On 3/27/16 2:46 PM, peter dalgaard wrote:
On 27 Mar 2016, at 22:05 , Mick Jordan <mick.jor...@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...
Thanks. I didn't look at the C code this time trusting the documentation ;-)
A related question is why are sys.parent/parent.frame so permissive in
their error checking? E.g:
> sys.parent(-1)
[1] 0
> sys.parent(-2)
[1] 0
> sys.parent(1)
[1] 0
> sys.parent(2)
[1] 0
> parent.frame(4)
<environment: R_GlobalEnv>
>
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