[Rd] incoherent treatment of NULL

[Martin Maechler]

>>>>> "WK" == Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk at idi.ntnu.no>
>>>>>     on Mon, 23 Mar 2009 09:52:19 +0100 writes:

    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> x = c()
    WK> dput(x)
    WK> # NULL
    WK> class(x) = 'integer'
    WK> dput(x)
    WK> # integer(0)

    WK> in both cases, x ends up with the value NULL (the no-value object).  in
    WK> both cases, dput explains that x is NULL.  in both cases, an attempt is
    WK> made to make x be an empty integer vector.  the first fails, because it
    WK> tries to modify NULL itself, the latter apparently does not and succeeds.

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

    WK> and also:

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

    WK> how come?  the behaviour can obviously be explained by looking at the
    WK> source code (hardly surprisingly, because it is as it is because the
    WK> source is as it is), and referring to the NAMED property (i.e., the
    WK> sxpinfo.named field of a SEXPREC struct).  but can the *design* be
    WK> justified?  can the apparent incoherences visible above the interface be
    WK> defended? 

    WK> why should the first example above be unable to produce an empty integer
    WK> vector? 

    WK> why is it possible to set a class attribute, but not a names attribute,
    WK> on c()? 

    WK> why is it possible to set the class attribute in c() to 'integer', but
    WK> not to 'foo'? 

    WK> why are there different error messages for apparently the same problem?


    WK> vQ


    WK> [1] search the rd archives for 'surprising behaviour of names<-'

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