Show syntax errors in knitr output? - knitr

Question

I'm writing a description of R syntax, and I'd like to include some statements that include syntax errors, e.g.
if (n >= 2)
{
# if TRUE
}
else
{
# if FALSE
}
(This is a syntax error at top level because the if is complete before the else is read.) I'm trying to put this into a knitr document, but
even with error=TRUE it fails, because that option only affects run-time errors, not syntax errors. Is there some reasonable way to get this to display what would be shown if I entered it at the console? I.e.
I'd like it to display something like
if (n >= 2)
{
# if TRUE
}
else
Error: unexpected 'else' in "else"
{
# if FALSE
}

Answer 1

Here's a function that does part of what I want: it reproduces the output from a syntax error, but it doesn't do all the markup that knitr would do, so the formatting is wrong.
parse_with_error <- function(text) {
reportError <- function(e) {
msg <- conditionMessage(e)
line <- as.numeric(sub("^text:([[:digit:]]*):.*", "\\1", msg))
col <- as.numeric(sub("^text:[[:digit:]]*:([[:digit:]]*):.*", "\\1", msg))
cat(text[1:line], sep="\n")
if (col > 1)
cat(paste(rep(" ", col-1), collapse=""))
cat("^\n")
err <- sub("^text:[[:digit:]]*:[[:digit:]]*: ([^[:cntrl:]]*)\n.*", "\\1", msg)
parseData <- getParseData(sf)
lastToken <- parseData[nrow(parseData), "text"]
cat(paste0("Error: ", err, ' in "', lastToken, '"'), "\n")
}
text <- unlist(strsplit(text, "\n"))
sf <- srcfile("text")
tryCatch(parse(text = text, keep.source = TRUE, srcfile=sf),
error = reportError)
}
This shows how it would be used:
parse_with_error(
"if (TRUE) {
# do TRUE
}
else {
# do FALSE
}")

Answer 2

You can use a separate R session to evaluate the code, e.g.
```{r, engine='Rscript', error=TRUE}
if (TRUE)
{
# if TRUE
}
else
{
# if FALSE
}
```