How to test if a macro is being expanded between \csname ... \endcsname, in (e-)upTeX?

Question

pdfTeX added some new primitives, based on e-TeX, including \ifincsname. When this primitive is being expanded between a pair of \csname ... \endcsname, the result of expansion will be true, otherwise false will be the result.

As the successor of pdfTeX, LuaTeX conserves \ifincsname as its primitive; in terms of XeTeX, it implements the \ifincsname primitive, too. However, in (e-)upTeX (and also ApTeX), there is not a primitive named or equalling to pdfTeX's \ifincsname.

Hence, here is the question: how to implement a macro to define if a macro is being expanded between \csname ...\endcsname, in (e-)upTeX.

'You can't': note that \ifincsname is a tricky thing at best - nesting can be a real issue. — Joseph Wright, Dec 05 '17 at 09:43
BTW, I don't know ApTeX: could you include (or comment) a link? — Joseph Wright, Dec 05 '17 at 09:43
@JosephWright ApTeX is short for Asiatic pTeX. The former name of it is pTeX-ng, which you might have idea about. — Ch'en Meng, Dec 05 '17 at 09:45
@JosephWright Thanks for the final answer, though it kills the question. : ( — Ch'en Meng, Dec 05 '17 at 09:46
Successor? I don't regard LuaLaTeX etc. as a successor to PdfTeX etc — , Dec 05 '17 at 10:13
@ChristianHupfer I get the successor statement from http://www.tex.ac.uk/FAQ-whatpdftex.html and http://www.tex.ac.uk/FAQ-luatex.html, as pdfTeX is frozen and future development is focused on LuaTeX. — Ch'en Meng, Dec 13 '17 at 02:54
Somewhat related, tex core - Are we inside an edef? - TeX - LaTeX Stack Exchange — user202729, Jul 06 '22 at 05:35

score 2 · Accepted Answer · edited Jul 06 '22 at 05:30

Short answer: you can't.

There is no way from TeX90/e-TeX primitives to tell that expansion is happening inside a \csname. One might imagine setting some flag, and replacing the 'real' \csname by a macro, but that would break expandability.

It's worth noting that \ifincsname is tricky to get right: there are various awkward edge case situations (for example nesting in \ifcsname) that make it a hard primitive to get right.

How to test if a macro is being expanded between \csname ... \endcsname, in (e-)upTeX?

1 Answers1