On Wed, 5 Sep 2018, Kevin Ushey wrote:
More generally, I think one of the issues is that R is not yet able to
decrement a reference count (or mark a 'shared' data object as
'unshared' after it knows only one binding to it exists). This means
passing variables to R closures will mark that object as shared:
x <- list()
.Internal(inspect(x)) # NAM(1)
identity(x)
.Internal(inspect(x)) # NAM(3)
I think for this reason users often resort to 'hacks' that involve
directly setting attributes on the object, since they 'know' only one
reference to a particular object exists. I'm not sure if this really
is 'safe', though -- likely not given potential future optimizations
to R, as Tomas has alluded to.
I think true reference counting has been implemented in the R sources,
but the switch has not yet been flipped to enable that by default.
Hopefully having that will make cases like the above work as expected?
Current R-devel built with reference counting by setting
CFLAGS="-O3 -g -Wall -pedantic -DSWITCH_TO_REFCNT"
gives
x <- list()
.Internal(inspect(x))
## @55ad788e3b28 19 VECSXP g0c0 [REF(1)] (len=0, tl=0)
identity(x)
## list()
.Internal(inspect(x))
## @55ad788e3b28 19 VECSXP g0c0 [REF(1)] (len=0, tl=0)
I'm moderately hopeful we'll be able to switch to this for 3.6.0 but
depends on finding enough time to sort out some loose ends.
Best,
luke
Thanks,
Kevin
On Wed, Sep 5, 2018 at 2:19 AM Iñaki Ucar <iu...@fedoraproject.org> wrote:
The bottomline here is that one can always call a base method,
inexpensively and without modifying the object, in, let's say,
*formal* OOP languages. In R, this is not possible in general. It
would be possible if there was always a foo.default, but primitives
use internal dispatch.
I was wondering whether it would be possible to provide a super(x, n)
function which simply causes the dispatching system to avoid "n"
classes in the hierarchy, so that:
x <- structure(list(), class=c("foo", "bar"))
length(super(x, 0)) # looks for a length.foo
length(super(x, 1)) # looks for a length.bar
length(super(x, 2)) # calls the default
length(super(x, Inf)) # calls the default
Iñaki
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Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
241 Schaeffer Hall email: luke-tier...@uiowa.edu
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