From Martin Maechler <maechler@stat.math.ethz.ch>  Wed Jan  7 08:29:02 1998
From: Martin Maechler <maechler@stat.math.ethz.ch> (Martin Maechler)
Date: Wed, 7 Jan 1998 09:29:02 +0100
Subject: Two more R/S differences
In-Reply-To: <199801070741.IAA08668@pc25.none> (message from Kurt Hornik on
 Wed, 7 Jan 1998 08:41:45 +0100)
Message-ID: <199801070829.JAA03989@sophie.ethz.ch>

(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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Two more R/S differences
Martin Maechler