(Another discussion out of R-core,
I think anyone in R-devel might have an opinion -- MM):
>>>>> "KH" == Kurt Hornik
<hornik@ci.tuwien.ac.at> writes:
>>>>> Martin Maechler writes:
>>>>> "KH" == Kurt Hornik
<Kurt.Hornik@ci.tuwien.ac.at> writes:
KH> * Bug or feature?
R> NULL + NULL
real(0)
R> NULL > NULL
Error: comparison is possible only for vector types
R> NULL & NULL
Error in NULL & NULL : operations are possible only for numeric or
logical types
KH> whereas S-PLUS:
>>>> NULL > NULL
NULL
>>>> NULL + NULL
NULL
>>>> NULL & NULL
NULL
>> NULL + NULL real(0) and NULL are not so different.. I think R is
>> better here...
>> The other two: R (once more) is stricter in order to help avoiding
>> buggy code.
>> In R,
>>> real(0) > integer(0)
>> logical(0)
>> Therefore, I don't see why NULL > NULL should work...
>> In sum, I think R behaves as we want, and the different behavior
>> should go into the FAQ and probably also be mentioned in some *.Rd
>> files.
KH> Again, I am not entirely convinced yet.
you are right, me neither anymore..
We should discuss this some more
KH> Why does adding two empty objects give something non-NULL? To me,
KH> this seems to violate `nihil nihilo fit' (the ``no-arbitrage
KH> condition'').
I think the paradigm is
NULL := list of length 0 + is sometimes coerced to vector of length 0
~~~~~~~~~
real(0) := numeric(0) = numeric vector of length 0
The real point of discussion is:
-------------------------------------------------------------------------
## In which situations should
## NULL silently be coerced to numeric(0) / logical(0) / character(0) ?
--------------------------------------------------------------------------
Currently, S does this silent coercion very often
while R does not.
I start thinking that maybe R should change here.
KH> Also, why is it o.k. to ADD two empty objects, but not to compare
KH> them? (Shouldn't two NULL objects be the same?)
KH> Btw, the code in chron that got me started on this was
KH> if(any(o1 != o2))
KH> trying to figure out whether two origins (o1, o2) were different.
KH> Under R, this code fails if at least one of the origins is NULL.
KH> What is the right way of coding this? (Surely not the
KH> if(any(o1 - o2))
KH> I changed it to :-))
All was ok, if both o1 and o2 where never NULL but rather numeric(0),
however see above.
-- Martin
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