Not display the content of one environment when its content contains one special macro

Question

I need to build one environment for proofs in my lessons.

\begin{proof}
    ...
\end{proof}

This environment could have one content containing one macro named \lines like in the following example. \lines{n} draws n horizontal rules on which the pupils can write during the lesson.

\begin{proof}
    bla, bla,...
    \lines{4}
    bla, bla,...
    \lines{7}
    bla, bla,...
\end{proof}

I would like to display,or not, proof environments that contains one or more macros \lines so as to have two versions of my lessons : one for the pupils and one showing only texts that I've typed.

Is it possible ?

More precisely, I want to not display the entire proof environment if the \lines macro is used.

Answer 1

You can use the environ package along with \newtoggle from the etoolbox package. The code below yields:

The environ package is used here so as to collect the body of the environment in the \BODY macro.

Further Enhancements:

• As mentioned in the comments, since the material in the environments containing the \lines macro is typeset (even though it may get discarded), any macros that have global side effects are still executed. These need to be disabled at the start of the \sbox so that they do not have any unintended side effects. Below I have disabled the \label and \stepcounter macros, but there may be many others that need to be disabled, depending on the application. The only macro that should not be disabled really is of course the \lines macro.

Notes:

• The \lines macro does not yet draw the lines. The appropriate code needs to be added.
• The solution here used the \newtoggle from the etoolbox package, as I find that syntax more readable. This can be adapted to the numerous other options for evaluating conditionals such as those mentioned in LaTeX conditional expression.

Code:

\documentclass{article}
\usepackage{amsthm}
\usepackage{environ}

\usepackage{etoolbox}
\newtoggle{LinesMacroUsed}%

\newcommand*{\lines}[1]{\global\toggletrue{LinesMacroUsed}}%

\newbox{\MyBody}
\NewEnviron{MyProof}{%
    \global\togglefalse{LinesMacroUsed}%
    \sbox{\MyBody}{%
        % Disable any commands here so that there are not any unwanted side effects
            \renewcommand{\ref}[1]{}%
            \renewcommand{\stepcounter}[1]{}%
        % Typeset the body onto a box to see if the \lines macro was called
            \vbox{\BODY}%
    }%
    \iftoggle{LinesMacroUsed}{%
    }{%
        \begin{proof}%
            \BODY% Using the \savebox here has issues for case of multiple lines
        \end{proof}%
    }%
}{}%

\begin{document}
\begin{MyProof}
    bla, bla,...
    \lines{4}
    bla, bla,...
    \lines{7}
    bla, bla,...
\end{MyProof}
%
\begin{MyProof}
    bla, bla, (without using lines macro)

    bla, bla, (without using lines macro)

    bla, bla, (without using lines macro)
\end{MyProof}
%
\begin{MyProof}
    bla, bla,...
    \lines{4}
    bla, bla,...
    \lines{7}
    bla, bla,...
\end{MyProof}
%
\end{document}

Answer 2

Using environ to grab the environment's body looks like the obvious thing to do. Then I suggest scanning the body, checking for a \lines token. This can be done easily using a macro with delimited argument if we're sure the \lines token will never be inside braces. Otherwise, using ted is a safe option: do a dummy substitution to count the number of occurences of \lines in the environment's body.

\documentclass[a4paper]{article}
\usepackage{amsthm}
\usepackage{environ, ted}

\makeatletter
\NewEnviron{Proof}{%
  \Substitute*{\BODY}{\lines}{}%
  \ifnum\ted@count=\z@
    \begin{proof}
      \BODY
    \end{proof}
  \fi
}
\makeatother

\newcommand*\lines[1]{(#1)}

\begin{document}

\begin{Proof}
  bli
  \begin{equation}
    2 + 2 = 4
  \end{equation}
  {\large \lines{1}}
  bli
\end{Proof}

\begin{Proof}
  bla
  \begin{equation}
    2 + 2 = 4
  \end{equation}
  bla
\end{Proof}

\arabic{equation}

\end{document}