Using key option to package that uses string cases

Question

What's wrong with the following package and source file? I get an error at line 14, after [Other], of mypackage.sty, and a "Missing \begin{document}" error resulting.

The source:

\documentclass{article}

\usepackage[what=this]{mypackage}

\usepackage{blindtext}

\begin{document}
\blindtext
\end{document}

The package mypackage.sty:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mypackage}

\RequirePackage{kvoptions}
\RequirePackage{xstring}

\DeclareStringOption{what}
\ProcessKeyvalOptions*

\IfStrEqCase{what}{%
    {this}{\dothis}%
    {that}{\dothat}%
    }%
    [Other]%

\newcommand{\dothis}{The magic word is ``this''}
\newcommand{\dothat}{The magic word is ``that''}

\endinput

I strongly suspect there's some simple syntactical error that I can't find. Or else there's something basic that I'm not understanding about either kvoptions or xstring

I think my syntax for \IfStrEqCase is OK, because the docs from texdoc xstring give the form:

\IfStrEqCase⟨[*]⟩{⟨string⟩}{% 
    {⟨string1⟩}{⟨code1⟩}% 
    {⟨string2 ⟩}{⟨code2 ⟩}%
    etc...
    {⟨stringN ⟩}{⟨codeN ⟩}}[⟨other cases code⟩]

My actual purpose

My actual purpose of kvoptions and xstring is for a complex document where:

• In one custom package read in as part of the preamble, I set a value for a key for one of several possible document text/math font choices. And in that same package, depending on that value, give the requisite commands to use that text/math font combination. (E.g., with cm as the font choice, which will be a default, the requisite command will simply be \usepackage{amsfonts}; with lucida as the font choice, instead, the requisite commands will be \usepackage[lucidasmallscale]{lucidabr} and \linespread{1.04}.)
• A second custom package read in as part of the preamble gives a large set of math command definitions, some of which depend on the math font being used. So this second package will take that key value as option, use it, inside a \IfStrEqCase expression, to alter the commands as needed. For example, for the cm choice, I specify \newcommand{\nsubset}{\not\subset} and \newcommand{\tendsto}{\rightarrow}; whereas for the lucida choice, since \nsubset is already defined, I don't need to define it, but I use tikz to create, as the definition for \tendsto, an arrow that better harmonizes with arrows appearing in tikz-cd diagrams.

The points of this seemingly complicated setup are:

1. To allow the font choice to be made by editing a single line in the first package, rather than having to comment and uncomment large blocks of text in source files.
2. To allow my second package, with all its math definitions, to be used not just in this particular document, but in any document that may need my specialized math definitions.

Answer 1

The first problem, which causes the error is that you are executing the \IfStrEqCase statement inside the package code to produce typeset output, however you can't do that before \begin{document}. I wrapped the test in a definition \mytest which can be used inside the document.

The other problem is that you are comparing what with this or that, which will never match and will go to the other branch. When you set up a key-value option <name> with kvoptions, the value passed to the option <name> will be stored in \<prefix>@<name>. The <prefix> can be changed, but the default is the package name, so mypackage, and <name> is what here, so instead of testing what you should test \mypackage@what. Changing that, the code prints The magic word is “this”.

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents}{mypackage.sty}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mypackage}
\RequirePackage{kvoptions}
\RequirePackage{xstring}
\DeclareStringOption{what}
\ProcessKeyvalOptions*
\def\mytest{%
  \IfStrEqCase{\mypackage@what}{%
      {this}{\dothis}%
      {that}{\dothat}%
      }%
      [Other]%
  }
\newcommand{\dothis}{The magic word is ``this''}
\newcommand{\dothat}{The magic word is ``that''}
\endinput
\end{filecontents}
\usepackage[what=this]{mypackage}
\usepackage{blindtext}
\begin{document}
\mytest

\blindtext
\end{document}

For this case in particular you could drop xstring and test if a macro \do<what> exists. If it does, use it, otherwise take the appropriate action. This definition for \mytest would produce the same output as the one with xstring:

\def\mytest{%
  \@ifundefined{do\mypackage@what}%
    {Other}%
    {\@nameuse{do\mypackage@what}}%
  }