[Rd] NULL == NULL

[Thomas Lumley]

On Fri, 15 Sep 2000, Martin Maechler wrote:

> gives an error in R.
> 
> In both S+3 and S+5, it "works" by returning NULL or logical(0),
> respectively.
> 
> As a consequence,
> 
>    all(x == y)
> 
> returns TRUE in Splus, when x <- y <- NULL
> but gives an error in R.
> 
> Do we have a good reason *not* to return  logical(0) ?

I think returning logical(0) is a good idea. Having all(NULL==NULL) be
TRUE is consistent with what we do for vacuous cases of min and max. 

However, a arguable reason for not doing it is that NULL is actually a
list with no elements rather than a vector with no elements, and you can't
compare lists with ==. On the gripping hand, you can't turn NULL into a
vector of length zero using eg unlist(), so the only way to get this
comparison to work is to have NULL==NULL.

	-thomas



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