How can I write code containing comments to a file, using an R function?

Question

I am trying to write an Rscript from within R. Using quote() with the $wholeSrcref attribute works well when on its own. However, additional unwanted lines are written when used within a function. Details are as provided below.

My first attempt was to quote() the expression, followed by writing the quoted expression to file, that is,

expr <- quote({
  a <- 1 # comment 1
  b <- 2 # comment 2
})
writeLines(tail(as.character(expr), -1), file)

However, comments in the code are not written to file. A workaround would be to use the $wholeSrcref attribute, as given by this answer . My second attempt:

library(magrittr)
expr <- quote({
  a <- 1 # comment 1
  b <- 2 # comment 2
})
attributes(expr)$wholeSrcref %>% as.character() %>%
  tail(-1) %>%  # remove '{'
  head(-1) %>%  # remove '}'
writeLines(file)

This works well. However, when wrapping this within a function, part of the function's body is also prepended to the quoted expression. That is, given the following code,

library(magrittr)
f <- function(dir) {
  # change directory to dir
  # do stuff

  expr <- quote({
    a <- 1 # comment 1
    b <- 2 # comment 2
  })

  attributes(expr)$wholeSrcref %>% as.character() %>%
    tail(-1) %>%  # remove '{'
    head(-1) %>%  # remove '}'
  writeLines(file)

  # do more stuff
}
f(dir="")

The file would contain

  # change directory to dir 
  # do stuff 

  expr <- quote({
    a <- 1 # comment 1
    b <- 2 # comment 2

How can I write only the quoted expression to file?

Answer 1

Well, the easiest approach would be to count the lines above the quote function and adjust the tail function to exclude these rows. Admittedly, it's not very elegant

f <- function(dir) {
  # change directory to dir
  # do stuff
  
  expr <- quote({
    a <- 1 # comment 1
    b <- 2 # comment 2
  })
  exclusion_line <- which(grepl("expr <- quote({",
                               attributes(expr)$wholeSrcref %>% as.character(),
                               fixed = TRUE))
  attributes(expr)$wholeSrcref %>% as.character() %>%
    tail(-exclusion_line) %>%  # remove '{'
    head(-1) %>%  # remove '}'
    writeLines(file)
  
  # do more stuff
}
f(dir="")

Answer 2

One way to approach this, as user starja pointed out in his answer , is to find the start of the quoted expression. In the example given in the question, one can search for "expr <- quote({" , as in:

  expr_ref <- attributes(expr)$wholeSrcref %>% as.character()
  exclusion_line <- grep("expr <- quote({", 
                         expr_ref, fixed = TRUE) 

  expr_ref %>%
    tail(-exclusion_line) %>%  # remove up till '{'
    head(-1) %>%  # remove '}'
    writeLines(file)

One caveat is the hard coded "expr <- quote({" . What if we wanted to use another variable name other than "expr" ?. We place the above code in a function and modify it with deparse(substitute(...)) , ie

write_expr <- function(expr, file) {

  expr_name <- deparse(substitute(expr))
  expr_ref <- attr(expr, "wholeSrcref") %>% as.character()
  exclusion_line <- grep(paste0("^\\s*", expr_name, "\\s*<-"), expr_ref)

 expr_ref %>%
    tail(-exclusion_line) %>%  # remove up till '{'
    head(-1) %>%  # remove '}'
    writeLines(file)
}

and modify the f to become

f <- function(dir) {
  # change directory to dir
  # do stuff
  
  some_expr <- quote({
    a <- 1 # comment 1
    b <- 2 # comment 2
  })
  write_expr(some_expr, file)

  # do more stuff
}

Now, upon calling f() , the file contains

    a <- 1 # comment 1
    b <- 2 # comment 2

as desired.