Writing expanded macro with space to file

Question

I'm trying to write an expanded macro to a file without the expansion eating the space after it.

I could replace the {} empty group with an explicit \space, but I want this to be fairly robust. Is this possible to do?

I'm guessing since TeX never gets to the layout portion of the process, it doesn't know what to do with the unexpandable {} tokens, which is why they show up during the \write?

Example

% tex test.tex
\def \test {Hello}
\openout 0 hello.txt
\write 0 {\test{} World}
\end

Contents of hello.txt

Hello{} World

Expected

Hello World

Background

In broad strokes, this project is trying to retrofit a sort of note/cross-reference system to a large document with many macros and edited by many people.

...
\def\proj{Foobar}
\def\rev{1.0}
\def\revstr{\proj{} v\rev}
\def\complicated{...} % essentially a bunch of ifs and concats
... % A bunch of macros scattered throughout
Body of document contains macros like \proj{} and \revstr. 
The definitions of these macros are tweaked and modified.
... % A bunch of text with \notes scattered throughout,
    % also tweaked/modified, added/removed
\note{Used for callouts to \proj{} and whatever else}
...

The notes are currently typeset with the document, and the task is to also put them into a file for processing by an external program. For the vast majority of cases, callouts just have text and macros: no formatting or anything that couldn't be represented in ASCII.

Manually going through all the macros to replace {} with \space is somewhat out of the question, due to the volume and going against the established workflow.

The fallback is just post-process the file once TeX is done with it. Seems like the ham-fisted approach though, since TeX has all the information necessary to boil down the text.

Maybe there is a way to pretend layout a page, but then strip the content out of the boxes and put it into a file?

Answer 1

Using {} in a write will produce {} that is the expected effect.

You could use \space It's not clear why you expect the normal TeX tokenization not to apply here since you are expecting \test to be tokenized and expanded as normal. If that was not the case, you could use

\write20{\noexpand\test zzz}

which would write \test zzz

But if you want \test to expand then

\write20{\test\space zzz}

is the simplest solution, although there areothers which can be useful sometimes, such as

\def\zz#1{\write20{\test#1zzz}}\zz{ }

which puts a space token, rather than \space which expands to a space token into the argument of \write.

You could also (and might be what you want here) treat space tokens verbatim not as normal TeX spaces so

{\catcode`\ =12\relax% space is a normal punctuation character
\write20{\test zzz}%
}% back to normal