Line break in a modified proof environment (with show/hide feature)

Question

In the recent post How to define a proof environment which can be displayed/hidden? [combining two TeX solutions], Werner helped me to redefine a proof environment which can be hidden by an optional boolean argument (showProof). Without an argument, somehow the line/paragraph breaking is not correct. How can one fix this?

\documentclass{scrartcl}

\usepackage{ifthen}
\usepackage{environ}
\usepackage{amsthm}

\newboolean{showProof}% for showing proofs
\setboolean{showProof}{true}% setting the switch
\newif\ifstarttheorem

% proof environment (with hide feature)
\makeatletter
\providecommand{\env@proof@save@env}{}
\providecommand{\env@proof@process}{}
\RenewEnviron{proof}[2][showProof]{
  \gdef\@tmp{#1}%
  \ifthenelse{\boolean{#1}}{\par\pushQED{\qed}%
    \normalfont\topsep2\p@\@plus2\p@\relax
    \trivlist\item[\hskip\labelsep\sffamily\bfseries Proof~#2]\gdef\mycurrenvir{proof}\global\starttheoremtrue\mbox{}\hfill\\*\ignorespaces
    \BODY}{\ignorespaces}
}[\ifthenelse{\boolean{\@tmp}}{%
  \gdef\mycurrenvir{\relax}
  \popQED\endtrivlist\@endpefalse
}{\ignorespacesafterend}]
\makeatother

\begin{document}
Just some text
\begin{proof}
  Just some text in a standard proof. As we can see, there is a problem.
\end{proof}
\begin{proof}[false]
  Proof with optional argument set to \texttt{false}. This proof correctly does
  not appear.
\end{proof}
\begin{proof}[showProof]{(Heading)}
  Proof with optional argument (\texttt{true}) and second argument ``(Heading)''
\end{proof}
\end{document}

Answer 1

I have a different proposal; instead of complicating your life, just redefine proof in such a way that \begin{proof}* doesn't print the proof. Instead of * you can choose whatever (single) character you prefer. After all, it's just a switch, so be it.

\documentclass{scrartcl}

\usepackage{xparse}
\usepackage{environ}
\usepackage{amsthm}

%% save the original macros
\let\amsthmproof\proof
\let\amsthmendproof\endproof

%% redefine proof
\RenewDocumentEnvironment{proof}{s}
 {\IfBooleanTF{#1}
   {\ignoreproof}
   {\amsthmproof}}
 {\IfBooleanTF{#1}
   {\endignoreproof}
   {\amsthmendproof}}

%% In case you want all proofs to appear, comment the above and
%% uncomment the following; a * following \begin{proof} will be ignored
%\RenewDocumentEnvironment{proof}{s}
% {\amsthmproof}
% {\amsthmendproof}

\NewEnviron{ignoreproof}{}

\begin{document}

Just some text

\begin{proof}
  Just some text in a standard proof.
\end{proof}

\begin{proof}*
  Proof with the asterisk appended. This proof correctly does not appear.
\end{proof}

\begin{proof}*[Secondary proof]
  Proof with the asterisk appended. This proof correctly does not appear.
\end{proof}

\begin{proof}[Proof of the main theorem]
  This is the proof of the main theorem.
\end{proof}

\end{document}

If you want a different character, say -, the first line should be

\RenewDocumentEnvironment{proof}{t-}

and the rest would be exactly the same.

---

A variant code for ignoring all proofs, starred or not; just uncomment the marked line if you want no proof at all.

\documentclass{scrartcl}

\usepackage{xparse}
\usepackage{environ}
\usepackage{amsthm}

%% save the original macros
\let\amsthmproof\proof
\let\amsthmendproof\endproof

%% a conditional for ignoring all proofs
\newif\ifignoreallproof
%\ignoreallprooftrue % <----- uncomment for ignoring all proofs

%% redefine proof
\ifignoreallproof
  \RenewDocumentEnvironment{proof}{s}
   {\ignoreproof}
   {\endignoreproof}
\else
  \RenewDocumentEnvironment{proof}{s}
   {\IfBooleanTF{#1}
     {\ignoreproof}
     {\amsthmproof}}
   {\IfBooleanTF{#1}
     {\endignoreproof}
     {\amsthmendproof}}
\fi

\NewEnviron{ignoreproof}{}

\begin{document}

Just some text

\begin{proof}
  Just some text in a standard proof.
\end{proof}

\begin{proof}*
  Proof with the asterisk appended. This proof correctly does not appear.
\end{proof}

\begin{proof}*[Secondary proof]
  Proof with the asterisk appended. This proof correctly does not appear.
\end{proof}

\begin{proof}[Proof of the main theorem]
  This is the proof of the main theorem.
\end{proof}

\end{document}

Answer 2

To reiterate from my comment, the problem with your MWE is the fact that your environment, as written, has one optional argument, and one mandatory argument. Your first use case fails to provide the mandatory argument (the label). Thus, the environment grabs the next thing, which is the first letter of your proof, and calls it the label. If your first use case is made as \begin{proof}{}, it gives the expected result.

Since you reply to Werner that "Yes, it's indeed like two optional arguments," I have taken that under advisement. I believe I have made it so that yourproof environment now takes two optional and zero mandatory arguments. Optional arguments are enclosed in square [] brackets, not {} braces.

However, note that, with two optional arguments, you can't specify a second optional argument without also specifying the first optional argument. Thus,

\begin{proof}[(Heading)]

is not a valid invocation. This, in the long run, could be more of a pain than what you started with.

\documentclass{scrartcl}

\usepackage{ifthen}
\usepackage{environ}
\usepackage{amsthm}

\newboolean{showProof}% for showing proofs
\setboolean{showProof}{true}% setting the switch
\newif\ifstarttheorem

% proof environment (with hide feature)
\makeatletter
\providecommand{\env@proof@save@env}{}
\providecommand{\env@proof@process}{}
\renewenvironment{proof}[1][showProof]{%
  \def\proofarg{#1}\proofhelper}{\endproofhelper}
\NewEnviron{proofhelper}[1][]{
  \gdef\@tmp{\proofarg}%
  \ifthenelse{\boolean{\proofarg}}{\par\pushQED{\qed}%
    \normalfont\topsep2\p@\@plus2\p@\relax
    \trivlist\item[\hskip\labelsep\sffamily\bfseries Proof~#1]\gdef\mycurrenvir{proof}\global\starttheoremtrue\mbox{}\hfill\\*\ignorespaces
    \BODY}{\ignorespaces}
}[\ifthenelse{\boolean{\@tmp}}{%
  \gdef\mycurrenvir{\relax}
  \popQED\endtrivlist\@endpefalse
}{\ignorespacesafterend}]
\makeatother

\begin{document}
Just some text
\begin{proof}
  Just some text in a standard proof. As we can see, there is a problem.
\end{proof}
\begin{proof}[false]
  Proof with optional argument set to \texttt{false}. This proof correctly does
  not appear.
\end{proof}
\begin{proof}[showProof][(Heading)]
  Proof with optional argument (\texttt{true}) and second argument ``(Heading)''
\end{proof}
\end{document}