How does dplyr::n function work?

Question

I am curious about how the function n from dplyr is programmed. When evaluating n in the dplyr env, all I get is this:

function () 
{
    stop("This function should not be called directly")
}
<environment: namespace:dplyr>

Maybe it's a silly question but, where is it defined? How come it works when called inside some other functions? In which env is it hidden?

Thanks for your help

Answer 1

As far as I understand, dplyr uses hybrid evaluation. That means it will evaluate some parts of the expression in C++ and others in R. n() is one of the functions that always gets handled by C++. This is why the function doesn't do anything in R directly, except for returning an error, since the function is never evaluated by R.

The relevant C++ code can be found on github.