ebox}{#1}%
  \begingroup
  \ifmp@frame
    \fboxrule=\dimexpr\mp@framesize\relax
  \else
    \fboxrule=0pt
  \fi
  \ifmp@shadow
    \shadowsize=\dimexpr\mp@shadowsize\relax
  \else
    \shadowsize=0pt
  \fi
  \setbox0=\hbox{%
    \fcolorbox{\mp@framecolor}{\mp@shadecolor}{%
      \hbox to\mp@boxsize{%
        \mp@alignright #2\mp@alignleft
      }%
    }%
  }%
  \hskip\shadowsize
  \color{\mp@shadowcolor}%
  \rule[-\dp0]{\wd0}{\the\dimexpr\ht0+\dp0\relax}%
  \llap{\raisebox{\shadowsize}{\box0\hskip\shadowsize}}%
  \endgroup
}
\makeatother

\begin{document}
\ebox[shadecolor=brown]{ebox1}
\ebox[framecolor=magenta,boxsize=2cm,align=right]{ebox2}
\ebox[shadow=false,framesize=1pt,framecolor=red,boxsize=1.5cm,align=left]{ebox3}
\ebox[framesize=1pt,framecolor=green,shadowcolor=blue]{ebox4}
\ebox[frame=false,shadow,shadowcolor=yellow,framesize=.5pt]{ebox5}
\end{document}

I am submitting version 0.5 of keyreader to CTAN. The syntax for \krddefinekeys is extended to meet your need:

\krddefinekeys[<pref>]{<fam>}[<mp>]{
  ord/<key>/<dft>/<f1>;
  cmd/<key>/<dft>/<f1>;
  bool/<key>/<dft>/<f1>;
  bool+/<key>/<dft>/<f1>/<f2>;
  choice/<key>/<dft>/<nominations>/<f1>;
  choice+/<key>/<dft>/<nominations>/<f1>/<f2>;
}

Here <f2> is, hopefully, the "handler" you wanted. This is now in the package but you won't find it in the documentation yet.

Example:

\krddefinekeys*[KV]{fam}[mp@]{%
  ord/keya/default-a/\def\x##1{#1*##1};
  bool/keyb/true/;
  bool+/keyc/true/\def\valueaccepted{#1}/\@latexerr{Invalid value '#1' for keyc}\@ehd;
  cmd/keyd/black;
  choice/keye/blue/green,blue,red,cyan/\def\emptycallback{};
  choice+/keyf/center/
    center.do=\let\mp@alignright\hfil\let\mp@alignleft\hfil,
    right.do=\let\mp@alignright\hfill\let\mp@alignleft\relax,
    left.do=\let\mp@alignright\relax\let\mp@alignleft\hfill,
    justified.do=\let\mp@alignright\relax\let\mp@alignleft\relax
    //
    \let\mp@alignright\hfil\let\mp@alignleft\hfil
    \@@warning{Invalid value for keyf; 'center' assumed}
  ;
}