export(x=list_of_data_frames, file="file%s.csv", ...)

#' Write a list of data.frames to multiple .csv files.
#'
#' @details The \code{path} may be one of three path specifications:
#' \itemize{
#'   \item{A character vector the same length as \code{x} in which case the
#'   filenames are specified as that character vector.}
#'   \item{A character scalar with a single "%%s" indicating where the name of
#'   \code{x} should be inserted.  (Note, \code{sprintf()} is not used to
#'   generate the output filename.)}
#'   \item{A character scalar where the name of \code{x} will be inserted before
#'   the file extension.}
#' }
#' 
#' Names of \code{x} must be unique, or \code{x} must not have a name.  If
#' \code{x} has no names, then integers with left-padded zeros are used.
#'
#' @param x A list of data.frames
#' @param path The filename specification (see the Details section)
#' @return The list of files created (invisibly)
#' @examples
#' \dontrun{
#' example_data <- list(A=data.frame(A=1), B=data.frame(B=1))
#' write_csv_list(example_data, file.path(tempdir(), "test-%s.csv"))
#' }
#' @export
write_csv_list <- function(x, path, ...) {
  if (!is.character(path)) {
    stop("`path` must be a character scalar or vector.")
  }
  if (is.data.frame(x)) {
    stop("`x` must be a list of data.frames, not a data.frame.")
  } else if (!is.list(x)) {
    stop("`x` must be a list of data.frames.")
  } else if (!all(sapply(X=x, FUN=is.data.frame))) {
    stop("`x` must be a list of data.frames, not a list including any other object classes.")
  }
  x_names <-
    if (is.null(names(x))) {
      gsub(
        pattern=" ",
        replacement="0",
        x=format(seq_along(x), justify="right"),
        fixed=TRUE
      )
    } else {
      names(x)
    }
  if (any(x_names %in% "")) {
    stop("All or none of the elements of `x` must be named, empty names are not allowed.")
  } else if (any(duplicated(x_names))) {
    stop("All names of `x` must be unique.")
  }
  find_percent_s <- strsplit(path[1], split="%s", fixed=TRUE)[[1]]
  paths <-
    if (length(path) == length(x)) {
      if (any(duplicated(path))) {
        stop("All values of `path` must all be unique.")
      }
      path
    } else if (length(path) != 1) {
      stop("`path` must either be a single character string or a vector the same length as `x`.")
    } else if (length(find_percent_s) > 2) {
      stop("`path` may only contain a single '%s'.")
    } else if (length(find_percent_s) == 2) {
      paste0(find_percent_s[1], x_names, find_percent_s[2])
    } else if (length(find_percent_s) == 1) {
      message(
        "When giving a single character string for `path`, it is recommended ",
        "to insert '%s' where the names of `x` should be inserted into the ",
        "path name.  The name of `x` will be inserted before the file ",
        "extension."
      )
      file_ext <- rio::get_ext(path)
      tmp_path <- gsub(pattern=paste0(".", file_ext, "$"), replacement="", x=path)
      paste0(tmp_path, x_names, ".", file_ext)
    }
  for (idx in seq_along(x)) {
    write_csv(x=x[[idx]], path=paths[[idx]], ...)
  }
  invisible(paths)
}