'identical' and the warning "ignoring non-pairlist attributes"

On Dec 11, 2010, at 11:23 , peter dalgaard wrote:

On Dec 11, 2010, at 08:23 , Niels Richard Hansen wrote:

Dear R developers

Using the 'foreach' package I encounter warnings like

Warning message: In identical(.combine, cbind) : ignoring non-pairlist
attributes

Warnings appear once in a new R-session when I run a particular piece of
code - and not again. I understand from the source code of 'identical'
that the function checks and believes that .combine and cbind have
non-pairlist attributes, but I don't understand how this can be the case

I have produced the following minimal example

data.frame()[rep(1,1e6), ] library(foreach)
identical(foreach:::defcombine, cbind)

resulting in

[1] FALSE Warning message: In identical(foreach:::defcombine, cbind) :
ignoring non-pairlist attributes

The warning is not related to the foreach package it seems. I get the
same kind of warning with e.g.

identical(spatial:::trmat, cbind)

in a new R-session. Many other functions I have tried do, however, not
result in the warning. The warning depends on the supposedly completely
unrelated data frame operation. It seems also to be related to the fact
that the data frame is empty and I extract a large number of columns out
of this empty data frame. I would highly appreciate help on this to get
rid of the warning in my own package.

Hm, I think you might have tickled a buglet here. I can debug it down to a
comparison of two CHARSXPs, both having CHARSXP attributes. I'm a getting a
little rusty on debugging at that level, but I don't think CHARSXPs are
supposed to _have_ attributes in any meaningful context, so perhaps
identical() just shouldn't check. On the other hand, it could also be the
sign of a memory overrun in the preceding memory-intensive operation.

Robert Gentleman pointed out offlist that attributes of CHARSXP are being
used for cacheing and hashing, so skipping the comparison is what makes
sense. I have fixed this for R-devel now (tampering with 2.12.1 RC for this
sort of reason is probably best avoided).