Re: another error message in LMTX

[Pablo Rodriguez <oinos@gmx.es>]

On 11/7/20 1:29 PM, Hans Hagen wrote:
> [...]

Many thanks for your in-depth explanation, Hans.

This sample triggers the error:

    \appendtoks
       \unexpanded\def\doverbatimspace{\char32\hskip\zeropoint}%
    \to \everyenableelements

    \setuptagging[state=start]

    \starttext
    \starttyping
    mtxrunjit --generate
    \stoptyping

    \startitemize[n]
    \startitem a\stopitem
    \stopitemize
    \stoptext

I have no idea why.

Many thanks for your help,

Pablo


> well, the clue is there: you have
>
> you have a \hskip or \vskip someplace that is not properly terminated by
> \relax so tex keeps scanning for optional plus|minus keywords and in
> your case runs into "mtxrunjit"
>
> in luametatex we have a slightly different keyword scanner
>
> - in traditional tex it will just push back the 'non valid' keyword and
> continue
>
> - but that actually obscures a potentential error when the \hskip is
> burried deep: maybe you are typesetting "minus" as is and then it *is* a
> valid keyword and probably an error wil be that no valid dimension is seen
>
> - so, you'rve just been lucky so far
>
> so, this new keyword scanner is more picky in the sense that when it
> notices a potential keywors start it will complain earlier which might
> actually catch potential errors ealier (in this case all follow up words
> that start with a 'p' or 'm'
>
> another change in scanning is that in many cases order no longer
> matters; for instance in scanning skips it's a 'plus' spec followed by a
> 'minus' spec but in rules it can be mized 'width', 'height', 'depths'
>   in any order and repetitive
>
> in luametatex (unless there is good reason) the order doesn't matter,
> and often repetition is ok (so, multiple plus and minus in a skip and in
> any order where the last one counts)
>
> this also permits for instance an order like:
>
>      \dontleavehmode
>      \hbox to 20pt xoffset  3cm{1\hss}
>      \hbox to 20pt             {\hss2\hss}
>      \hbox to 20pt xoffset -3cm{\hss3}
>
> where previously the 'to' spec always came last and multiple 'to' are
> okay (offsets etc are a lmtx feature, and it means many more keywords
> have to be scanned, which was one reason for cooking up a better
> mechanism, even if right from the start that was already rather optimized).
>
> now, in principle this brings the possibility to support partial
> keywords, so 'xo' 'xoff' etc (in fact i only need to change a few lines
> of code) which can be handy when for instance one passes many
> attributes) but first i want the main machinery to be stable
>
> actually, this was already there for a while; the good news is that all
> this doesn't harm performance (we actually gain some here, not that it's
> normally noticeable)
>
> so: more flexibility, better performance, earlier interception but of
> course the later might demand users to fix code (i actually had to fix
> context in one spot for this lookahead issue).
>
> (I'll add \relaxes to the after handling but that just obscures the issue.)


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