added cache_engines object and cache_engine function to make API for …

…engine-specific caches

NAMESPACE

@@ -26,6 +26,7 @@ export(all_labels)
 export(all_patterns)
 export(all_rcpp_labels)
 export(asis_output)
+export(cache_engines)
 export(clean_cache)
 export(combine_words)
 export(current_input)

R/block.R

@@ -72,6 +72,7 @@ call_block = function(block) {
         params$engine != 'Rcpp') {
       if (opts_knit$get('verbose')) message('  loading cache from ', hash)
       cache$load(hash, lazy = params$cache.lazy)
+      cache_engine(params$engine, hash)
       if (!params$include) return('')
       if (params$cache == 3) return(cache$output(hash))
     }

R/engine.R

@@ -31,6 +31,24 @@
 #' names(knit_engines$get())
 knit_engines = new_defaults()
 
+
+#' Cache engines of other languages
+#'
+#' This object controls how to load cached environments from languages
+#' other than R (when the chunk option \code{engine} is not \code{'R'}).
+#' Each component in this object is a function that takes the current path
+#' to the chunk cache and loads it into the language environment.
+#'
+#' The cache engine function has one argument \code{cache_path}. The
+#' argument is the path to the current chunk cache with the chunk's
+#' hash, but without any file extension.
+#'
+#' The cache engine function should load the cache environment and should
+#' know the extension appropriate for the language.
+#' @export
+#' @examples cache_engines$set(python = reticulate::load_python_session)
+cache_engines = new_defaults()
+
 #' An output wrapper for language engine output
 #'
 #' If you have designed a language engine, you may call this function in the end
@@ -178,6 +196,19 @@ eng_python = function(options) {
   }
 }
 
+eng_python_cache = function(options) {
+  if (isFALSE(options$python.reticulate)) {
+    eng_interpreted(options)
+  } else {
+    if (!loadable('reticulate')) warning2(
+      "The 'python' engine in knitr requires the reticulate package. ",
+      "If you do not want to use the reticulate package, set the chunk option ",
+      "python.reticulate = FALSE."
+    )
+    reticulate::load_python_session(options)
+  }
+}
+
 ## Java
 #  e.g. see http://cran.rstudio.com/package=jvmr
 
@@ -649,6 +680,12 @@ get_engine = function(name) {
   }
 }
 
+cache_engine = function(name, cache_path) {
+  cache_fun = cache_engines$get(name)
+  if (!is.function(cache_fun)) return()
+  cache_fun(cache_path)
+}
+
 # possible values for engines (for auto-completion in RStudio)
 opts_chunk_attr$engine = as.list(sort(c('R', names(knit_engines$get()))))
 opts_chunk_attr[c('engine.path', 'engine.opts')] = list('character', 'character')