[suggestion] option purl="comment"

[stla]

Hello knitr developers,

I've modified hook_purl in order to have the option purl="comment". With this option, the code is commented in the output script, even when it is evaluated.

I'm just sharing in case you think this is a good idea (see example below) and you want to add this modification in the package.

My code:

hook_purl2 <- function (before, options, envir)
{
  knitEnv <- getFromNamespace(".knitEnv", "knitr")
  if (before || knitr:::isFALSE(options$purl) || options$engine != "R")
    return()
  output = knitEnv$tangle.file
  if (knitr:::isFALSE(knitEnv$tangle.start)) {
    assign("tangle.start", TRUE, envir=knitEnv)
    unlink(output)
    params = knitEnv$tangle.params
    if (length(params))
      writeLines(params, output)
    assign("tangle.params", NULL, envir=knitEnv)
  }
  code = options$code
  if (knitr:::isFALSE(options$eval) || identical(options$purl, "comment"))
    code = knitr:::comment_out(code, "# ", newline = FALSE)
  if (is.character(output)) {
    cat(knitr:::label_code(code, options$params.src), file = output,
        sep = "\n", append = TRUE)
  }
}

Below is an example of application that illustrates the reason why I did this modification:

<<response-knitexpand, purl="comment">>=
outResp <- NULL
for (iResponse in seq_len(nResponses)){
  outResp <- c(outResp, knit_expand(responseTemplateFile))
}
@

In this chunk, I run knit_expand. Of course I want this code to be evaluated, but I don't want it in the script extracted by hook_purl. However I use purl="comment" instead of purl=FALSE. In this way I get the code as comments in the script. This is helpful to see that the report ran knit_expand at this place.