R-alpha: (minor?) S-R inconsistency: NULL =~= list() -- useful is.ALL function

[Thomas Lumley]

On Fri, 15 Aug 1997, Martin Maechler wrote:
> In S, 
> 	NULL
> and
> 	list()
> are not the same.
> 
> In R they are (I think).
> 

The thing that really broke your code is an incompatibility I reported a
while back that people didn't seem to think was serious.  

Comparisons involving NULL give an error in R, but give NA in S.  I didn't
have an example at the time except to point out that returning NA would at
least allow correct short-circuiting of & and | conditions. 

I think in match() there is a much stronger argument for returning NA (or
the nomatch value).  After all, the description of match says

             If x[i] is found to equal table[j] then the value
             returned in the i-th position of the return value is j.
             If no match is found, the value is nomatch.

Surely if table==NULL then the correct return value is nomatch.

Thomas Lumley
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