By default, `names<-` alters S4 objects

This is basically a case of a user error that is not being caught:

On 5/14/11 3:47 PM, Herv? Pag?s wrote:

Hi,

I was stumped by this. The two S4 objects below looked exactly the same:

a1

An object of class "A"
Slot "aa":
integer(0)

a2

An object of class "A"
Slot "aa":
integer(0)

str(a1)

Formal class 'A' [package ".GlobalEnv"] with 1 slots
..@ aa: int(0)

str(a2)

Formal class 'A' [package ".GlobalEnv"] with 1 slots
..@ aa: int(0)

But they were not identical:

identical(a1,a2)

[1] FALSE

Then I found that one had a "names" attribute but not the other:

names(attributes(a1))

[1] "aa" "class" "names"

names(attributes(a2))

[1] "aa" "class"

names(a1)

NULL

names(a2)

NULL

Which explained why they were not reported as identical.

After tracking the history of 'a1', I found that it was created with
something like:

setClass("A", representation(aa="integer"))

[1] "A"

a1 <- new("A")
names(a1) <- "K"
names(a1)

NULL

So it seems that, by default (i.e. in the absence of a specialized
method), the `names<-` primitive is adding a "names" attribute to the
object. Could this behaviour be modified so it doesn't alter the object?

Eh? But you did alter the object. Not only that, you altered it in what
is technically an invalid way: Adding a names attribute to a class that
has no names slot.

The modification that would make sense would be to give you an error in
the above code. Not a bad idea, but it's likely to generate more
complaints in other contexts, particularly where people don't
distinguish the "list" class from lists with names (the "namedList" class).

A plausible strategy:
1. If the class has a vector data slot and no names slot, assign the
names but with a warning.

2. Otherwise, throw an error.

(I.e., I would prefer an error throughout, but discretion ....)

Comments?

John

Thanks,
H.