On 8/17/2019 10:56 AM, Henri Menke wrote:

On 17/08/19 8:48 PM, Hans Hagen wrote:

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On 8/17/2019 9:19 AM, Henri Menke wrote:

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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)

I can accept this answer. Just for a little context, the question was triggered by this:

https://tex.stackexchange.com/questions/504501/global-variant-of-csname-endc...

In short: Having thousands of

\expandafter\gdef\csname foo\endcsname{}

inside a group (as happens for xmltex), can lead to a save_stack overflow. One way around it is to do

\begingroup\expandafter\endgroup\expandafter\gdef\csname foo\endcsname{}

Sure, just group. But actually, if one needs that many csnames one can wonder about the approach. One can bump the save stack just like one also might have to bump the hash (extra) size (either of them can overflow). Also, probably a bit of extra grouping can happen at a different level, not for each csname but for in this case an xml element, which is also more efficient

The \expandafter inside the group will pull the evaluation of \csname into the group which will discard the save_stack at the \endgroup, thus avoiding the build-up. However, this construction is a bit hard to understand so I was wondering whether

well, instead of this: \begingroup\expandafter\endgroup\expandafter\gdef\csname foo\endcsname{} one can just use this: \begingroup\expandafter\gdef\csname foo\endcsname{}\endgroup which is less tokens, less pushing/poping and therefore a litle faster (but often neglectable compared to other things that tex/macros do in most cases) but of course it looks less 'cool' and 'expert' and creates less 'awe' .. so let's add another one: {\expandafter}\expandafter\gdef\csname foo\endcsname{} this one is performance wise close to the second case (normal grouping) but it might look more puzzling which is why i should wrap it: \def\defcsname {{\expandafter}\expandafter\def \csname} \def\gdefcsname{{\expandafter}\expandafter\gdef\csname} which then is about as efficient as the first alternative with two \expandafter usage using \begingroup\endgroup (okay, efficiency depends of course on the engine too, and probably on the cpu as well) (you can argue that \expandafter and \noexpand and \futurelet ... were added to tex so that one could boost his resume ... the more you use in sequence the more expert you are; but you can also argue that they add some charm to tex, a nice playground and such)

\expandafter\gdef\begincsname foo\endcsname{}

could be used instead to elide the save_stack (which doesn't work because \begincsname does not actually build a \csname).

it does when it's known and then it puts something in the input (a token), but when unknown it doesn't so you effectively get \def{} which is not what you want (ok, maybe some weird usage where { is defined as macro does, which actually can make sense when one handles xml with active characters). Hans ----------------------------------------------------------------- Hans Hagen | PRAGMA ADE Ridderstraat 27 | 8061 GH Hasselt | The Netherlands tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl -----------------------------------------------------------------