Empty lines in hidden part

Question

I have defined the following environment to insert a solution in my text:

\newif\ifsolution
\def\solution#1{\ifsolution {\color{blue}\textbf{Oplossing:} #1} \else \relax\fi} 

If I add \solutiontrue to the file, it adds the solutions in blue, but if I put \solutionfalse, it omits them altogether. So far, so good.

The problem I have though is that in the solution itself, I can't seem to be able to insert blank lines, which I really need. For instance, if I put

\solution{This is the first line of the solution.

This is the second line. QED}

I receive an error. I understand that instead I have to write

\solution{This is the first line of the solution.
This is the second line. QED}

Now given that the blank lines are unmissable, is there a way to avoid the errors?

Answer 1

I'm not quite sure if this is really what you wanted. But here is a way to allow blank lines in your macro input and omit the solution while keeping it's space as a wildcard in the output:

\documentclass{article}
\usepackage{xcolor}

\newif\ifsolution
\long\def\solution#1{%
  \setbox0\vbox{#1}
  \vbox to \ht0 {
    \ifsolution {\color{blue}\textbf{Oplossing:} #1} \else \relax\fi}}
\solutiontrue
%\solutionfalse

\setlength\parindent{0em}

\begin{document}
(before solution)

\solution{%
  This is the first line of the solution.

  This is the second line. QED
}

(after solution)
\end{document}

with \solutiontrue

with \solutionfalse

---

The reason for the blank lines not working in macro input by default lies in the TeX fundamentals. By design Knuth did not allow that behaviour of macro parameters. Only by adding the \long primitive you can overcome this (what some people would call a limitation). In this question you find a good discussion of this topic.

Answer 2

I intended your question in a different way, what about this solution with \makecell?

\documentclass{article}
\usepackage{color}
\usepackage{makecell}
\newif\ifsolution
\def\solution#1{\ifsolution {\color{blue}\textbf{Oplossing:} #1} \else \relax\fi}
\solutiontrue  
\begin{document}
 \solution{\makecell[l]{This is the first line of the solution.\vspace{3ex}\\   
    This is the second line. QED}}
\end{document}