Re: Commands with arguments before in ConTeXt

["Jairo A. del Rio" <jairoadelrio6@gmail.com>]

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I'll take a look at it. Thank you a lot, Taco.

Regards,

Jairo :)

El lun., 29 de jun. de 2020 a la(s) 05:36, Taco Hoekwater (
taco@elvenkind.com) escribió:

> Hi,
>
> Oversimplifying:
>
> TeX (and derivatives like luatex) read the input file as a list of
> operators with optional suffix arguments.
>
> In normal (text) mode, there is no expression state maintained except
> inside the handling of such optional arguments.
>
> It follows that, if there is a top-level text input like
>
>   a\bold{a}
>
> TeX treats this as (assuming ’simple’ macro definitions):
>
>   command a     (no arguments)
>   command \bold (a macro with one one braced argument)
>
> And by the time “\bold" is seen, the  “a” command has already been
> processed completely.
>
>
> In math mode, there *is* an expression state maintained, and that is why
> \over and \atop work.
>
>
> For further information and actual details, you can just look at the
> luatex source (it is in C):
>
>
> https://github.com/TeX-Live/luatex/blob/trunk/source/texk/web2c/luatexdir/tex/maincontrol.c
>
> The function
>
>   void main_control(void)
>
> is the one that does all the heavy lifting (using a dispatch table).
>
> Taco
>
> > On 29 Jun 2020, at 12:15, Jairo A. del Rio <jairoadelrio6@gmail.com>
> wrote:
> >
> > Hans, after your explanation I'm actually curious now about details, but
> my knowledge is too limited now (maybe reading source codes would be
> better? Worse?). I do a bit of C, but I don't know about Pascal at all and
> I'm not sure where to start from in order to understand TeX better. Well,
> at least I know why that feature isn't supported. I'll be thankful for any
> references. Thank you very much.
> >
> > Jairo :)
> >
> > El lun., 29 de jun. de 2020 a la(s) 02:28, Hans Hagen (j.hagen@xs4all.nl)
> escribió:
> > On 6/28/2020 10:48 PM, Jairo A. del Rio wrote:
> > > I've read the following is not possible in TeX
> > >
> > > \def#1\macro{blabla#1}
> > >
> > > where arguments come before. The only partial exceptions are commands
> > > like \atop or \over, which are in fact primitives. Is there a way to
> do
> > > this in ConTeXt?
> > >
> > > Could it be a feature request for LuaMetaTeX? I've seen Hans
> > > experimenting a lot with new primitives and new possibilities for
> > > arguments, like #0 and co., so I ask in case it's not too nonsensical
> to
> > > propose it. Regards
> > Every \foo will be looked up, so by the time \macro in:
> >
> >    bla bla {\bf xxx}\macro{xxx}
> >
> > is seen, the {\bf xxx} is already passed and processed. TeX never looks
> > back, which actually would make for a pretty complex multipass parsing
> > and expansion management (forward control and backward: \expandafter
> > would then also have an \expandbefore companion). Even in the simple
> > case: should it keep track of quantities done (grouped, single token,
> > box, etc.) and then in retrospect see it as #1 (them being nodes by now
> > and not tokens)?
> >
> > So, why in math but not in text? The \atop and \over (those are
> > basically all the same command but with a different treatment
> > afterwards) are an exception: (1) tex knows that is is in math mode, and
> > in math mode the { } are not really arguments but defines some stuff
> > handled together. Much processing (not all) is delayed to a second pass,
> > so {1}\over{2} internally becomes \over{1}{2} and even that is kind of
> > tricky because there are math styles involved (which makes for some hard
> > coded behaviour that in the perspective of luametatex i try to get more
> > grip on). Now, in order to handle this one (!) exception to lookahead
> > parsing, special tracking happens in math mode, the previous math
> > grouped stuff is registered and adapted to the \over when seen,
> > otherwise it stays as is. This exception also maked the code somewhat
> > messier because there are several spots where it has to be dealt with
> > (also think of saving and restoring states). Just imagine that there
> > were more such commands. Believe me, you really don't want to know the
> > details.
> >
> > So the answer is "Can't be done without overhauling the whole concept".
> >
> > Now, I know that the narrative is that context is a moving target
> > (contrary to other macro packages that don't / can't change because
> > users / publishers expect them to behave the same forever) so one can
> > argue that for context we can follow a drastic different route, but even
> > then, we can't shoot ourselves in the foot too often. I know that some
> > people (read: Alan) love these {{}\foo{}} syntaxes but live would have
> > been simpler if even \over has not be in there with prefix notation
> > (there is a reason why macro package have \frac like variants).
> >
> > 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
> > -----------------------------------------------------------------
> >
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> ___________________________________________________________________________________
>
> Taco Hoekwater
> Elvenkind BV
>
>
>
>
>
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