Declare package options and loading packages leads to "options section" error

Question

I'm trying to create a package with some options. However, whenever I declare options and load other packages, I receive the following error:

LaTeX Error: \RequirePackage or \LoadClass in Options Section.

What is going on here and/or how do I declare package options and load packages?

Here is a minimal example replicating the problem:

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{mystyle.sty}
\newcommand{\mycmd}{new}% Define some command
\DeclareOption{change}{\renewcommand{\mycmd}{renew}}% Package option `change` updates \mycmd
\RequirePackage{amsmath}% Load amsmath
\end{filecontents*}

\usepackage{mystyle}
\begin{document}
\mycmd% This should be "new"
\end{document}

Answer 1

At the moment you use \DeclareOption, your style file is divided into three parts:

1. Pre-options Section (above first \DeclareOption);

2. Options Section (between first \DeclareOption and \ProcessOptions); and

3. Post-options Section (after \ProcessOptions).

Whatever is contained within the pre-options section has already been processed. So, the only problem arises with content beyond this point.

Since the possibility exists that you may want to conditionally load a package (or class) based on some option, and the fact that once a package (or class) is loaded you can't "undo" the action, \RequirePackage (also \usepackage) is made a invoke an error if used within the options section. Here's an example of how one may decide to load a package conditionally:

\newif\if@loadhyperref %\@loadhyperreffalse
\DeclareOption{hyperref}{\@loadhyperreftrue}
\if@loadhyperref
  \RequirePackage{hyperref}
\fi

You'll have to \ProcessOptions before \RequirePackage (in the above case, outside the if-condition), for the above code to be successful. In general, issue \ProcessOptions in order to put yourself in the post-options section before you can load any more packages (or classes).

In the specific example, the following change to mystyle.sty should suffice:

\newcommand{\mycmd}{new}
\DeclareOption{change}{\renewcommand{\mycmd}{renew}}
\ProcessOptions% Process package options
\RequirePackage{amsmath}% Load amsmath