R: How far does it go? (Plus venting) - r

Question

I have an object called defaultPacks, containing the names of packages installed on all the computers I use. Much abbreviated:
defaultPacks <- c(
"AER",
"plyr",
"dplyr"
)
I want to save this object to file in a shared directory all of them can reach. I am using Dropbox for this, with sync always paused when R is running.
save(defaultPacks,
file.path("C:","Users","andrewH","Dropbox","R_PROJ","sharedSettings.rdata"))
Then I want to load the object and install the packages the names of which are in the object defaultPacks.
SyncPacks <- function(fileString){
defaultPacks <- load(file=fileString)
install.packages(defaultPacks, repos="http://cran.us.r-project.org")
}
SyncPacks(file.path("C:","Users","andrewH","Dropbox","R_PROJ","sharedSettings.rdata")
If I do this, I get a warning:
Warning in install.packages: package ‘defaultPacks’ is not available (for R version 3.2.1)
I look what is in defaultPacks immediately after I load and assign it: the string "defaultPacks". So it seems to loading just be a string rather than an object.
So I go back to my save, and try
save(get(defaultPacks), file.path(etc.))
This gives me an different error:
Error in save(get("defaultPacks"), file = file.path("C:", "Users", "andrewH", :
object ‘get("defaultPacks")’ not found.
Then I tried dynGet() with the same result.
So where before it was treating a symbol as a string, now it is treating a function as a string.
So I try the list option for save:
save(list = defaultPacks, file = file.path(etc))
And get yet another error:
Error in save(list = defaultPacks, file = file.path("C:", "Users", "andrewH", :
objects ‘AER’, ‘plyr’, ‘dplyr’, (etc.) not found
So where before I couldn't get to my character vector, now I am shooting right past it, evaluating defaultPacks to find the strings, and then treating each string as a symbol, and evaluating it to its (nonexistent) object.
So, I want to know how to make this work. But I am asking for something more than that. I have this problem, or an analogous problem, all the time. After several years of using R, I still have it a couple of times a week. I don't know how many steps of evaluation R is going to take on any given occasion. I hand a function an object name, and the function treats it as a string. I hand a function a string, and the R function converts it to a symbol and tries to evaluate it. Here, I don't understand why the save function does not save the object I gave it, and then give it back with load.
I've read the discussions on scoping in ten different R books, from Chambers "Software for Data Analysis" to Wickham's "Advanced R." Twice. Four times in some cases. I know about the four environments of a function, and the difference between the call stack and the chain of environmental parents. And yet, it is clear that I am missing something basic. It is not just that I don't know why save does not take a name in its ... argument and save it as an object (unless the problem is at the load end). I don't know how I can know. The function description says, of the ...s, "the names of the objects to be saved (as symbols or character strings)." So why is it saving a name as a string? Or why is load returning a string, if save saved an object? And how could I predict that?
Experienced R programmers, I know you can tell in advance how a given R function is going to treat one of its arguments. You know how far it will be evaluated. You can make it go as far as you want it to, and then STOP. You don't have to write str()'s into your functions every time you want to figure out what the heck it thinks its arguments mean. How do you do it?
Bloody "R Inferno". It's an understatement.

Answer 1

One way of seeing the problem is to note that the value of defaultPacks changes from before to after these operations.
> fname = tempfile()
> orig = defaultPacks = c("AER", "plyr", "dplyr")
> save(defaultPacks, file=fname)
> defaultPacks = load(fname)
> identical(orig, defaultPacks)
[1] FALSE
The problem starts with an understanding of what save() does. From ?save, the object that is saved is named defaultPacks and it has value c("AER", "plyr", "dplyr"). save() could save multiple objects, each with a name and associated value, so it somehow has to save the name of each object.
load() restores the objects that save() has written, and returns (from ?load) a "character vector of the names of objects created". In this case load() restores (creates in the global environment) the symbol defaultPacks, populates it with the character vector of default packages, and returns the name (i.e., character vector of length 1 "defaultPacks") of the object(s) it has restored. The return value then overwrites the restored value, and we have defaultPacks = "defaultPacks".
install.packages doesn't do anything fancy with it's first argument, which from ?install.packages is a "character vector of the names of packages whose current versions should be downloaded". The character vector happens to be the symbol defaultPacks, but the error comes from the value of the symbol, which is the character vector "defaultPacks".
save() and load() more or less have to work the way they do to support multiple objects. On the other hand saveRDS() and readRDS() (ok, why read instead of load?) have a contract to save a single object. The name of the saved object does not need to be stored to be able to recover the values associated with it. So saveRDS(defaultPacks, fname); defaultPacks = readRDS(fname) works, and in particular the value of defaultPacks before and after this series of operations remains unchanged.
> orig = defaultPacks = c("AER", "plyr", "dplyr")
> saveRDS(defaultPacks, fname)
> defaultPacks = readRDS(fname)
> identical(orig, defaultPacks)
[1] TRUE
Without meaning to be too much of a jerk, the answer to the question "Experienced R programmers...how do you do it?" the answer is implied by the ? above -- by carefully reading the manual. Also, there are not that many places in base R code where evaluation is non-standards -- formulas and library are the main culprits -- so recognizing what the problem is not can help to focus on what is actually going on.