[Rd] package slot of generic "[" and missing env at target?

Hin-Tak Leung htl10 at users.sourceforge.net
Fri Jun 22 00:02:00 CEST 2012


Since upgraded to R 2.15, I have a problem with duplicate S4 class name no longer works (the reason for having duplicate S4 class names is just software forks - they are largely identical but don't have an inheritence relationship, and will never have such). 

This is happening with "library(mypackage2)" :

Error: package slot missing from signature for generic ‘[’
and classes myclass, ANY, ANY, ANY
cannot use with duplicate class names (the package may need to be re-installed)

I have done "debug(methods:::.pkgMethodLabel)" and found that curiously, it is invoked for the two copies of the "[" generic with signature "classes myclass, ANY, ANY, ANY", ones with 
packageSlot(method at target) sets to c("mypackage2", "methods", "methods", "methods"), the other just c("", "", "",""), instead of  
c("mypackage1", "methods", "methods", "methods").

So in the R debugger, I modified the packageSlot() directly, and get library(mypacakge2) to finish loading, a few times (once in a while R craps out with some error message about LANGSXP - I know it is probably a bad idea to modify data structures within the debugger, so that's probably expected).

So my next step was simply to patch R/library/methods to downgrade the stop() to a warning(), as well as inserting the missing packageSlot values there and then.

This got both packages loaded. But then, some actual use of "[", it gives this error instead:

Error in .methodPackageSlots(def, label, table, dupl, isCurrent) : 
  trying to get slot "target" from an object of a basic class ("environment") with no slots

and when I downgrade this error to a warning again, modifying (src/main/attrib.c) I got "[" dispatched to the base class (as well as stack imbalance - the latter is expected and "asking for it"...).

Anyway, I am thinking:

- doing "debug(methods:::.pkgMethodLabel)", I see two "[" of signature "classes myclass, ANY, ANY, ANY", one comes with packageSlot with the correct stuff and one with just 4 "", that seems to be a bug?

- from the warning at the beginning: 
"A specification for class “myclass” in package ‘mypackage1’ seems equivalent to one from package ‘mypackage2’ and is not turning on duplicate class definitions for this class"

It would appear that there is a "correct" way of turning on duplicate class definition, but I have not found any documentation to that effect.
I see some discussion about class unions, and inheritances, but in this case, neither are appropriate - the two packages are just forks and have no union nor inheritance relationship; but it is desirable to load both, to check for differences and bugs.

- I am also curious why only "[" triggers the problem, but not "[<-" or another of the other duplicated methods. Obviously subsetting is being treated specially, as is mentioned a few times in R internals; but I was expecting subassignment to be affected also, and that does not seems to be the case.

Anyhow, it is an extremely useful feature to be able to load a past version of a package simultaneously just by renaming the package name and namespace, to check for regression and change-in-behavior, i.e. newly added bugs; so I am hoping to get this working again.


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