Re: [possible patch] Remove the '\\[0pt]' string from the last line of a verse block in LaTeX export

[Ihor Radchenko]

Juan Manuel Macías <maciaschain@posteo.net> writes:

>>> If I'm not mistaken, in TeX '\\' can only be used in horizontal mode:
>>> that is, within a paragraph. At the end of an environment like verse
>>> (\end{verse}) you are forced to enter vertical mode, and a new paragraph
>>> starts.
>>
>> Is it true for any environment? Or just some?
>
> In principle, nothing prevents one from defining an environment for use
> in horizontal mode (within the paragraph). E.g.:
> ...
> But the usual thing is that the beginning/end of an environment changes to
> vertical mode, with \par or something more elaborate.

This is annoying.
ox-latex has some transcoders that unconditionally add \\ at the
end: clocks and planning lines. If they happen to be at the end of an
environment, it can be problematic.

> hmm... I don't know if this should be considered a bug. We may think
> that \\[0pt] should never do anything, but we must keep in mind that the
> end of the verse environment is the end of a paragraph, and it changes
> to vertical mode. And the end of a paragraph is an illogical place to
> put that command. But it seems that it also alters things when it is at
> that point. Check out this reply from David Carlisle
> (https://tex.stackexchange.com/a/82666):

The very reason we use \\[0pt] is because it supposed to prevent
interpreting [...] at the new line/transcoded element as argument.

You demonstrated that it is yet not always safe enough.

May it be better to use something like

\newcommand\nothing{}
\newcommand{\safenewline}{\\\nothing}

And then use \safenewline instead of \\[0pt]

In my tests,

\begin{center}
\begin{tabular}{ll}
[t] & s\safenewline
[I] & A\safenewline
[m] & kg\safenewline
\end{tabular}
\end{center}

Does not suffer from misinterpreting new line as argument.

> If there are environments that redefine \\, such as verse or tabular,
> probably putting the optional argument of \\ with a value of 0pt at the
> end of verse alters the calculation of the normal space after the
> environment, making it shorter. I think that here we would not have a
> LaTeX bug, because the syntax of the verse environment itself says that
> the last line should not carry any \\ mark. 

AFAIU, \safenewline should still use re-defined version of \\ according
to the context.

> At the end of the tables I have not noticed any side effects. But in the
> export of the verse block, I would be in favor of somehow eliminating
> that last \\[0pt].

`org-latex-verse-block' already has a giant regexp replacement:

      ;; In a verse environment, add a line break to each newline
      ;; character and change each white space at beginning of a line
      ;; into a normal space, calculated with `\fontdimen2\font'.  One
      ;; or more blank lines between lines are exported as a single
      ;; blank line.  If the `:lines' attribute is used, the last
      ;; verse of each stanza ends with the string `\\!', according to
      ;; the syntax of the `verse' package. The separation between
      ;; stanzas can be controlled with the length `\stanzaskip', of
      ;; the aforementioned package.  If the `:literal' attribute is
      ;; used, all blank lines are preserved and exported as
      ;; `\vspace*{\baselineskip}', including the blank lines before
      ;; or after CONTENTS.

We may as well strip the trailing \\[0pt] there.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
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