srcref attribute is NULL

[howardbaik]

The code chunk immediately preceding Section 6.2.2 Primitive functions currently returns NULL, but it should display the source code of the function instead:

> attr(f02, "srcref")
function(x, y) {
  # A comment
  x + y
}

[yjunechoe]

A similar problem in 20.2.3 Gotcha: function(), where the first code chunk gives away the "gotcha" because the srcref appears to be stripped:

x <- 10
y <- 20
f <- eval(function(x, y) !!x + !!y))
f
#> function (x, y) 
#> 10 + 20

It should instead show the actual "gotcha" which is the preservation of the bang-bangs:

f
#> function(x, y) !!x + !!y

[greystat]

I was quite puzzled by this as well -- after some digging, it seems like this is an issue with knitr's evaluation order when compiling markdown files. Seems like something about knitr stripping the source reference metadata at the front end when you start the knit, so that by the time you get to the render of that actual chunk it's been reset to NULL.

I'm not certain of why it happens, but I did manage to find a workaround that outputs correctly when knitted by using parse(..., keep.source = TRUE) and wrapping it in eval() directly:

code_txt <- "sum_xy <- function(x, y) {
  # A comment
  x + y
}
"

# Parse the code with srcref preserved
exprs <- parse(text = code_txt, keep.source = TRUE)
eval(exprs)

# Now sum_xy has an srcref!
attr(sum_xy, "srcref")