In R, every function argument may be 1) ignored, 2) accessed as an AST, 3) passed to another function, or 4) accessed as a value, or some combination thereof. N

In R, every function argument may be 1) ignored, 2) accessed as an AST, 3) passed to another function, or 4) accessed as a value, or some combination thereof. Note that 1, 2, and 3 don’t result in the expression being evaluated, as function arguments are lazy in R. There’s no way to look at only a function’s signature and determine whether a given argument is going to be evaluated never, once, or multiple times, or conditionally (though the vast majority of the time it’s “once”). On the plus side it does allow for some extremely elegant libraries to be written.