[NTG-context] The odd semantics of \begincsname

Hans Hagen j.hagen at xs4all.nl
Sat Aug 17 10:48:18 CEST 2019

On 8/17/2019 9:19 AM, Henri Menke wrote:
> Dear list,
>
> According to the LuaTeX documentation:
>
>      “The \begincsname primitive is like \csname but doesn’t create a
>      relaxed equivalent when there is no such name.”
>
> I thought it would be possible to use this fact to skip the \relax-ed
> definition when \def-ining a new control sequence, but the following MWE
> fails with \inaccessible:
>
>      \expandafter\gdef\csname yes\endcsname{}
>      \expandafter\gdef\begincsname no\endcsname{}
>      \bye
>
> Is this a bug or is this behaviour intended?  Could this be fixed by
> making manufacture_csname aware whether it is in a def_cmd context or
> not?
[sorry to those who are not interested in these low level issues, just skip]

intended ... it expands to basically nothing so you get no token
representing a 'name' after the gdef .. the expansion is pushed in from
of whatever comes next (which could be another \expandafter for instance)

you suggest that if \begincsname could behave differently when it's
after a \def, \gdef, (and then quite some more definition related
commands), it could behave differently but it not an option

for instance (as mentioned) there can be more than one expansion going
on after these define commands, like expanding a macro that itself
expands to \csname so one has several \expandafters before the gdef
then); there is actually no looking back in scanning tokens unless a
token has been scanned already and looking forward would involve
expansion so a circular mess

an option could be not to push something on the save stack (a side
effect of creating the csname, which has a little impact on performance
and nesting) but removing that bit might give other side effects (e.g.
for successive reassignments inside a group, maybe even mixed local and
global); i did a quick test with that and it gives quite incompatible
output in ConTeXt so that's definitely a no-go (adding all kind fo
saveguards and checks in the engine doesn't pay off, especially not for
something that never was a problem)

some time ago i considered a convenience command \[e]defcsname, as it
saves a few tokens (no gain in performance as all the related things
still need to happen); but even that one would probably create the name
in the same way

so ... this is the way it is ... (i must admit that it never gave me any
issues so whatever triggered the question, there's probbaly a way around
it)

Hans

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