created a eng_r function which exposes the R engine

DESCRIPTION

@@ -4,6 +4,7 @@ Title: A General-Purpose Package for Dynamic Report Generation in R
 Version: 1.31.5
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name", comment = c(ORCID = "0000-0003-0645-5666")),
+    person("Abhraneel", "Sarma", role = "ctb"),
     person("Adam", "Vogt", role = "ctb"),
     person("Alastair", "Andrew", role = "ctb"),
     person("Alex", "Zvoleff", role = "ctb"),

NEWS.md

@@ -2,6 +2,8 @@
 
 ## NEW FEATURES
 
+- The internal code to process R code chunks was factored out into the language engine `R`, which can be obtained via `knit_engines$get('R')` now (thanks, @abhsarma, #1963).
+
 - Added arguments `dir` and `envir` to `load_cache()` to specify the working directory and environment into which the cached objects are loaded. By default, all cached objects are loaded into the global environment, which may not be desirable (thanks, @LTLA, #1905).
 
 - The internal function `html_screenshot()`, which takes screenshots of HTML widgets and Shiny apps in **knitr** documents, now prefers the **webshot2** package over the **webshot** package. This is because the backend of the **webshot** package is PhantomJS, which [has been archived since 2018](https://github.com/ariya/phantomjs/issues/15344). If **webshot** is still preferred, use the chunk option `opts_chunk$set(webshot = "webshot")` (thanks, @atusy #1918, @LouisStAmour #1858).

R/block.R

@@ -101,25 +101,42 @@ cache2.opts = c('fig.keep', 'fig.path', 'fig.ext', 'dev', 'dpi', 'dev.args', 'fi
 cache0.opts = c('include', 'out.width.px', 'out.height.px', 'cache.rebuild')
 
 block_exec = function(options) {
+  if (options$engine == 'R') return(eng_r(options))
+
   # when code is not R language
-  if (options$engine != 'R') {
-    res.before = run_hooks(before = TRUE, options)
-    engine = get_engine(options$engine)
-    output = in_dir(input_dir(), engine(options))
-    if (is.list(output)) output = unlist(output)
-    res.after = run_hooks(before = FALSE, options)
-    output = paste(c(res.before, output, res.after), collapse = '')
-    output = knit_hooks$get('chunk')(output, options)
-    if (options$cache) {
-      cache.exists = cache$exists(options$hash, options$cache.lazy)
-      if (options$cache.rebuild || !cache.exists) block_cache(options, output, switch(
-        options$engine,
-        'stan' = options$output.var, 'sql' = options$output.var, character(0)
-        ))
-      }
-    return(if (options$include) output else '')
+  res.before = run_hooks(before = TRUE, options)
+  engine = get_engine(options$engine)
+  output = in_dir(input_dir(), engine(options))
+  if (is.list(output)) output = unlist(output)
+  res.after = run_hooks(before = FALSE, options)
+  output = paste(c(res.before, output, res.after), collapse = '')
+  output = knit_hooks$get('chunk')(output, options)
+  if (options$cache) {
+    cache.exists = cache$exists(options$hash, options$cache.lazy)
+    if (options$cache.rebuild || !cache.exists) block_cache(options, output, switch(
+      options$engine,
+      'stan' = options$output.var, 'sql' = options$output.var, character(0)
+    ))
   }
+  if (options$include) output else ''
+}
 
+#' Engine for R
+#'
+#' This function handles the execution of R code blocks (when the chunk option
+#' \code{engine} is \code{'R'}) and generates the R output for each code block.
+#'
+#' This engine function has one argument \code{options}: the source code of the
+#' current chunk is in \code{options$code}. It returns a processed output that
+#' can consist of data frames (as tables), graphs, or character output. This
+#' function is intended for advanced use to allow developers to extend R, and
+#' customize the pipeline with which R code is executed and processed within
+#' knitr.
+#'
+#' @param options A list of chunk options. Usually this is just the object
+#'   \code{options} associated with the current code chunk.
+#' @noRd
+eng_r = function(options) {
   # eval chunks (in an empty envir if cache)
   env = knit_global()
   obj.before = ls(globalenv(), all.names = TRUE)  # global objects before chunk

R/engine.R

@@ -756,7 +756,7 @@ knit_engines$set(
   c = eng_shlib, cc = eng_shlib, fortran = eng_shlib, fortran95 = eng_shlib, asy = eng_dot,
   cat = eng_cat, asis = eng_asis, stan = eng_stan, block = eng_block,
   block2 = eng_block2, js = eng_js, css = eng_css, sql = eng_sql, go = eng_go,
-  python = eng_python, julia = eng_julia, sass = eng_sxss, scss = eng_sxss
+  python = eng_python, julia = eng_julia, sass = eng_sxss, scss = eng_sxss, R = eng_r
 )
 
 cache_engines$set(python = cache_eng_python)