Conditional environment using package option

Question

I asked this question, which was solved but didn't help me with the issue I was trying to fix. Seems I fell for the XY problem, so I'll make it more explicit.

What I want: a package that has a variable called solutions and an environment \begin{solution}\end{solution} which when solutions is set will shade the background of the text that is inside it and when not set will remove everything (basically as if I hadn't written anything at all).

What I've tried so far:

\ProvidesPackage{solutions}

\newif\ifsolutions
\solutionsfalse
% If solution is false the environment should blank whatever is inside it
% If true it should shade the background gray
\DeclareOption{solutions}{\solutionstrue}
\ProcessOptions\relax
\newenvironment{solution}{
    \ifsolutions
       #1 $\delta$ 
    \else 
    \fi
}{
    \ifsolutions
       #1 $\delta$  % playing with \delta just to see something
    \else   
    \fi
}

Just using a \delta because I thought it would have been easier to see. But this doesn't work at all since I don't think there is a way to remove whats inside the environment from within it. I need something modifying it from outside.

Answer 1

Based on \NewDocumentEnvironment, \colorbox this is rather straight forward. Please keep in mind that \colorbox doesn't allow page breaks inside your solutions.

\documentclass[]{article}

\begin{filecontents}[overwrite]{solutions.sty}
\RequirePackage{xcolor}

\newif\ifsolutions
\solutionsfalse
% If solution is false the environment should blank whatever is inside it
% If true it should shade the background gray
\DeclareOption{solutions}{\solutionstrue}
\ProcessOptions\relax

\NewDocumentEnvironment{solution}{m +b}
  {%
    \ifsolutions
      \expandafter\@secondoftwo
    \fi
    \@gobble
      {%
        #1 $\delta$
        \par
        \noindent
        \colorbox[gray]{0.9}
          {%
            \begin{minipage}{\dimexpr\linewidth-2\fboxsep\relax}%
              #2%
            \end{minipage}%
          }%
        \par
        #1 $\delta$
      }%
  }
  {}
\end{filecontents}

\usepackage[solutions]{solutions}

\begin{document}
You had a question
\begin{solution}{a}
  This is a solution to your problem.
\end{solution}
\end{document}

On how this works:

The +b argument of a \NewDocumentEnvironment will collect the contents of that environment as an argument, in the definition you can then refer to said contents by #2 (in this case, as it's the second argument).

The \ifsolutions is evaluated, but the contents aren't immediately put inside that \if... conditional, as that can get unstable in TeX. Instead we put an argument after the \if...\fi part, and if the conditional is true execute that argument (\expandafter\@secondoftwo first expands the \fi closing the conditional block, then executes \@secondoftwo that's defined as \providecommand\@secondoftwo[2]{#2}). If the conditional is false everything up to \fi will be removed and \@gobble will gobble up the contents (\@gobble is defined as \providecommand\@gobble[1]{}).

---

A variant that displays another symbol if \ifsolutions is false:

\documentclass[]{article}

\begin{filecontents}[overwrite]{solutions.sty}
\RequirePackage{xcolor}

\newif\ifsolutions
\solutionsfalse
% If solution is false the environment should blank whatever is inside it
% If true it should shade the background gray
\DeclareOption{solutions}{\solutionstrue}
\ProcessOptions\relax

\providecommand\@secondofthree[3]{#2}

\NewDocumentEnvironment{solution}{m +b}
  {%
    \ifsolutions
      \expandafter\@secondofthree
    \fi
    \@secondoftwo
      {%
        #1 $\delta$
        \par
        \noindent
        \colorbox[gray]{0.9}
          {%
            \begin{minipage}{\dimexpr\linewidth-2\fboxsep\relax}%
              #2%
            \end{minipage}%
          }%
        \par
        #1 $\delta$
      }%
      {$\alpha$}% this shows up if \ifsolutions is false
  }
  {}
\end{filecontents}

\usepackage[solutions]{solutions}

\begin{document}
You had a question
\begin{solution}{a}
  This is a solution to your problem.
\end{solution}
\end{document}

Answer 2

This is similar to Skillman's answer in that it uses \NewDocumentEnvironment and \colorbox. Since questions are usually typeset as lists, however, this tries to adapt to an enumerate or itemize environment, if applicable. To do this, the width of the minipage inside the box is adjusted to account for the indentation of the list and paragraph indentation is restored for paragraphs after the first¹.

\begin{filecontents}[overwrite]{solutions.sty}
\ProvidesPackage{solutions}

\newif\ifsolutions
\solutionsfalse
% If solution is false the environment should blank whatever is inside it
% If true it should shade the background gray
\DeclareOption{solutions}{\solutionstrue}
\ProcessOptions\relax
\RequirePackage{calc}
\RequirePackage{xcolor}
\newlength\normalparindent
\AddToHook{begindocument}{\setlength\normalparindent{\parindent}}
\NewDocumentEnvironment {solution} { +b }
{%
  \ifsolutions
    \par
    \colorbox{gray!15}{%
      \begin{minipage}{\linewidth-\@totalleftmargin-2\fboxsep}
        \AddToHookNext{para/after}{%
          \setlength\parindent{\normalparindent}%
        }%
        #1
      \end{minipage}%
    }%
  \fi
}{}
\end{filecontents}
\documentclass{article}
\usepackage[solutions]{solutions}
% \usepackage{solutions}
\begin{document}
\begin{enumerate}
  \item a question
    \begin{solution}
      a solution
    \end{solution}
  \item another question
    \begin{solution}
      a somewhat longer solution which goes into much more detail and requires line-breaking, paragraphs and similar bells, whistles and such-like.

      here's a new paragraph just for the sake of making a paragraph, really, since there's no topical content to this as it is all properly classified as `waffle'.

      \begin{center}
        \begin{tabular}{ll}
          \hline
          one & two \\
          \hline
        \end{tabular}
      \end{center}

      \begin{equation}
        v = ut + \frac{1}{2}at^{2}
      \end{equation}
    \end{solution}
\end{enumerate}
\end{document}

If you need to allow page breaks within solutions, I would look at the tcolorbox package which has a library for enabling breakable boxes.

_{¹Whether this is preferred to boxes which span the text width depends on personal preference and use case, so adjusting the alignment isn't necessarily better.}

Answer 3

I ended up using both answers to put something together that fits what I want to

\ProvidesPackage{solutions}

\newif\ifsolutions
\solutionsfalse
% If solution is false the environment should blank whatever is inside it
% If true it should shade the background gray
\DeclareOption{solutions}{\solutionstrue}
\DeclareOption{nosolutions}{\solutionsfalse}
\ProcessOptions\relax
\RequirePackage{xcolor}
\NewDocumentEnvironment{solution} { +b }
{%
  \ifsolutions
    \par
    \noindent
    \colorbox{gray!15}
      {%
        \begin{minipage}{\dimexpr\linewidth-2\fboxsep\relax}%
          #1%
        \end{minipage}%
      }%%
  \fi
}{}