Toggling Phantom In Macros (plain TeX)

Question

I am using plain TeX / pdfTeX.

Is there a way to build a set of macros, say \showOn / \showOff, such that

1. I can nest them

2. for a given piece of text, it "looks up the parse tree", find the first \showOn/\showOff parent, and gets displayed if and only if it's on?

For example:

Cat \showOff { dog \showOn {red \showOff {blue} green } superman} wonderwoman

Would result in:

Cat [spacing for dog] red [spacing for blue] green [spacing for superman] wonderwoman

So, I know that I can do the spacing without display part through using phantom - however, is there any way to turn phantom on/off in a nested manner?

Right now, I have:

\def\showOn#1{{#1}}
\def\showOff#2{\phantom{#2}}
\showOff {\showOn {foo}}
\bye

However, this clearly fails the nesting since \showOff {\showOn {foo}} will NOT output foo.

Edit:

My fault for not including this earlier. Color changing does not work. The document changes colors all over the place (in pdfTeX).

Answer 1

An answer which does not rely on colors and allows verbatim text.

\catcode 64 11
\newif\if@inSO@
\def\ShowOff{\afterassignment\ShowOff@\setbox\z@\hbox}
\def\ShowOff@{\@inSO@true\aftergroup\ShowOff@@}
\def\ShowOff@@{\hskip\wd\z@}
\def\ShowOn{\if@inSO@\expandafter\ShowOn@\fi}
\def\ShowOn@#{\egroup\bgroup\aftergroup\ShowOn@@\let\next= }
\def\ShowOn@@{\ShowOff\bgroup}
\catcode 64 12

abcdefghijklmnop
coffin

abc\ShowOff{defg\ShowOn{hi\ShowOff{jk}lm}n}op
\ShowOn{co\ShowOff{f\ShowOn{f}i}n}

\end

How does \phantom work? It stores the text in a box, then creates an empty box with the same width, height and depth. In a similar way, I'll collect the text that should be hidden in box 0, and produce a horizontal space of the same width as that box. All text that is to be displayed is typeset directly.

Let us analyse for example \ShowOff{abc} First, it becomes

\afterassignment\ShowOff@ \setbox\z@\hbox{abc}

TeX remembers \ShowOff@ for use after the next assignment, namely \setbox. The placement of the "after assignment" token in the case of \setbox is a little bit tricky; in our case it ends up after the opening brace following \hbox. Ignore \@inSO@true for the moment. \aftergroup\ShowOff@@ will place \ShowOff@@ as soon as the box that is being build ends. This macro, in turn, inserts a horizontal skip of the size of the box.

Now, \ShowOn cancels this hiding by ending the box of \ShowOff early, namely with the \egroup in \ShowOn@. The horizontal skip is then inserted there. We can then typeset the argument of \ShowOn, and resume hiding afterwards. I could have defined \def\ShowOn@#1{\egroup#1\ShowOff\bgroup}, but this grabs the argument early, forbidding verbatim. Instead, I used \aftergroup again: the argument of \ShowOn comes as a group, {...}. Somehow, we want to insert \aftergroup\ShowOn@@ after the opening brace. Open a brace (\bgroup), and remove the one that the user put (\let\next=).

I still have to explain the @inSO@ switch. If \ShowOn appears outside any \ShowOff command, it should just do nothing to its argument. Otherwise, it acts as described in the previous paragraph.

EDIT: I tried to add support for math mode below (didn't incorporate it above because the macros become ugly).

\catcode`@=11
\def\ShowOff
  {%
    \v@true\h@true
    \ifmmode
      \expandafter\mathpalette\expandafter\ShowOff@math
    \else
      \afterassignment\ShowOff@\setbox\z@\hbox
    \fi
  }
\def\ShowOff@{\let\ShowOn\ShowOn@\aftergroup\finph@nt}
\long\def\ShowOff@math#1{%
  \setbox\z@\hbox\bgroup$\m@th #1%
  \bgroup
    \aftergroup\ShowOff@math@
    \long\def\ShowOn##1{\egroup##1\ShowOff@math#1\bgroup}%
    \let\next= }
\def\ShowOff@math@{$\egroup\finph@nt}
\let\ShowOn\relax
\def\ShowOn@#{\egroup\bgroup\aftergroup\ShowOn@@\let\next= }
\def\ShowOn@@{\ShowOff\bgroup}
\catcode`@=12

% Tests.
$abcdefg^h_{ijkl}mnop$ $co^{ffi}n$

$abc\ShowOff{defg\ShowOn{^h_{i\ShowOff{jk}l}m}n}op$
$\ShowOn{co^{\ShowOff{f\ShowOn{f}i}}n}$

abcdefghijklmnop coffin

abc\ShowOff{defg\ShowOn{hi\ShowOff{jk}lm}n}op
\ShowOn{co\ShowOff{f\ShowOn{f}i}n}

\bye

Answer 2

\input color
\def\showOn#1{\textcolor{black}{#1}}
\def\showOff#1{\textcolor{white}{#1}} 
Cat \showOff { dog \showOn {red \showOff {blue} green } superman} wonderwoman
\bye

Update

Here is the solution that does not assume the text is black. It assumes, however, that the background is white. Also, any explicit color change inside \showOff makes the text visible.

\input color
\catcode`\@=11
\newif\ifinside@showoff
\def\showOff#1{\textcolor{white}{\inside@showofftrue#1}}
\def\showOn#1{\ifinside@showoff
  \bgroup\inside@showofffalse\reset@color#1\egroup\color{white}%
  \else#1\fi}
\color{green} Cat \showOff { dog \showOn {\textcolor{red}{red}
\showOff {blue} green } superman} wonderwoman 
\bye