fixes #322: allows Rmd chunks using {lang} engine format (#350)

Allow Rmd chunks using {lang} engine format

Rmarkdown format allows the use of code blocks in the following format

```{some_engine}
# not necessarily R syntax
foo = bar
```

These blocks should be filtered out before linting the remaining R-code blocks.

A couple of python blocks were added to test.Rmd: one with R-like syntax and one with syntax that can't be parsed as R.

The function `defines_knitr_engine` was extracted from `extract_r_source` and extended. This tests for the presence of {r engine = some_engine} and {some_engine} in the start line of a code chunk. The names of valid engines are obtained from `knitr::knit_engines`

Fixes #322

NEWS.md

@@ -58,6 +58,7 @@
 * Allow for any number of `#` to start a comment. Useful in ESS (#299, @prosoitos)
 * New equals_na_linter() (#143, #326, @jabranham)
 * Fixed plain-code-block bug in Rmarkdown (#252, @russHyde)
+* Fixed bug where non-R chunks using {lang} `engine format` were parsed from R-markdown (#322, @russHyde)
 
 # lintr 1.0.2 #
 * Fix tests to work with upcoming testthat release.

R/extract.R

@@ -26,13 +26,12 @@ extract_r_source <- function(filename, lines) {
   Map(
     function(start, end) {
       if (
-
         # block contains at least one line of code
         start + 1 < end &&
 
         # block does not set an engine (so is r code)
-        !rex::re_matches(lines[start],
-          rex::rex(boundary, "engine", any_spaces, "="))) {
+        !defines_knitr_engine(lines[start])
+      ) {
         output[seq(start + 1, end - 1)] <<- lines[seq(start + 1, end - 1)]
       }
     },
@@ -68,13 +67,29 @@ filter_chunk_end_positions <- function(starts, ends) {
   }
 
   positions <- sort(c(starts = starts, ends = ends))
-  code_start_indexes <- which(grepl("starts", names(positions)))
+  code_start_indexes <- grep("starts", names(positions))
   code_ends <- positions[1 + code_start_indexes]
 
   stopifnot(all(grepl("ends", names(code_ends))))
   code_ends
 }
 
+defines_knitr_engine <- function(line) {
+  engines <- names(knitr::knit_engines$get())
+
+  # {some_engine}, {some_engine label, ...} or {some_engine, ...}
+  bare_engine_pattern <- rex::rex(
+    "{", or(engines), one_of("}", " ", ",")
+  )
+  # {... engine = "some_engine" ...}
+  explicit_engine_pattern <- rex::rex(
+    boundary, "engine", any_spaces, "="
+  )
+
+  rex::re_matches(line, explicit_engine_pattern) ||
+  rex::re_matches(line, bare_engine_pattern)
+}
+
 replace_prefix <- function(lines, prefix_pattern) {
   if (is.null(prefix_pattern)) {
     return(lines)

tests/testthat/knitr_formats/test.Rmd

@@ -34,3 +34,19 @@ a[0]=1
 Plain code blocks can be written after three or more backticks
 - R Markdown: The Definitive Guide. Xie, Allaire and Grolemund (2.5.2)
 ```
+
+Calls to a non-R knitr-engine using {engine_name} syntax.
+
+```{python}
+# Python that looks like R
+a = list()
+b = {2}
+print(a)
+```
+
+```{python}
+# Python that's definitely not R
+a = []
+a.append(2)
+print(a)
+```