Print custom object in rmarkdown and knitr (PDF and HTML)

Question

Let's say I have a package with a function that returns an S3 object:

new_myclass <- function() {
  return(structure(list(a=1, b=2), class = "myclass"))
}

I also have two functions that take a myclass object and return respectively a HTML representation and a LaTeX representation of the object.

myclass2html <- function(obj) { return("<p>MyClass object</p>")}
myclass2latex <- function(obj) { return("\begin{em}MyClass\end{em} object $x$")}

What functions/methods should I define to provide consistent and transparent knitr and rmarkdown support? I would like to support both .Rmd files and .R files with a header like:

#'---
#' title: My document
#' output: pdf_output
#'---

So far my approach goes through the knit_print method:

knit_print.myclass <- function(x, ...) {
  rmarkdown_fmt <- rmarkdown::metadata$output
  knitr_fmt <- knitr::opts_knit$get("out.format")
  # should I use these heuristics with both variables?
  if (rmarkdownfmt == "pdf_document") {
    return(knitr::asis_output(myclass2latex(x)))
  }
  if (knitr_fmt %in% c("html", "markdown")) {
    return(knitr::asis_output(myclass2html(x)))
  } else {
    stop("Format not supported!")
  }
}

My main issue is that there may be two variables rmarkdown::metadata$output and knitr::opts_knit$get("out.format") that may or may not be defined (depending on whether or not rmarkdown is being used). I find this confusing.

Is knit_print the right method to customize for this purpose?
What is the right way to know the output format? Is there a get_output_format function that tells me the output format?

To put the question in context, I am working on the condformat package that allows to visualize DataFrames with conditional formatting rules.

EDIT: So far I have been doing my own voodoo to detect the output format:

#' @importFrom rmarkdown metadata
#' @importFrom knitr opts_knit
guess_output_format <- function() {
  rmd_output <- tryCatch({rmarkdown::metadata$output},
                         error = function(e) {NULL})
  if (is.null(rmd_output)) {
    rmd_output = ""
  }
  if (is.list(rmd_output)) {
    rmd_output <- names(rmd_output)[1]
  }
  if (rmd_output == "pdf_document") {
    return("latex")
  } else if (rmd_output %in% c("html_document", "html_vignette")) {
    return("html")
  } else if (rmd_output != "") {
    stop("Unsupported rmarkdown output format:", rmd_output)
  }
  # No rmarkdown, let's try with knitr:
  format <- knitr::opts_knit$get("out.format")
  if (format %in% c("html", "markdown")) {
    return("html")
  } else if (format %in% c("latex")) {
    return("latex")
  } else {
    stop("Format not supported!")
  }
}

EDIT: Further discussion here: https://github.com/rstudio/rmarkdown/issues/649

zeehio zeehio · Accepted Answer · 2017-10-25T10:53:42

With knitr 1.18 this is much simpler:

guess_output_format <- function() {
  if (knitr::is_html_output()) {
    return("html")
  } else if (knitr::is_latex_output()) {
    return("latex")
  } else {
    return("unsupported")
 }
}

Related issue: https://github.com/rstudio/rmarkdown/issues/649

Print custom object in rmarkdown and knitr (PDF and HTML)

1 Answers