% (This file is public domain.)
% Demonstrate how Eplain can be used to include a TeX source file
% verbatim, typesetting comments in colored italic typewriter type.

% Load Eplain and LaTeX's color.sty package.
\input eplain
\beginpackages \usepackage{color} \endpackages
\nopagenumbers % Disable page numbers.
\font\commentfont = cmitt10 % Font for the comments (italic \tt).
% We'll define some `protected' macros with `@' in their names.
\makeatletter
% Define an equivalent of Eplain's \letreturn, to be able to assign
% various actions to the (active) percent character.
\begingroup \makeactive\%
  \gdef\letpercent{\let%}
\endgroup
% The listing hook to be called in \setuplistinghook, see below.  It
% makes `%' active and assigns it a `start comment' action.
\def\hlightcommentslisting{\makeactive\% \letpercent\start@comment}%
% This is what `%' is aliased to before a comment is started.
\def\start@comment{%
  \leavevmode % Needed in the very first line of the input to process
              % the new par (possibly inserting line number) before we
              % kick in with the color and stuff.
  \begingroup % Isolate color and font changes and the redefinitions.
    \commentfont
    \color[cmyk]{0.28,1,1,0.35}%
    \percentchar            % Produce the actual `%' and
    \letpercent\percentchar % make all following `%'s do the same.
    \letreturn\end@comment  % Call \end@comment at end-of-line.
}
% \end@comment (alias for ^^M inside a comment) will end the comment
% started by \start@comment.  We make ^^M active temporarily so that
% the active version of ^^M gets "frozen" into \end@comment.
\begingroup \makeactive\^^M % Avoid ^^M's from here on.
  \gdef\end@comment{\endgroup ^^M}%
\endgroup
\resetatcatcode % Make `@' again inaccessible for use in macro names.

% Define \setuplistinghook to setup comments highlighting, line
% numbering and omitting the last (empty) line of input.
\def\setuplistinghook{\hlightcommentslisting \linenumberedlisting
                      \nolastlinelisting}
% Isn't this fun?  This file typesets itself, with the extra bonus of
% the pretty-printed comments and numbered source lines!
\listing{lscommnt}
\bye