On 11/13/2009 3:03 PM, Trishank Karthik Kuppusamy wrote:
On Nov 13, 2009, at 2:47 PM, Duncan Murdoch wrote:
Inconsistent with what happens for lists:
> x <- list()
> x$b
NULL
and attributes:
> attr(x, "b")
NULL
Ah, I see. I would claim that the same argument for default safety should apply
here too.
I have mixed feelings about this. If you follow the rule in your
programs that setting x to NULL acts the same as not having x at all,
then things are fine. (Sometimes that's impossible, but it is what
happens when you do the list assignment x$b <- NULL). Use NA or some
other special value to signal missing, and NULL will usually cause a
visible error soon after if you mess up.
It is already a little stricter than $ on a list:
> x$longname <- 1
> x$long
[1] 1
> e$longname <- 1
> e$long
NULL
I apologize that I cannot say that this is a good idea for reasons of safety
and readability.
I think the list behaviour is a bad design, but it's been in the
language forever, so we're stuck with it. It's related to the bad
design of function calls, where arguments can similarly be abbreviated.
Duncan Murdoch
so I supposed we could make it even more strict, but there is an awful lot of
code out there that uses tests like
if (!is.null(x <- e$b)) { do something with x }
and all of that would break.
Unfortunately, such code does make it harder to detect programming errors.
I understand should the hands of R be tied by backwards-compatability; bad
habits are hard to break.
Thanks for your time.
-Trishank
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
