Support #<< to highlight lines in revealjs?

[etiennebacher]

I like the way to highlight lines in revealjs slides with the chunk option code-line-numbers, but one drawback to me is that the line numbers are hardcoded. This means that if I want to modify the chunk later, I have to change the line numbers, which is a bit annoying.

Before using revealjs, I used xaringan which supported the special comment #<< to highlight specific lines, like this:

---
output: 
  xaringan::moon_reader:
    nature: 
      highlightLines: true
---

```{r}
# Higlight the following line
print("a") #<<
```

Would it be possible to support something like this?

[mcanouil]

It's unlikely to happen because this is Pandoc/Reveal.js internal framework.
This would require to write a pre-processor script (which you could write if that is something you really need, see https://quarto.org/docs/projects/scripts.html#pre-and-post-render) that will read the code cells, detect the symbol #<<, figure out the line number, set the code cell option code-line-numbers.

but one drawback to me is that the line numbers are hardcoded

So is your #<< at the end of the line.

[etiennebacher]

but one drawback to me is that the line numbers are hardcoded

So is your #<< at the end of the line.

Yes but if I add or remove lines above print("a") #<<, the line will still be highlighted. With hardcoded line numbers, if I add or remove lines above, I have to change the line number to highlight

[mcanouil]

I can see how it can be a little inconvenient, but for presentation, do you have to change that often your code and the highlighting?

I feel the issue might be more on the process to build your slides rather than on the tool to build them. 🤔

[etiennebacher]

For sure it's not the most important feature ever, it just makes it a bit more comfortable. And since it was something available in xaringan I thought it might be implemented in revealjs as well. But if it's hard to implement then it's ok.

And regarding the process of making slides, it's also due to the fact that I don't teach a lot so I probably don't have the most efficient workflow, but sometimes you're like "I should add a comment in this chunk of code" and if you have a lot of chunks then it's a bit annoying to change all line numbers to highlight.

[mcanouil]

Such feature can be implemented, but should it? (I am not the one that can answer that)
knitr hooks and/or pre-render script could be used but again is this time worth spending?
When I make slides, the highlighting, etc. is one of the final step and only once, so I never had the issue to change so often the highlighting/etc. that it became a hassle.

For your information, xaringan is built on top of Remark.js while Quarto is using Pandoc which uses Reveal.js.
Remark.js and Reveal.js are two totally different framework.

[mcanouil]

I believe the "code annotation" feature addresses this, see https://quarto.org/docs/authoring/code-annotation.html (if not feel free to reopen).