Re: .Bl -tag -width ".mdoc macro" not recognised sometimes(?)

[наб <nabijaczleweli@nabijaczleweli.xyz>]

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Hi!

On Sun, Jun 05, 2022 at 09:43:13PM +0200, Ingo Schwarze wrote:
> Nab wrote on Sun, Jun 05, 2022 at 08:28:24PM +0200:
> > On Sun, Jun 05, 2022 at 07:55:39PM +0200, Jan Stary wrote:
> 
> >> On Jun 05 18:16:35, nabijaczleweli@nabijaczleweli.xyz wrote:
> >> Quoting mdoc(7):
> >> 
> >> 	The -width and -offset arguments accept
> >> 	macro names as described for Bd -offset,
> >> 	scaling widths as described in roff(7),
> >> 	or use the length of the given string.
> >> 
> >> Is your -width a macro name (such as Ds) as descibed for -offset? No.
> >> Is it a scalinbg width as described in roff(7)? No.
> >> Apparently, mandoc uses the length of the given string then.
> >> AFAICT, that's what you told it to do.
> 
> > Ah, yeah, that's fair; this is, apparently, a groffism:
> >   If ⟨string⟩ starts with a ‘.’ (dot) immediately followed by a valid
> >   -mdoc macro name, interpret ⟨string⟩ and use the width of the result.
> 
> I think you are right that this is a groff extension.
> I did quick testing with Cynthia's final version of the 4.4BSD mdoc(7)
> macros and they don't appear to support a full macro line (including
> the leading dot) as a -width argument, or rather, they just take the
> string length of the line, like mandoc(1) does.

My quick skim of 4.4BSD-Lite2 mdoc aligns with this, yeah.

> Then again, Cynthia's version of the macros is very old and mandoc(1)
> mostly aims for groff compatibility.  So supporting this feature is
> desirable.  But it looks seriously complicated given the existing
> mandoc code structure.  So it's certainly not a short-term task and
> maybe not even feasible in the medium term but rather a long-term
> goal.  Besides, the priority is moderate at best because i don't
> recall having seen this often in practice.

It's /incredibly/ useful for printing to PDF (and replaces banging out
the fonts and funny spaces manually in the -width argument),
but the value add for nroff is little (since every character in every
font is the same size; still pretty nice to not have to manually render
the macros though).

> >   Almost all lists in this document use this option.
> I must admit i share Jan's curiosity in which real-world document
> this feature is used.

This whole segment is taken directly from groff_mdoc(7) off Bullseye,
and I use it for my manuals (although that usually matches the natural
text width well enough; this tr example is an edge-case).

> In general, if a feature is used in important, widely used manual
> pages, that raises the priority of supporting it - even though in
> this case, admittedly, even a very high priority would help little
> to overcome the significant difficulty of implementing it.

Not that it's the be-all end-all, but DCS[1] says:
  116 files grepped (95 results)
(if you filter by src:groff, that accounts for
 65 files grepped (65 results))
this includes mandoc:
-- >8 --
mdocml_1.14.6-1/TODO

- When the -width string contains macros, the macros must be rendered
  before measuring the width, for example
    .Bl -tag -width ".Dv message"
  in magic(5), located in src/usr.bin/file, is the same
  as -width 7n, not -width 11n.
-- >8 --
continuing from CVS:
-- >8 --
  The same applies to .Bl -column column widths;
  reported again by Nicolas Joly Thu, 1 Mar 2012 13:41:26 +0100 via wiz@ 5 Mar
  reported again by Franco Fichtner Fri, 27 Sep 2013 21:02:28 +0200
  reported again by Bruce Evans Fri, 17 Feb 2017 21:22:44 +0100 via bapt@
  loc ***  exist ***  algo ***  size **  imp ***
-- >8 --

There's also one result for
  -column ".
from manpages-l10n.

Most of them are relatively simple:
  .Cm restrict_loggedin_tty Ns = Ns Ar ttyglob*
  .Ev DNSDB_API_KEY , APIKEY
  .Fl bullet
this affects them, but, well, not much (45/30, 26/21, 10/7, respectively),
my 59/13 is, decidedly, a 3.26-fold edge-case.

I'd say that the shorthand assessment from the TODO and yours, above,
are unfortunately accurate.

[1]: https://codesearch.debian.net/search?q=-width+%22.&literal=1

> Yours,
>   Ingo

Best,
наб

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