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

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.