make it possible to set options without persisting

.Rbuildignore

@@ -7,3 +7,4 @@
 ^scenarios/
 ^configure.R$
 ^NEWS.md$
+^\.Rprofile$

R/options.R

@@ -138,37 +138,45 @@ get_opts <- function(options = NULL, simplify = TRUE, project = NULL) {
 
 make_setter <- function(name) {
   force(name)
-  function(x) {
+  function(x, persist = TRUE) {
     if (missing(x)) return(get_opts(name))
-    else do.call(set_opts, setNames(list(x), name))
+    else setOptions(setNames(list(x), name), persist = persist)
   }
 }
 
 ##' @rdname packrat-options
 ##' @name packrat-options
 ##' @export
 set_opts <- function(..., project = NULL) {
+  setOptions(list(...), project = project)
+}
+
+setOptions <- function(options, project = NULL, persist = TRUE) {
 
   project <- getProjectDir(project)
   optsPath <- packratOptionsFilePath(project)
 
-  if (!file.exists(optsPath)) {
+  if (persist && !file.exists(optsPath)) {
     dir.create(dirname(optsPath), recursive = TRUE, showWarnings = FALSE)
     file.create(optsPath)
   }
-  dots <- list(...)
-  validateOptions(dots)
-  keys <- names(dots)
-  values <- dots
+
+  validateOptions(options)
+  keys <- names(options)
+  values <- options
   opts <- read_opts(project = project)
   for (i in seq_along(keys)) {
     if (is.null(values[[i]]))
       opts[keys[[i]]] <- list(NULL)
     else
       opts[[keys[[i]]]] <- values[[i]]
   }
-  write_opts(opts, project = project)
-  updateSettings(project)
+
+  write_opts(opts, project = project, persist = persist)
+
+  if (persist)
+    updateSettings(project)
+
   invisible(opts)
 }
 
@@ -227,7 +235,7 @@ readOptsFile <- function(path) {
 read_opts <- function(project = NULL) {
   project <- getProjectDir(project)
   path <- packratOptionsFilePath(project)
-  if (!file.exists(path)) return(invisible(NULL))
+  if (!file.exists(path)) return(list())
   opts <- readOptsFile(path)
   if (!length(opts)) return(list())
   opts[] <- lapply(opts, function(x) {
@@ -244,7 +252,8 @@ read_opts <- function(project = NULL) {
   opts
 }
 
-write_opts <- function(options, project = NULL) {
+write_opts <- function(options, project = NULL, persist = TRUE) {
+
   project <- getProjectDir(project)
   if (!is.list(options))
     stop("Expecting options as an R list of values")
@@ -274,6 +283,10 @@ write_opts <- function(options, project = NULL) {
   # Update the in-memory options cache
   assign("options", options, envir = .packrat)
 
+  # Write options to disk
+  if (!persist)
+    return(invisible(TRUE))
+
   sep <- ifelse(
     unlist(lapply(options, length)) > 1,
     ":\n",