r - Global variable in a package - which approach is more recommended?

Question

I do understand that generally global variables are evil and I should avoid them, but if my package does need to have a global variable, which of these two approaches are better? And are there any other recommended approaches?

1. Using an environment visible to the package

   pkgEnv <- new.env()  
   pkgEnv$sessionId <- "xyz123"
   

2. Using options

   options("pkgEnv.sessionId" = "xyz123")
   

I know there are some other threads that ask about how to achieve global variables, but I haven't seen a discussion on which one is recommended

Answer 1

Some packages use hidden variables (variables that begin with a .), like .Random.seed and .Last.value do in base R. In your package you could do

e <- new.env()
assign(".sessionId", "xyz123", envir = e)
ls(e)
# character(0)
ls(e, all = TRUE)
# [1] ".sessionId"

But in your package you don't need to assign e. You can use a .onLoad() hook to assign the variable upon loading the package.

.onLoad <- function(libname, pkgname) {
    assign(".sessionId", "xyz123", envir = parent.env(environment()))
}

See this question and its answers for some good explanation on package variables.