Allow to load external highlighting theme for Pandoc HTML and PDF

[Francis-Hsu]

I have a customized pandoc syntax highlighting theme that I would like to use for my Rmd file: my.theme. For PDF output this can be achieved easily by using the following YAML header:

---
title: "Test"
author: "Me"
date: "2/8/2021"
output:
  pdf_document:
    pandoc_args: "--highlight-style=my.theme"
---

Unfortunately the same wouldn't work for html_document. While the console output shows --highlight-style=my.theme was successfully passed to pandoc, the pandoc_html_highlight_args() utility attached an argument --no-highlight to the end which then overtakes the previous argument. Stating highlight: my.theme is not an option either since pandoc_html_highlight_args() will do a match.arg check. The only work around I can think of is to use trace() to intercept the pandoc_args inside render() before it was passed to pandoc_convert(), and this is obviously a terrible idea.

A fix shouldn't be too complicated though. Maybe an option customized can be added to the output of html_highlighters()? In which case pandoc_html_highlight_args() will stop attaching further arguments so that --highlight-style can be processed correctly.

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[atusy]

I'd suggest something like this so that users can easily specify custom definition like highlight: my.theme.

highlight <- tryCatch(
	match.arg(highlight, html_highlighters()),
	error = function(e) {
		if (file.exists(highlight)) return(highlight)
		stop(e$message, ", or a custom definition file.")
	}
)

[cderv]

Thank you for this suggestion !

This is currently a limitation that should be overcome in this (rather large) PR. It is not specific to this issue, but it refactor the whole highlighting logic and thus a .theme file will work if provided in highlight argument once this PR is merged. (see #1941 (comment)).

There is currently not an easy work around due to the order of flags passed to Pandoc. Instead of hacking with trace() you can modify the arguments in a custom format. This is a temporary workaround for your case waiting for the aforementioned PR.

html_doc <- function() {
  base_format <- rmarkdown::html_document(highlight = NULL)
  pandoc_args <- base_format$pandoc$args
  pandoc_args[pandoc_args == "--no-highlight"] <- "--highlight-style=my.theme"
  base_format$pandoc$args <- pandoc_args
  base_format
}

rmarkdown::render("test.Rmd", html_doc())

[cderv]

@royfrancis for HTML you can now use the highlight argument to pass a .theme file. For PDF not yet but you still get your trick for now.

I'll reopen this one and edit title as I have done 50% of the work. I'll do PDF support soon

[cderv]

And support has now been adding pdf_document() - you can use now this

---
title: "Test"
author: "Me"
date: "2/8/2021"
output:
  pdf_document:
    highlight: my.theme
---

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