Adding \comment or \begin{comment} to \AtBeginDocument?

Question

For Partial compilation commands/environment?, I would like to add \begin{comment} or \comment at the start of the document (so I could turn it off elsewhere in the code).

So, even if in the larger scope of things it is useless - in this MWE, as an exercise, I'd like the whole text to be blanked, basically, conditionally through the command line (the result would be a blank page, or even no pdf).

EDIT: However, I'd want it done in such a way, so that I could arbitrarily add \end{comment} / \begin{comment} later on it the document, and have only that snippet compile.

\documentclass[11pt]{book}
\usepackage{lipsum}

\ifx\doskip\relax
  \typeout{DOSKIP}
  \usepackage{etoolbox}
  \usepackage{comment}
  % https://tex.stackexchange.com/questions/14135/how-to-automatically-add-text-immediately-after-begindocument
  %\AtBeginDocument{\comment}         % ! Extra \endgroup.
  %\AtBeginDocument{\begin{comment}}  % Runaway argument? ! File ended while scanning use of \next.
  %\AfterEndPreamble{\comment}        % ! Extra \endgroup.
  %\AfterEndPreamble{\begin{comment}} % Runaway argument? ! File ended while scanning use of \next.
  %\AtEndDocument{\endcomment}%{\end{comment}}
  % desperate try (also fails w/ ! Extra \endgroup.):
  \protected\def\startcomment{\expandafter\comment}
  \AtEndPreamble{
\startcomment
  } % ! Extra \endgroup.
\fi

\begin{document}

\chapter{Some chapter}
\section{Section One}
\lipsum[1-3]

% later I might want to use here:
% \end{comment}
% \lipsum[5] % this would be typeset
% \begin{comment} % from this point on, again blanked

\end{document}

The idea is that the document would be "blanked" if Latex is called on the command line via
pdflatex "\let\doskip\relax\input{test.tex}"
... however, clearly there are some grouping errors; otherwise without the intervention, just using pdflatex test.tex, works fine.

Any ideas how this could be implemented?

---

EDIT2: closer, but not yet there: I've seen in kpsewhich comment.sty "Other 'comment' environments are defined by ... \excludecoment{versionb} ; These environments are used as \begin{versiona} ... \end{versiona} with the opening and closing commands again on a line of their own. This is not a LaTeX environment: for an included comment, the \begin and \end lines act as if they don't exist. In particular, they don't imply grouping ..."

So in the below example, which uses \excludecomment{versionb}, the begin{document} is succesfully overridden, the first part is skipped, then comment is interrupted, \lipsum[5] is typeset, and document compiles without error - but beyond that, it is very difficult to properly close either non-interrupted (fully blanked doc) or continue with blanking after once it's been interrupted:

\documentclass[11pt]{book}
\usepackage{lipsum}
\usepackage{trace}

\ifx\doskip\relax
  \typeout{DOSKIP}
  \usepackage{etoolbox}
  \usepackage{comment}
  \excludecomment{versionb}
  %\AtEndPreamble{ % here causes ! LaTeX Error: Missing \begin{document}.
  \AtBeginDocument{%
    % \expandafter\begin{versionb}\relax
    \begin{versionb}
  } % 
  %\AtEndDocument{\end{versionb}} % ! LaTeX Error: \begin{document} ended by \end{versionb}.
\fi

% \traceon
\begin{document}

\chapter{Some chapter}
\section{Section One}
\lipsum[1-3]

\end{versionb}
\lipsum[5]
% \begin{versionb}

%% \end{versionb} % can this be implicit, as per \AtEndDocument?
\end{document}
%%ENNDDD

A \typeout can be added, to this macro in comment.sty:

 \gdef\ProcessCommentLine#1^^M{\def\test{#1}
      \typeout{test: \meaning\test, \meaning\expandafter \csname End\CurrentComment Test\endcsname}

... so if there isn't a \end{versionb} before the \lipsum[5], we can see that the problem occurs because lines are attempted to be read beyond end of file:

...
test: macro:->\end{document}, \expandafter\end{versionb}
test: macro:->%%ENNDDD, \expandafter\end{versionb}
test: macro:->, \expandafter\end{versionb}
)
Runaway argument?
! File ended while scanning use of \next.
<inserted text> 
                \par 
<*> \let\doskip\relax\input{test.tex}

So, I'm still in need of help on how to make this code work for all circumstances...

Answer 1

You can use the following code somewhere in the preamble after \documentclass.

\def\doskipA{\begin{document}\end{document}}
\ifx\doskip\relax \expandafter\doskipA\fi

Answer 2

Ok, I finally got somewhere, though maybe not ideal. The below MWE behaves like this:

• If compiled with pdflatex test.tex, the comment stuff is ignored, and the document compiles fully; but there will be these two warnings generated in the log (don't know how to fix that):

  (\end occurred inside a group at level 2)
  
  ### semi simple group (level 2) entered at line 86 (\begingroup)
  ### semi simple group (level 1) entered at line 86 (\begingroup)
  ### bottom level...
  

• If compiled with pdflatex "\let\doskip\relax\input{test.tex}"; then there is an \excludecomment defined, called SKIPLINES, and \begin{document} starts it automatically; the code compiles without warnings, and only the segment between \end{SKIPLINES} and \begin{SKIPLINES} (note the order) which here is \lipsum[5] will be typeset. Also messages like these will be output to terminal:

  ...
  test: macro:->, macro:->\end{SKIPLINES}
  noend macro:-> macro:->\end{document}
  test: macro:->\end{document}, macro:->\end{SKIPLINES}
  yesend
  

I guess, this requires \excludecomment so that additional (?) grouping is avoided. However, there are still some hacks that are needed, and so the code includes modified versions of {comment}'s \excludecomment and \ProcessCommentLine. In particular, as @UlrikeFischer noted, if it happens that Latex is in "comment mode" when reading \end{document}, it will be skipped, resulting with errors; so the redefined \ProcessCommentLine additionally checks for \end{document} as a token for stopping comment mode, and then re-executes the proper \end{document} if that condition is encountered.

This means, one should be able to, in \doskip mode, to add \end{SKIPLINES} and \begin{SKIPLINES} at will, to define parts of the compilable regions, without needing to worry whether they are closed properly for the \end{document}: a single pair of \end{SKIPLINES} and \begin{SKIPLINES} should work, just as well as a doc with none of them (or with an additional \end{SKIPLINES}).

Obviously, this is not a truly proper solution (this one does not work with \inputted files), so I hope a more erudite answer will eventually emerge; but for now, here is the MWE (and sorry for the mess):

\documentclass[11pt]{book}
\usepackage{lipsum}
\usepackage{trace}

\newif\ifSKIPLINESAct % if active
\ifx\doskip\relax
  \typeout{DOSKIP}
  \usepackage{etoolbox}
  \usepackage{comment}
  \usepackage{xstring}
  \def\excludecommentB
   #1{\message{Excluding comment '#1'}%
      %\csarg\newif{if#1Act}% declare globally; http://tex.stackexchange.com/questions/228994/handling-a-conditionally-defined-newif-nested-in-an-ifx-conditional
      \csarg\def{#1}{\endgroup \message{Excluding '#1' comment.}%
          \csname #1Acttrue\endcsname
          \begingroup
             \DefaultCutFileName \def\ProcessCutFile{}%
             \def\ThisComment####1{}\ProcessComment{#1}}%
      \csarg\def{After#1Comment}{%
        \global\csname #1Actfalse\endcsname% must globalize - inside a group here!
        \CloseAndInputCutFile \endgroup}
      \CommentEndDef{#1}}
  % redef this too:
  \csarg\xdef{EndDocTest}{\string\end\string{document\string}}
%   \showme\EndDocTest
  {\catcode`\^^M=12 \endlinechar=-1 %
   \gdef\xComment#1^^M{\ProcessCommentLine}
   \gdef\ProcessCommentLine#1^^M{\def\test{#1}
        \typeout{test: \meaning\test, \expandafter\meaning \csname End\CurrentComment Test\endcsname}
        \csarg\ifx{End\CurrentComment Test}\test
            \edef\next{\endgroup\noexpand\EndOfComment{\CurrentComment}}%
        \else \ThisComment{#1}\let\next\ProcessCommentLine
        \fi%
        % also add for end document; use \IfStrEq else catcodes could be a problem
        \IfStrEq*{\EndDocTest}{\test}{%
            %\traceon%
            \typeout{yesend}%
            \def\next{%\endgroup\noexpand\EndOfComment{\CurrentComment}%
            \endgroup\noexpand\EndOfComment{\noexpand\CurrentComment}% % must add \noexpand to \CurrentComment, else it leaks and gets typeset in the doc!
            %\SKIPLINESActfalse% no; use:
            \AfterSKIPLINESComment% % both sets false and ends group
            \enddocument%
            }%
        }{\typeout{noend \meaning\test\space\meaning\EndDocTest}%\ThisComment{#1}\let\next\ProcessCommentLine% don't execute here, just typeout
        }%
        \next}
  }
  \excludecommentB{SKIPLINES} % define, also \ifSKIPLINESAct
  %\AtEndPreamble{ % here causes ! LaTeX Error: Missing \begin{document}.
  \AtBeginDocument{%
    % \expandafter\begin{SKIPLINES}\relax
    \begin{SKIPLINES}
  } % ! Extra \endgroup.
  %\AtEndDocument{\end{SKIPLINES}} % ! LaTeX Error: \begin{document} ended by \end{SKIPLINES}. % redefine instead?
  %% no need for this:
  % \global\let\oldenddocument\enddocument
  % \gdef\enddocument{% redefine
  %   %\ifSKIPLINESAct\expandafter\end\expandafter{SKIPLINES}\fi
  %   \oldenddocument%
  % }
\else
  % still need this, when calling without \doskip:
  \makeatletter
  \newenvironment{SKIPLINES}{%
  \begingroup\def\@currenvir{document}%
  }{%
  % comment.sty: % sabotage LaTeX's environment testing \begingroup\def\@currenvir{#1}\end{#1}
  \begingroup\def\@currenvir{SKIPLINES}%\endgroup
  }
  % \def\endSKIPLINES{\begingroup\def\@currenvir{SKIPLINES}} % nope
  \makeatother
\fi

% \traceon
\begin{document}

\chapter{Some chapter}
\section{Section One}
\lipsum[1-3]

% \traceon
\end{SKIPLINES}
% \traceoff
\lipsum[5]
\begin{SKIPLINES} % will cause ### semi simple group (level 2) entered at line 90 if without \doskip and unclosed, but doc will compile

\lipsum[10]

% \end{SKIPLINES} % if this is uncommented when \doskip, it will allow the next typeout to execute (else they will be gobbled)

% these print to stdout only if \end{SKIPLINES} occured last previously:
\typeout{\meaning\endSKIPLINES}
\ifx\doskip\relax
  \ifSKIPLINESAct\typeout{SKIPLINES Active}\else\typeout{no SKIPLINES}\fi%
\else
\fi

\end{document}
%%ENNDDD