Re: groff_mm : problems with .DS

[Damian McGuckin]

On Fri, 23 Oct 2020, Anton Shepelev wrote:

> Does  not  the  current  behavior  agree  with   the
> groff_mm manpage, which states for .DS that:
>
>   `rindent shortens the line length by that amount.
>
> If you don't pass the `rindent' as the third parame-
> ter, you get the same line length,  but  indented to
> the right.

The 'rindent' is optional. AT&T's DWB MM never had this option. You only 
use 'rindent' if you want to over-ride the standard display indent, i.e. 
that which you get with

        .DS 1

The default Display indent is stored in the 2-letter number register Si
which defaults to 5 standard units. Note that 'groff' uses the notation

        \n[Si]

to extrapolate even a 2-letter number register (instead of \n(Si as used 
in AT&T nroff/troff) so please excuse me if your prefer the older form.

Currently

        .DS

by itself does not indent, i.e. it implies

        .DS 0

which uses the

        linelength of the text prior to when '.DS' appears

whereas

        .DS 1

uses the

        linelength of the text when '.DS' appears LESS \n[Si]

So, if one starts with main body text line-length of say 70, the use of 
that '.DS 1' results in a line-length of 65.

So, in no-fill mode, the current MM (or groff_mm) works as documented,
as expected, i.e. as AT&T DWB MM has worked for 40+ years.

If I then ask for fill mode in my indented display, i.e.

        .DS 1 1

I still expect a line-length of 65 BUT with the text inside the display 
being filled. Sure, fill mode works. But unfortunately, the line-length 
reverts to that used in the document prior to the use of that 'DS' Which 
is wrong.

> If it is not the intended behavior, then it is a a very long-standing 
> bug in Groff's implementation of MM that I have taken for granted more 
> than ten years. I think I can fix it if you can't once I am sure it *is* 
> a bug.

I have not used fill-mode in a '.DS'. in 15 years so it may well be 
long-standing.  It is most definitely a bug.

My document is 40 years old and with AT*T troff with 'MM' does the right 
thing. I do not use 'DS' in a nested way as AT&T's MM did not support such 
usage.

Stay safe - Damian

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