Tweak handling of mathjax argument in gitbook template

[cderv]

This could resolve #915

It is only a suggestion for modificaiton in bookdown only. There are other solution to deal with that more broadly for all the formats:

• tweak rmarkdown pandoc_mathjax_args in pre processor
• (maybe) Use a lua filter to change the Meta information if possible (just an idea for now)

But on this PR, here is what has changed based on the analysis in #915 (comment)

# on the branch custom-mathjax from this PR
pkgload::load_all()
#> Loading bookdown

temp_file <- tempfile(fileext = ".Rmd")
xfun::write_utf8(c(
  "---",
  "title: Test",
  "---",
  "",
  "# test",
  "",
  "$1+1$"
), temp_file)

render_gitbook <- function(...) {
  # we insure pandoc 2.7.3 for now
  rmarkdown::find_pandoc(version = "2.7.3")
  res <- xfun::in_dir(tempdir(),
               rmarkdown::render(
                 temp_file, "bookdown::gitbook",
                 output_options = list(...),
                 quiet = TRUE)
  )
  html <- xfun::read_utf8(res)
  i <- grep("<!-- dynamically load mathjax for compatibility with self-contained -->",
       html)
  if (!length(i)) return("no Mathjax")
  start_s <- grep("<script>", html)
  end_s <- grep("</script>", html)
  xfun::raw_string(html[start_s[start_s > i][1]:end_s[end_s>i][1]])
}

custom_mathjax <- "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"

This PR changes nothing at the fact gitbook and mathjax does not work when self_contained = TRUE

render_gitbook(self_contained = TRUE)
#> Warning: MathJax doesn't work with self_contained when not using the rmarkdown
#> "default" template.
#> [1] "no Mathjax"
render_gitbook(self_contained = TRUE, mathjax = custom_mathjax)
#> Warning: MathJax doesn't work with self_contained when not using the rmarkdown
#> "default" template.
#> [1] "no Mathjax"

Custom url now works. See the value of src below

render_gitbook(mathjax = custom_mathjax)
#> <script>
#>   (function () {
#>     var script = document.createElement("script");
#>     script.type = "text/javascript";
#>     var src = "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js";
#>     if (src === "" || src === "true") src = "https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-MML-AM_CHTML";
#>     if (location.protocol !== "file:")
#>       if (/^https?:/.test(src))
#>         src = src.replace(/^https?:/, '');
#>     script.src = src;
#>     document.getElementsByTagName("head")[0].appendChild(script);
#>   })();
#> </script>

it works also with mathjax local now. See src below

render_gitbook(mathjax = "local")
#> <script>
#>   (function () {
#>     var script = document.createElement("script");
#>     script.type = "text/javascript";
#>     var src = "libs/mathjax-local/MathJax.js?config=TeX-AMS-MML_HTMLorMML";
#>     if (src === "" || src === "true") src = "https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-MML-AM_CHTML";
#>     if (location.protocol !== "file:")
#>       if (/^https?:/.test(src))
#>         src = src.replace(/^https?:/, '');
#>     script.src = src;
#>     document.getElementsByTagName("head")[0].appendChild(script);
#>   })();
#> </script>

with no mathjax asked, it is still working - no mathjax is used.

render_gitbook(mathjax = NULL)
#> [1] "no Mathjax"

with custom template, we can decide to pass the mathjax-url variable for use in custom template

library(htmltools)
html_content <- div(
  # hack to work with previous function render_gitbook
  HTML("<!-- dynamically load mathjax for compatibility with self-contained -->"),
  tags$script(
    p("this is mathjax variable: $mathjax$"),
    br(),
    p("this is mathjaxurl variable: $mathjaxurl$"),
    br(),
    p("this is mathjax-url variable: $mathjax-url$"),
    br(),
    p("this is math variable: $math$")
  )
)
template <- tempfile(fileext = ".html")
xfun::write_utf8(as.character(html_content), template)

custom templates can use pandoc mathjax-url variable, that corresponds
but it is not used by pandoc - So I guess we should choice what should happen here.

• choose what to do when custom mathjax AND template are used

render_gitbook(mathjax = custom_mathjax, template = template)
#>   <script>
#>     <p>this is mathjax variable: true</p>
#>     <br/>
#>     <p>this is mathjaxurl variable: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js</p>
#>     <br/>
#>     <p>this is mathjax-url variable: https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js</p>
#>     <br/>
#>     <p>this is math variable: <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js?config=TeX-AMS_CHTML-full" type="text/javascript"></script></p>
# same for mathjax = local
render_gitbook(mathjax = "local", template = template)
#>   <script>
#>     <p>this is mathjax variable: true</p>
#>     <br/>
#>     <p>this is mathjaxurl variable: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js</p>
#>     <br/>
#>     <p>this is mathjax-url variable: libs/mathjax-local/MathJax.js?config=TeX-AMS-MML_HTMLorMML</p>
#>     <br/>
#>     <p>this is math variable: <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js?config=TeX-AMS_CHTML-full" type="text/javascript"></script></p>

still does work with self_contained = TRUE

render_gitbook(mathjax = custom_mathjax, template = template, self_contained= TRUE)
#> Warning: MathJax doesn't work with self_contained when not using the rmarkdown
#> "default" template.
#>   <script>
#>     <p>this is mathjax variable: </p>
#>     <br/>
#>     <p>this is mathjaxurl variable: </p>
#>     <br/>
#>     <p>this is mathjax-url variable: </p>
#>     <br/>
#>     <p>this is math variable: </p>
#>   </script>

with default, the pandoc mathjax default and custom template one are used.

render_gitbook(template = template)
#>   <script>
#>     <p>this is mathjax variable: true</p>
#>     <br/>
#>     <p>this is mathjaxurl variable: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js</p>
#>     <br/>
#>     <p>this is mathjax-url variable: </p>
#>     <br/>
#>     <p>this is math variable: <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js?config=TeX-AMS_CHTML-full" type="text/javascript"></script></p>

we see that with a custom template it does not change a lot

About other format

It seems html_chapter uses default.html that does use the math pandoc variable. I suppose because self_contained is forced to FALSE.

This is one option for gitbook format too if we don't want to support self_contained gitbook with mathjax (as it is not working anyway for now it seems)

@yihui I am waiting on your thoughts on all that to see what we do