Re: Absolute spacing redux

[G. Branden Robinson]

At 2021-10-06T00:31:56-0400, Peter Schaffter wrote:
> I've been frustrated by groff's absolute spacing (.sp |<dist>) for
> decades.
> 
> First off, <dist> is calculated from the first baseline of the
> vertical spacing currently in effect when .sp |<dist> is called, not
> from the top edge of the printer medium or PDF/PostScript display.

Thank you for explaining this.  I haven't revised this part of our
Texinfo manual yet, and one of the reasons is that I haven't been able
to make complete sense of the "|" modifier, especially in light of
threads like we had in July[1].

> I believe groff needs a dedicated request for unambiguous absolute
> spacing.  One that says, "Advance from the top of the printer sheet
> to the location I say and print whatever comes next right there."
> The same goes for .mk/.rt.  (Preparing mom 2.5, I was surprised to
> discover that .rt suffers from the same problem, one of the reasons
> I've decided to bring this up.)
> 
> What say ye?  I'd like to put this in as feature request, but
> there's not much point if others don't see its utility.

I didn't play with your demo yet, but the first thing that comes to my
mind is that a _request_ might not be the best syntactical place to
locate this extension from a language design standpoint.  Might it make
more sense as a new measurement modifier?  As "|" means "absolute" (in
its idiosyncratic way) maybe the reckoning you're proposing could use a
prefix of "@".  It doesn't need composability with "|", if I understand
you, and outside of the few places where "|" is already recognized, it
doesn't need to be exposed anywhere else.

The other question that troubles me, as many of you will have predicted,
is one of lexicon.  What are we going to _call_ these "|" and "@"
modifiers?  We can take over "absolute positioning" for "@" if we want,
and document our deviation from CSTR #54 in a footnote[2].  Maybe the
harder problem is what to then call the "|" modifier that we will have
to continue to support indefinitely.

Regards,
Branden

[1] https://lists.gnu.org/archive/html/groff/2021-06/msg00104.html
[2] It's termed the "absolute position indicator", bottom of page 9 (PDF
    page 11), CSTR #54, 1992 revision.

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