A versatile re-source file function for R

rs.r

#' (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,
            blank.lines.skip = FALSE
        )

        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)
    }
})