[Rd] incoherent treatment of NULL

[Martin Maechler]

>>>>> "WK" == Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk at idi.ntnu.no>
>>>>>     on Mon, 23 Mar 2009 10:56:37 +0100 writes:

    WK> Martin Maechler wrote:
    >>>>>>> "WK" == Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk at idi.ntnu.no>
    >>>>>>> 
    >>>>>>> 
    WK> somewhat related to a previous discussion [1] on how 'names<-' would
    WK> sometimes modify its argument in place, and sometimes produce a modified
    WK> copy without changing the original, here's another example of how it
    WK> becomes visible to the user when r makes or doesn't make a copy of an
    WK> object:
    >> 
    WK> x = NULL
    WK> dput(x)
    WK> # NULL
    WK> class(x) = 'integer'
    WK> # error: invalid (NULL) left side of assignment
    >> 
    >> does not happen for me in R-2.8.1,  R-patched or newer
    >> 
    >> So you must be using your own patched version of  R ?
    >> 

    WK> oops, i meant to use 2.8.1 or devel for testing.  you're right, in this
    WK> example there is no error reported in > 2.8.0, but see below.

ok

 [...... omitted part no longer relevant ........]

    WK> however, the following has a different pattern:
    >> 
    WK> x = NULL
    WK> dput(x)
    WK> # NULL
    WK> names(x) = character(0)
    WK> # error: attempt to set an attribute on NULL
    >> 

    WK> i get the error in devel.

Yes,  NULL is NULL is NULL !   Do read  ?NULL !   [ ;-) ]

more verbously,  all NULL objects in R are identical, or as the
help page says, there's only ``*The* NULL Object'' in R,
i.e., NULL cannot get any attributes.

    WK> x = c()
    WK> dput(x)
    WK> # NULL
    WK> names(x) = character(0)
    WK> # error: attempt to set an attribute on NULL
    >> 

    WK> i get the error in devel.

of course!  
   [I think *you* should have noticed that  NULL and c()  *are* identical]

    WK> and also:
    >> 
    WK> x = c()
    WK> class(x) = 'integer'
    WK> # fine
"fine" yes; 
here, the convention has been to change NULL into integer(0);
and no, this won't change, if you find it inconsistent.


    WK> class(x) = 'foo'
    WK> # error: attempt to set an attribute on NULL
    >> 

    WK> i get the error in devel.

No, not if you evaluate the statements above (where 'x' has
become  'integer(0)' in the mean time).

But yes, you get in something like

    x <- c();  class(x) <- "foo"

and I do agree that there's a buglet : 
The error message should be slightly more precise,
--- improvement proposals are welcome ---
but an error nontheless

    WK> it doesn't seem coherent to me:  why can i set the class, 

you cannot set it, you can *change* it.

    WK> but not names
    WK> attribute on both NULL and c()?  why can i set the class attribute to
    WK> 'integer', but not to 'foo', as i could on a non-empty vector:

    WK> x = 1
    WK> class(x) = 'foo'
    WK> # just fine

mainly because 'NULL is NULL is NULL' 
(NULL cannot have attributes)

    WK> i'd naively expect to be able to create an empty vector classed 'foo',

yes, but that expectation is wrong

    WK> displayed perhaps as

    WK> # speculation
    WK> x = NULL
    WK> class(x) = 'foo'
    WK> x
    WK> # foo(0)

    WK> or maybe as

    WK> x
    WK> # NULL
    WK> # attr(, "class")
    WK> # [1] "foo"

    WK> vQ

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