Verbatim write in Plain TeX

Question

I am trying to write verbatim text into a file in Plain TeX. In LaTeX I could use the verbatim package. My goal is using Asymptote in Plain TeX. So I need something that works like \verbatim@start from verbatim.sty.

MWE:

% Plain LuaTeX
\catcode`@=11
% from latex.ltx
\long\def\@ifnextchar#1#2#3{%
    \let\reserved@d=#1%
    \def\reserved@a{#2}%
    \def\reserved@b{#3}%
    \futurelet\@let@token\@ifnch}
\def\@ifnch{%
    \ifx\@let@token\@sptoken
    \let\reserved@c\@xifnch
    \else
    \ifx\@let@token\reserved@d
    \let\reserved@c\reserved@a
    \else
    \let\reserved@c\reserved@b
    \fi
    \fi
    \reserved@c}
\def\:{\let\@sptoken= } \:  % this makes \@sptoken a space token
\def\:{\@xifnch} \expandafter\def\: {\futurelet\@let@token\@ifnch}

\def\@makeother#1{\catcode`#1=12\relax}

Let's try to write.

\newwrite\file
\immediate\openout\file=temp.tmp\relax

\def\verbatimwrite{\begingroup\catcode`\^^M=12\relax
    %\let\do\@makeother\dospecials%%%%%%% PROBLEM! (A)
    \@verbatimwrite}

\begingroup
\catcode`\^^M=12 %
\gdef\@verbatimwrite#1^^M{% Every line must be written separately to save the line breaks.
    \immediate\write\file{\unexpanded{#1}}%
    \@ifnextchar\endverbatimwrite\endgroup\@verbatimwrite}% detect the end of verbatim (B)
\endgroup

\let\endverbatimwrite\relax

\verbatimwrite
123#;^{t}
äöüß

4567\undefined
\endverbatimwrite

\immediate\closeout\file

\bye

There are 2 problems (also as comments in the code):

• (A): Where do I have to change all the other catcodes? It should be possible for example to write unbalanced braces. If this is done correctly I think the \unexpanded command is not needed any more.
• (B): Do you have an idea how to improve the detection of \endverbatimwrite? I think my method is really poor.

Thanks.

Answer 1

You were changing the catcodes at the right moment. The problem was that after \let\do\@makeother \dospecials the guard \endverbatimwrite is not a control sequence anymore, but the backslash character plus a sequence of 16 letters. This made your code fail because your \@ifnextchar test looks for the control sequence \endverbatimwrite, which is never found.

As for detecting \endverbatimwrite, you don't need to search for it after every line (you can, but you don't have to). The biggest problem is, again, due to the catcode changes: you don't have \endverbatimwrite anymore, you have \ e n d v e r b a t i m w r i t e, which is completely different (and much harder to find with \@ifnextchar; you'd need a loop of some sort, more or less like listings detects the end of the environment). It's much easier to just define a delimited macro that looks for the string \endverbatimwrite with the correct catcodes.

Usually the algorithm for a verbatim environment (the simple ones, of course) is: 1) Grab arguments (optional arguments, file name to write or, in this case, the write stream); 2) \begingroup; 3) \catcodes; 4) Grab the contents of the command/environment; and 5) \endgroup and process the grabbed characters (the order of these last two may vary).

Here's a modified version of your code (based some of it form here):

\catcode`@=11

\def\@makeother#1{\catcode`#112\relax}
\def\verbatimwrite#1{%
  \begingroup
    \catcode`\^^M=13
    \newlinechar`\^^M
    \let\do\@makeother \dospecials
    \verbwrite@cleanup#1}
\begingroup
  \lccode`\?=`\\
  \lowercase{\endgroup% \
\def\verbwrite@other#1#2?endverbatimwrite{%
  \immediate\write#1{\unexpanded\expandafter
    {\verbwrite@strip@newline#2\relax}}%
  \endgroup}}
\begingroup
  \catcode`\^^M=13
  \gdef\verbwrite@cleanup#1#2^^M{%
    \if\relax\detokenize{#2}\relax
    \else \errmessage{Wrong input `#2' discarded}%
    \fi
    \verbwrite@other#1}
  \gdef\verbwrite@strip@newline#1^^M\relax{#1}
\endgroup

Let's try to write.

\newwrite\file
\immediate\openout\file=temp.tmp\relax

\verbatimwrite\file
123#;^{t}
äöüß}

4567\undefined{
\endverbatimwrite

\immediate\closeout\file

\bye

The output file contains:

123#;^{t}
äöüß}

4567\undefined{