surprising behaviour of names<-

(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