How to create hide-able environments

Question

I am relatively new to writing LaTeX packages. I would like to create an environment so that anything contained in that environment can be ignored when producing output. The desired behavior would be the following:

\documentclass{article}
\usepackage[hide]{MyPackage}
\begin{document}
This should always show up.
\begin{Hidden}
This should show up unless the [hide] option is used.
\end{Hidden}
\end{document}

One possible use case: if I write a problem set for a class, I can write the solutions into the same document wrapped in the Hidden environment. That way, I can compile with the [hide] option to produce the problem set for my students, and I can recompile without the [hide] option to produce the solution set.

Here's the code I currently use to accomplish this. It was at some point borrowed from a package offering similar functionality, but unfortunately I can't remember the original package.

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{MyPackage}
\RequirePackage{ifthen}
\newboolean{MyPackage@hidden}
\setboolean{MyPackage@hidden}{false}
\DeclareOption{hide}{\setboolean{MyPackage@hidden}{true}}
\ProcessOptions\relax
\newenvironment{Hidden}
    {\ifthenelse{\boolean{MyPackage@hidden}}
    {\setbox\z@\vbox\bgroup}{}}
    {\ifthenelse{\boolean{MyPackage@hidden}}{\egroup}{}}

This code has been working somewhat, but there are three main issues. The first is that there are errors when I put figures inside the Hidden environment. (This is a pretty common need when writing solutions to homework problems.) The following code does not compile:

\documentclass{amsart}

\usepackage{amsmath}
\usepackage{tikz}
\usepackage[hidden]{hideexample}

\begin{document}

This always shows up.

\begin{Hidden}
\begin{figure}[h]
\begin{tikzpicture}

\draw (0,0) circle (2cm);

\end{tikzpicture}
\caption{This shows up when not hidden, but causes an error when the [hide] option is used.}
\end{figure}
\end{Hidden}

\end{document}

The second issue is that I would like to implement multiple hide-able environments. (Expanding on the homework use case, I might also want to create notes to the grader about how to assign partial credit. So I would want to be able to independently hide/show the solutions and the grader comments.) I have no idea how to adapt the code above to do this. I never really understood what the code does, just that it (partially) worked.

The third "issue" isn't really a problem with the functionality, but another feature. It would be really convenient if, instead of [hide]/[show] options, there was a [solutions] option (or something similar) so that compiling the document would produce both the homework set and the solution set at the same time. (So, I would be working on HW.tex, and compiling would produce HW.pdf and HW_solutions.pdf at the same time.) Is something like this possible?

Thanks in advance.

---

Follow-up: After modifying Werner's answer below, I'm using the code

\if@hidden
  \long\gdef\hide[#1]#2\ehide{\vspace{#1}}%
\else
  \long\gdef\hide[#1]#2\ehide{#2}%
\fi

This allows the following use:

\hide[4in]
%stuff
\ehide

This shows %stuff when I'm not using the global [hide] option. When I am using the [hide] option, %stuff is hidden, and 4 inches of blank vertical space are shown instead (as a placeholder for %stuff). The only issue is that this usage makes it look like [4in] is an optional argument. Since \def requires an exact calling match, the argument is actually required. How can I make this argument optional (i.e., with 0in as the default value)?

Answer 1

It seems like you are trying to hide fairly common document elements. For that I would suggest capturing the environment contents using environ.

\documentclass{article}

\usepackage{filecontents}

\begin{filecontents*}{MyPackage.sty}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{MyPackage}
\RequirePackage{environ}
\newif\if@hidden% \@hiddenfalse
\DeclareOption{hide}{\global\@hiddentrue}
\ProcessOptions\relax
\NewEnviron{Hidden}
  {\if@hidden\else\BODY\fi}
\end{filecontents*}

\usepackage[hide]{MyPackage}

\begin{document}

This should always show up.
\begin{Hidden}
This should show up unless the [hide] option is used.
\end{Hidden}

\end{document}

I've used basic TeX conditionals rather than xifthen, but that's up to you. See Why is the ifthen package obsolete?.

---

If you wish to hide content using an abbreviated form, you'll have to specify it as such in a verbatim way. That is, if you want \hide and \ehide to define your start- and end-clause for capturing content to hide/not, then you have to use them as-is. Here's an example that includes an optional argument to \hide so you can specify a vertical gap to be left under the [hide] option:

\documentclass{article}

\usepackage{filecontents}

\begin{filecontents*}{MyPackage.sty}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{MyPackage}
\newif\if@hidden% \@hiddenfalse
\DeclareOption{hide}{\global\@hiddentrue}
\ProcessOptions\relax
\gdef\hide{\@ifnextchar[\hide@{\hide@[]}}
\if@hidden
  \long\gdef\hide@[#1]#2\ehide{\ifx\relax#1\relax\else\par\rule{0pt}{#1}\fi}% Gobble content between \hide...\ehide
\else
  \long\gdef\hide@[#1]#2\ehide{#2}% Release content between \hide...\ehide (ignore [.]
\fi
\end{filecontents*}

\usepackage[hide]{MyPackage}

\begin{document}

This should always show up.
\hide[2in]
This should show up unless the [hide] option is used.
\ehide

This should also show up.
\hide
This should show up unless the [hide] option is used.
\ehide

This will show up.
\end{document}

Note that nesting in these cases won't work.