Set an Output Hook Convert Control Sequences to HTML in Rmarkdown

This is a convenience function designed for use within an rmarkdown document. It overrides the knitr output hooks by using knitr::knit_hooks$set. It replaces the hooks with ones that convert Control Sequences into HTML. In addition to replacing the hook functions, this will output a <STYLE> HTML block to stdout. These two actions are side effects as a result of which R chunks in the rmarkdown document that contain CSI SGR are shown in their HTML equivalent form.

Usage

set_knit_hooks(
  hooks,
  which = "output",
  proc.fun = function(x, class) html_code_block(to_html(html_esc(x)), class = class),
  class = sprintf("fansi fansi-%s", which),
  style = getOption("fansi.css", dflt_css()),
  split.nl = FALSE,
  .test = FALSE
)

Arguments

hooks: list, this should the be knitr::knit_hooks object; we require you pass this to avoid a run-time dependency on knitr.
which: character vector with the names of the hooks that should be replaced, defaults to 'output', but can also contain values 'message', 'warning', and 'error'.
proc.fun: function that will be applied to output that contains CSI SGR sequences. Should accept parameters x and class, where x is the output, and class is the CSS class that should be applied to the <PRE><CODE> blocks the output will be placed in.
class: character the CSS class to give the output chunks. Each type of output chunk specified in which will be matched position-wise to the classes specified here. This vector should be the same length as which.
style: character a vector of CSS styles; these will be output inside HTML >STYLE< tags as a side effect. The default value is designed to ensure that there is no visible gap in background color with lines with height 1.5 (as is the default setting in rmarkdown documents v1.1).
split.nl: TRUE or FALSE (default), set to TRUE to split input strings by any newlines they may contain to avoid any newlines inside SPAN tags created by to_html(). Some markdown->html renders can be configured to convert embedded newlines into line breaks, which may lead to a doubling of line breaks. With the default proc.fun the split strings are recombined by html_code_block(), but if you provide your own proc.fun you'll need to account for the possibility that the character vector it receives will have a different number of elements than the chunk output. This argument only has an effect if chunk output contains CSI SGR sequences.
.test: TRUE or FALSE, for internal testing use only.

Value

named list with the prior output hooks for each of which.

Details

The replacement hook function tests for the presence of CSI SGR sequences in chunk output with has_ctl, and if it is detected then processes it with the user provided proc.fun. Chunks that do not contain CSI SGR are passed off to the previously set hook function. The default proc.fun will run the output through html_esc, to_html, and finally html_code_block.

If you require more control than this function provides you can set the knitr hooks manually with knitr::knit_hooks$set. If you are seeing your output gaining extra line breaks, look at the split.nl option.

Note

Since we do not formally import the knitr functions we do not guarantee that this function will always work properly with knitr / rmarkdown.