[Rd] Unexpected I(NULL) output

[Martin Maechler]

>>>>> Florent Angly <florent.angly at gmail.com>
>>>>>     on Tue, 20 Dec 2016 13:42:37 +0100 writes:

    > Hi all,
    > I believe there is an issue with passing NULL to the function I().

    > class(NULL)  # "NULL"  (as expected)
    > print(NULL)   # NULL  (as expected)
    > is.null(NULL) # TRUE  (as expected)

    > According to the documentation I() should return a copy of its input
    > with class "AsIs" preprended:

    > class(I(NULL))  # "AsIs"  (as expected)
    > print(I(NULL))   # list()  (not expected! should be NULL)
    > is.null(I(NULL)) # FALSE  (not expected! should be TRUE)

    > So, I() does not behave according to its documentation. 

yes.

    > In R, it is
    > not possible to give NULL attributes, but I(NULL) attempts to do that
    > nonetheless, using the structure() function. Probably:
    > 1/ structure() should not accept NULL as input since the goal of
    > structure() is to set some attributes, something cannot be done on
    > NULL.

I tend to agree.  However if we gave an error now, I notice that
even our own code, e.g., in stats:::formula.default()  would fail.

Still, I think we should consider *deprecating*  structure(NULL, *),
so it would give a *warning* (and continue working otherwise)
(for a while before giving an error a year later).

    > 2/ I() could accept NULL, but, as an exception, not set an "AsIs"
    > class attribute on it. This would be in line with the philosophy of
    > the I() function to return an object that is functionally equivalent
    > to the input object.

If we'd adopt 2, the I(.) function would become slightly more
complicated and slower...  but possibly not practically
noticeable.

A last option would be

3/  The help page for I() could note what happens in the NULL case.

That would be the least work for everyone,
but at the moment, I tend to agree that '1/' is worth the pain to
have R's structure() become more consistent.

Martin Maechler
ETH Zurich

    > My sessionInfo() returns:
    >> sessionInfo()
    > R version 3.3.2 (2016-10-31)
    > Platform: x86_64-w64-mingw32/x64 (64-bit)
    > Running under: Windows 7 x64 (build 7601) Service Pack 1

    > locale:
    > [1] LC_COLLATE=German_Switzerland.1252
    > LC_CTYPE=German_Switzerland.1252
    > LC_MONETARY=German_Switzerland.1252 LC_NUMERIC=C
    > [5] LC_TIME=German_Switzerland.1252

    > attached base packages:
    > [1] stats     graphics  grDevices utils     datasets  methods   base

    > Best regards,

    > Florent

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