On 8/17/2019 10:56 AM, Henri Menke wrote:
On 17/08/19 8:48 PM, Hans Hagen wrote:
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)
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-endcsname
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
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