Re: table borders don't span entire width

[Ingo Schwarze <schwarze@usta.de>]

Hi Anthony,

Anthony J. Bentley wrote on Fri, Feb 08, 2019 at 04:18:37PM -0700:

> The missing characters ring a bell. I'm pretty sure I investigated this
> before, came to the same conclusion (that needed box drawing characters
> don't exist), and then must have forgotten all about it.

> Ingo Schwarze writes:

>> So in particular, the following are missing:
>>
>>  * double right
>>  * double down
>>  * single left with double right
>>  * single left down with double right
>>  * single left with double right down
>>  * double left
>>  * double left with single right
>>  * double left with single right down
>>  * double left down with single right
>>
>> and so on...
>>
>> With heavy instead of double, such combinations exist.

> That's a real shame. I looked in the Unicode standard but couldn't find
> any rationale, only a comment that the box drawing section of Unicode
> exists for compatibility with historic systems. I guess there wasn't
> anything out there that used double boxes in this way.
> 
> https://www.unicode.org/versions/Unicode11.0.0/ch22.pdf

>> So, should i leave this untouched, or am i missing something?

> Leave it as is, I guess. But we should document this limitation in
> tbl(7) since it's non-obvious and seems to have come up multiple times.

Done, see the commit below.

> Or just draw twice as many lines, like groff attempts poorly with
> doublebox...

No, that would look horrible and waste vertical screen space for
double horizontal lines, and less importantly, horizontal screen
space for double vertical lines.  In particular in tables, screen
space is often a scarce resource.

Besides, representing double lines as heavy lines is *logically*
just fine.  It only mismatches the intent of the author in a minor
presentational detail - or maybe even not at all because the tbl(7)
language does not provide any syntax for "heavy", so the assumption
that authors use "double" to express that they want a line emphasized
seems quite safe.

Yours,
  Ingo


Log Message:
-----------
add a BUGS section explaining the situation with box and line drawing
in UTF-8 output; suggested by bentley@

Modified Files:
--------------
    mandoc:
        tbl.7

Revision Data
-------------
Index: tbl.7
===================================================================
RCS file: /home/cvs/mandoc/mandoc/tbl.7,v
retrieving revision 1.32
retrieving revision 1.33
diff -Ltbl.7 -Ltbl.7 -u -p -r1.32 -r1.33
--- tbl.7
+++ tbl.7
@@ -1,7 +1,7 @@
 .\"	$Id$
 .\"
 .\" Copyright (c) 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
-.\" Copyright (c) 2014, 2015, 2017, 2018 Ingo Schwarze <schwarze@openbsd.org>
+.\" Copyright (c) 2014,2015,2017,2018,2019 Ingo Schwarze <schwarze@openbsd.org>
 .\"
 .\" Permission to use, copy, modify, and distribute this software for any
 .\" purpose with or without fee is hereby granted, provided that the above
@@ -438,3 +438,17 @@ reference was written by
 .An Kristaps Dzonsons Aq Mt kristaps@bsd.lv
 and
 .An Ingo Schwarze Aq Mt schwarze@openbsd.org .
+.Sh BUGS
+In
+.Fl T
+.Cm utf8
+output mode, heavy lines are drawn instead of double lines.
+This cannot be improved because the Unicode standard only provides
+an incomplete set of box drawing characters with double lines,
+whereas it provides a full set of box drawing characters
+with heavy lines.
+It is unlikely this can be improved in the future because the box
+drawing characters are already marked in Unicode as characters
+intended only for backward compatibility with legacy systems,
+and their use is not encouraged.
+So it seems unlikely that the missing ones might get added in the future.
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