sys.function(0)

On 27 Mar 2016, at 22:05 , Mick Jordan <mick.jordan at oracle.com> wrote:

As I understand
https://stat.ethz.ch/R-manual/R-devel/library/base/html/sys.parent.html
sys.function(n) returns the function associated with stack frame n.
Since frame 0 is defined as .GlobalEnv which is not associated with a
function, I would expect this to always return NULL. However, it does not:

sys.function()

NULL

f <- function(x) sys.function(x)
f(0)

function(x) sys.function(x)

f(1)

function(x) sys.function(x)

f(2)

Error in sys.function(x) : not that many frames on the stack

Why the different behavior when sys.function(0) is called inside another
function?

This is a documentation bug. The case "which = 0" differs between sys.frame() and sys.call()/sys.function(). For the latter, it means the current call/function, whereas sys.frame(0) is always the global envir. It is pretty clear from the underlying C code that the three functions treat their argument differently:

R_sysframe has

     if (n == 0)
         return(R_GlobalEnv);

     if (n > 0)
         n = framedepth(cptr) - n;
     else
         n = -n;

whereas the other two (R_syscall and R_sysfunction) omit the special treatment for n==0. Without this, n==0, comes out unchanged from the if-construct, indicating that one should go 0 frames up the stack (same as n==framedepth(cptr)).

Obviously, it won't work to document the "which" argument identically for all three functions...