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
> -----------------------------------------------------------------
>