R writing style - require vs. ::

OK, we're all familiar with double colon operator in R. Whenever I'm about to write some function, I use require(), but I was always thinking about using :: instead. Using require in custom functions is better practice than library, since require returns warning and FALSE, unlike library, which returns error if you provide a name of non-existent package.

On the other hand, :: operator gets the variable from the package, while require loads whole package (at least I hope so), so speed differences came first to my mind. :: must be faster than require.

And I did some analysis in order to check that - I've written two simple functions that load read.systat function from foreign package, with require and :: respectively, hence import Iris.syd dataset that ships with foreign package, replicated functions 1000 times each (which was shamelessly arbitrary), and... crunched some numbers.

Strangely (or not) I found significant differences in terms of user CPU and elapsed time, while there were no significant differences in terms of system CPU. And yet more strange conclusion: :: is actually slower! Documentation for :: is very blunt, and just by looking at sources it's obvious that :: should perform better!

require

#!/usr/local/bin/r

## with require
fn1 <- function() {
  require(foreign)
  read.systat("Iris.syd", to.data.frame=TRUE)
}

## times
n <- 1e3

sink("require.txt")
print(t(replicate(n, system.time(fn1()))))
sink()

double colon

#!/usr/local/bin/r

## with ::
fn2 <- function() {
  foreign::read.systat("Iris.syd", to.data.frame=TRUE)
}

## times
n <- 1e3


sink("double_colon.txt")
print(t(replicate(n, system.time(fn2()))))
sink()

Grab CSV data here. Some stats:

user CPU:     W = 475366    p-value = 0.04738  MRr =  975.866    MRc = 1025.134
system CPU:   W = 503312.5  p-value = 0.7305   MRr = 1003.8125   MRc =  997.1875
elapsed time: W = 403299.5  p-value < 2.2e-16  MRr =  903.7995   MRc = 1097.2005

MRr is mean rank for require, MRc ibid for ::. I must have done something wrong here. It just doesn't make any sense... Execution time for :: seems way faster!!! I may have screwed something up, you shouldn't discard that option...

OK... I've wasted my time in order to see that there is some difference, and I carried out completely useless analysis, so, back to the question:

"Why should one prefer require over :: when writing a function?"

=)