[Rd] surprising behaviour of names<-

[Wacek Kusnierczyk]

Stavros Macrakis wrote:
>>> (B) you cannot (easily) predict whether or not x will be modified
>>> destructively
>>>       
>> that's fine, thanks, but i must be terribly stupid as i do not see how
>> this explains the examples above.  where is the x used by something else
>> in the first example, so that 'names<-'(x, 'foo') does *not* modify x
>> destructively, while it does in the other cases?
>>
>> i just can't see how your explanation fits the examples -- it probably
>> does, but i beg you show it explicitly.
>>     
>
> I think the following shows what Peter was referring to:
>
> In this case, there is only one pointer to the value of x:
>
> x <- c(1,2)
>   
>> "names<-"(x,"foo")
>>     
>  foo <NA>
>    1    2
>   
>> x
>>     
>  foo <NA>
>    1    2
>
> In this case, there are two:
>
>   
>> x <- c(1,2)
>> y <- x
>> "names<-"(x,"foo")
>>     
>  foo <NA>
>    1    2
>   
>> x
>>     
> [1] 1 2
>   
>> y
>>     
> [1] 1 2
>   

that is and was clear to me, but none of my examples was of the second
form, and hence i think peter's answer did not answer my question. 
what's the difference here:

    x = 1
    'names<-'(x, 'foo')
    names(x)
    # NULL

    x = c(foo=1)
    'names<-'(x, 'foo')
    names(x)
    # "foo"

certainly not something like what you show.   what's the difference here:

    x = 1
    'names<-'(x, 'foo')
    names(x)
    # NULL
  
    x = 1:2
    'names<-'(x, c('foo', 'bar'))
    names(x)
    # "foo" "bar"

certainly not something like what you show.

> It seems as though `names<-` and the like cannot be treated as R
> functions (which do not modify their arguments) but as special
> internal routines which do sometimes modify their arguments.
>   

they seem to behave somewhat like macros:

    'names<-'(a, b)

with the destructive 'names<-' is sort of replaced with

    a = 'names<-'(a, b)

with a functional 'names<-'.  but this still does not explain the
incoherence above.  my problem was and is not that 'names<-' is not a
pure function, but that it sometimes is, sometimes is not, without any
obvious explanation.  that is, i suspect (not claim) that the behaviour
is not a design feature, but an incident.

vQ