Source only part of a file

My R workflow is usually such that I have a file open into which I type R commands, and I’d like to execute those commands in a separately opened R shell.

The easiest way of doing this is to say source('the-file.r') inside R. However, this always reloads the whole file which may take considerable time if big amounts of data are processed. It also requires me to specify the filename again.

Ideally, I’d like to source only a specific line (or lines) from the file (I’m working on a terminal where copy&paste doesn’t work).

source doesn’t seem to offer this functionality. Is there another way of achieving this?

Solution

Using the right tool for the job …

As discussed in the comments, the real solution is to use an IDE that allows sourcing specific parts of a file. There are many existing solutions:

For NeoVim, there’s R.nvim.
For Emacs, there’s ESS.
And of course there’s the excellent stand-alone RStudio IDE.

As a special point of note, all of the above solutions work both locally and on a server (accessed via an SSH connection, say). R can even be run on an HPC cluster — it can still communicate with the IDEs if set up properly.

… or … not.

If, for whatever reason, none of the solutions above work, here’s a small module^[gist] that can do the job. I generally don’t recommend using it, though.¹

#' (Re-)source parts of a file
#'
#' \code{rs} loads, parses and executes parts of a file as if entered into the R
#' console directly (but without implicit echoing).
#'
#' @param filename character string of the filename to read from. If missing,
#' use the last-read filename.
#' @param from first line to parse.
#' @param to last line to parse.
#' @return the value of the last evaluated expression in the source file.
#'
#' @details If both \code{from} and \code{to} are missing, the default is to
#' read the whole file.
rs = local({
    last_file = NULL

    function (filename, from, to = if (missing(from)) -1 else from) {
        if (missing(filename)) filename = last_file

        stopifnot(! is.null(filename))
        stopifnot(is.character(filename))
        
        force(to)
        if (missing(from)) from = 1

        source_lines = scan(filename, what = character(), sep = '\n',
                            skip = from - 1, n = to - from + 1,
                            encoding = 'UTF-8', quiet = TRUE)
        result = withVisible(eval.parent(parse(text = source_lines)))
    
        last_file <<- filename # Only save filename once successfully sourced.
        if (result$visible) result$value else invisible(result$value)
    }
})

Usage example:

# Source the whole file:
rs('some_file.r')
# Re-soure everything (same file):
rs()
# Re-source just the fifth line:
rs(from = 5)
# Re-source lines 5–10
rs(from = 5, to = 10)
# Re-source everything up until line 7:
rs(to = 7)

¹ Funny story: I recently found myself on a cluster with a messed-up configuration that made it impossible to install the required software, but desperately needing to debug an R workflow due to a looming deadline. I literally had no choice but to copy and paste lines of R code into the console manually. This is a situation in which the above might come in handy. And yes, that actually happened.