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

[Martin Maechler]

>>>>> "TL" == Thomas Lumley <thomas@biostat.washington.edu> writes:

    TL> 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 :-)

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

and we had decision quite a while ago that  is.null(list()) should give
FALSE {and this is S compatible}.

I've already committed fix for  the wrong comment above :

  cvs diff -r 1.3 src/library/base/man/NULL.Rd

  ...

  diff -r1.3 -r1.3.22.1
  24c24
  < is.null(list())    # TRUE (not so in S)
  ---
  > is.null(list())    # FALSE (on purpose!)


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

    TL> -thomas

    TL> Thomas Lumley
    TL> Assistant Professor, Biostatistics
    TL> University of Washington, Seattle

Martin Maechler <maechler@stat.math.ethz.ch>	http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum  LEO D10	Leonhardstr. 27
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1228			<><
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