What's the benefit of defining a macro that expands to itself?

Question

While was looking at the source code for pgffor, I noticed the macro \pgffor@stop is defined as

\def\pgffor@stop{\pgffor@stop}

I think the macro is used to stop argument parsing. If that's the case, what is the benefit of defining it recursively? Won't that cause an infinite loop if it is ever (accidentally) expanded? Why isn't it defined as {} or maybe even \relax?

Yes, it would cause an infinite loop if it is expanded but then something went wrong which it shouldn't have. \pgffor@stop is only used to detect something which usually will also gobble it up and not actually expand it. You can also use a macro that is never defined but that also \ifx-equals against other undefined macros (whch shouldn't be around but could be). In addition to what Joseph wrote, it also is almost impossible for a user to accidently trigger it. — Qrrbrbirlbel, Oct 28 '22 at 12:22
With \def\pgffor@stop{} this will break very easily: \foreach \x in {1,2,,4}{``\x'' }. — Qrrbrbirlbel, Oct 28 '22 at 12:28
Near duplicate of programming - What are expl3 quarks? When, where, why and how should I use them? - TeX - LaTeX Stack Exchange except for the name — user202729, Oct 28 '22 at 13:10
In argument parsing, \def\foo#1\bar{#1} will pull text until it reaches \bar so you want to make sure \bar is never actually used inside #1. If \bar is a quartk, it can't be used as text without crashing, only as a delimiter. — John Kormylo, Oct 28 '22 at 14:33

Joseph Wright · Accepted Answer · 2022-10-28T12:17:29.273

The macro is \ifx equal to itself even if stored inside another macro

\def\some@store{\pgffor@stop}
\ifx\some@store\pgffor@stop
 % TRUE
\fi

This makes it useful for fast tests that are used with macros-as-variables.

Note that if you need to be sure of a unique ID, you can't use \relax or {}, at least if you are testing using \ifx. (With a modern TeX system you can do an expandable string test, so you can check the name of the token.)

\def\pgffor@stop@bad{}
\def\foo#1{\ifx#1\pgffor@stop@bad ...}
\foo\@empty % Oops

What's the benefit of defining a macro that expands to itself?

1 Answers1