Re: incorrect @math spacing for PDF output with an empty macro

[Vincent Lefevre]

On 2022-10-25 14:42:20 +0100, Gavin Smith wrote:
> On Tue, Oct 25, 2022 at 02:41:48PM +0200, Vincent Lefevre wrote:
> > @w doesn't work correctly in math mode:
> > 
> > ------------------------------------------------------------
> > \input texinfo    @c -*-texinfo-*-
> > @documentencoding UTF-8
> > 
> > @node Top
> > @node Test
> > 
> > Formatting test.
> > 
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > zzzzzzzzzzzzzzzzzz @math{a + b}
> > 
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
> > 
> > @bye
> > ------------------------------------------------------------
> > 
> > One can see that with @w, spaces do not stretch.
> 
> Yes, but I wouldn't count this as "incorrect".

Perhaps not incorrect, but this can give an inconsistent style
if an author does not always use @w (and I don't think that such
an author is to blame, as @w is not supposed to change the style
except forbidding line breaks).

> Note if you want a fair comparison, you should have the same paragraph
> indent in both paragraphs, using @noindent:
> 
>   @noindent
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   zzzzzzzzzzzzzzzzzz @math{a + b}
>   
>   @noindent
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@math{a + b}}

Paragraphs except the first one ("Formatting test." above; did
you see it?) are expected to use the same indentation rule. So
the @noindent should not be necessary for the comparison.

> Note it is not an issue with @math, but with @w generally, as you
> can see with the following input:
> 
>   @noindent
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   zzzzzzzzzzzzzzzzzz @asis{a + b}
>   
>   @noindent
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
>   zzzzzzzzzzzzzzzzzz @w{@asis{a + b}}
> 
> In theory, there could be better output if we could change the size
> of spaces inside of @w, but I doubt that there is a good way of
> achieving this in TeX.

I had looked at tex.stackexchange.com and found

  https://tex.stackexchange.com/questions/574783/stretch-spaces-within-a-box

but the accepted answer seems incorrect, or not answering the
general issue. In the comments:

  @DonaldArseneau IMO if the only reason for wanting it to remain
  boxed is to prevent line breaks, then maybe one could also insert
  huge penalties everywhere. (Or I imagine one could do something
  fancy in LuaTeX with whatsits or whatever, that does a
  post-linebreak check to see whether there has been a line break
  “inside”.) – ShreevatsaR

  @ShreevatsaR Yes, exactly what I think. Package nolbreaks.sty does
  the penalties in a LaTeX context. To do literal adjustment of spaces
  in a box, one could reset the box in certain contexts, but
  paragraphs are very difficult. The only potential, I think, is with
  some luaTeX post-processing of the nodes. – Donald Arseneau

So

https://mirror.ibcp.fr/pub/CTAN/macros/latex/contrib/nolbreaks/nolbreaks.sty

might be a solution if applicable in the context of Texinfo (it
is quite short and in the public domain, so it could be adapted
if need be).

> Is there actually anything wrong with the way the output looks for
> your document?

I don't think so, but the solution should remain valid in the future.
That said, for the MPFR manual, using a macro without @w for TeX and
with @w for the other outputs should not be an issue. The good thing
is that TeX does not seem to introduce line breaks in math formulas
if they are not really necessary. Well, I had noticed in the past
that the default was not perfect, IMHO, but

  \binoppenalty=10000
  \relpenalty=10000

solved the issue in all my LaTeX documents.

However, for the Info output at least (and I assume that this is
also the case for the HTML output), @w is really necessary. See
the differences:

@@ -1586,8 +1586,8 @@
           mpfr_rnd_t RND)
      Set ROP to the square root of OP rounded in the direction RND.  Set
      ROP to −0 if OP is −0, to be consistent with the IEEE 754 standard
-     (thus this differs from ‘mpfr_rootn_ui’ and ‘mpfr_rootn_si’ with N
-     = 2).  Set ROP to NaN if OP is negative.
+     (thus this differs from ‘mpfr_rootn_ui’ and ‘mpfr_rootn_si’ with
+     N = 2).  Set ROP to NaN if OP is negative.
 
  -- Function: int mpfr_rec_sqrt (mpfr_t ROP, mpfr_t OP, mpfr_rnd_t RND)
      Set ROP to the reciprocal square root of OP rounded in the
@@ -1595,8 +1595,8 @@
      NaN if OP is negative.  Warning!  Therefore the result on −0 is
      different from the one of the rSqrt function recommended by the
      IEEE 754 standard (Section 9.2.1), which is −Inf instead of +Inf.
-     However, ‘mpfr_rec_sqrt’ is equivalent to ‘mpfr_rootn_si’ with N =
-     -2.
+     However, ‘mpfr_rec_sqrt’ is equivalent to ‘mpfr_rootn_si’ with
+     N = −2.
 
  -- Function: int mpfr_cbrt (mpfr_t ROP, mpfr_t OP, mpfr_rnd_t RND)
  -- Function: int mpfr_rootn_ui (mpfr_t ROP, mpfr_t OP, unsigned long

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)