R/list_merge.R

In ropenscilabs/robotstxt: A 'robots.txt' Parser and 'Webbot'/'Spider'/'Crawler' Permissions Checker

#' @keywords internal
#' @author Kun Ren <mail@renkun.me>
reduce <- function(f, x, init, ...) {
  y <- init
  for (xi in x) y <- f(y, xi, ...)
  y
}

#' Merge a number of named lists in sequential order
#'
#'
#' @author Kun Ren <mail@renkun.me>
#'
#'
#' The function merges a number of lists in sequential order
#' by \code{modifyList}, that is, the later list always
#' modifies the former list and form a merged list, and the
#' resulted list is again being merged with the next list.
#' The process is repeated until all lists in \code{...} or
#' \code{list} are exausted.
#'
#' @details
#' List merging is usually useful in the merging of program
#' settings or configuraion with multiple versions across time,
#' or multiple administrative levels. For example, a program
#' settings may have an initial version in which most keys are
#' defined and specified. In later versions, partial modifications
#' are recorded. In this case, list merging can be useful to merge
#' all versions of settings in release order of these versions. The
#' result is an fully updated settings with all later modifications
#' applied.
#' @param ... named lists
#' @importFrom utils modifyList
list_merge <- function(...) {
  lists <- list(...)
  if (any(vapply(lists, function(x) is.null(names(x)), logical(1L))))
    stop("All arguments must be named list", call. = FALSE)
  reduce(modifyList, lists, list())
}