[Rd] is.null(list()) is FALSE ?

[Thomas Lumley]

On Tue, 21 Mar 2000, Torsten Hothorn wrote:

> 
> On R-1.0.0, debian GNU/Linux, 
> 
> R> is.null(list())
> FALSE
> 
> conflicts with the help for ?is.null:
> 
> Examples:
> 
>      is.null(list())    # TRUE (not so in S)
>      is.null(integer(0))# F
>      is.null(logical(0))# F
>      as.null(list(a=1,b='c'))
> 
> and, I would like is.null(list()) to be TRUE :-)

My guess is that this dates from old-style pairlists, where list() could
easily be NULL. Now a list is internally a vector, so list() is a vector
of length zero and so is stored as a VECTOR node.

Also, a difficulty with storing list() as NULL is that you can't set
attributes on NULL:
R> a<-NULL
R> attr(a,"foo")<-"bar"
Error: attempt to set an attribute on NULL
R> a<-list()
R> attr(a,"foo")<-"bar"
R> a
list()
attr(,"foo")
[1] "bar"


	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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