[tex-k] Bug-report for the TeXbook: Not all non-primitive control-sequences are defined, ultimately, in terms of the primitive ones.

[Heiko Oberdiek]

Hello,

On 2022-12-13 19:39, Doug McKenna wrote:
> Just to add some complexity/nuance to the discussion ...
> 
> Is the empty control sequence
> 
>      \csname\endsname
> 
> constructed from two TeX primitives a TeX primitive or not?

It is an undefined control sequence with the meaning of \relax
due to a side effect of the primitive \csname that undefined
commands are getting defined with the meaning of \relax.

\expandafter\show\csname\endcsname

Result:
 > \csname\endcsname=\relax.

e-TeX has \ifcsname that checks, if a command sequence exists
*without* defining it in the undefined case:

\ifcsname\endcsname
   \message{1=exists}
\else
   \message{1=undefined}
\fi
\csname\endcsname  % the "empty control sequence" gets defined
\ifcsname\endcsname
   \message{2=exists}
\else
   \message{2=undefined}
\fi
\end

Result:
   1=undefined 2=exists

> The following typesets "AB" (using plain.tex and pdftex in TL 2021) without error:
> 
> \def\test{A\csname\endcsname B}
> \test
> \end

Same effect as
   \def\test{A\relax B}
   \test
   \end

Yours sincerely
   Heiko