[Rd] class(<matrix>) |--> c("matrix", "arrary") [was "head.matrix ..."]

Martin Maechler m@ech|er @end|ng |rom @t@t@m@th@ethz@ch
Mon Nov 11 10:40:11 CET 2019

>>>>> Duncan Murdoch 
>>>>>     on Sun, 10 Nov 2019 11:48:26 -0500 writes:

    > On 10/11/2019 9:17 a.m., Bryan Hanson wrote:
    >>> On Nov 10, 2019, at 3:36 AM, Martin Maechler <maechler using stat.math.ethz.ch> wrote:
    >>>>>>>> Gabriel Becker
    >>>>>>>> on Sat, 2 Nov 2019 12:37:08 -0700 writes:
    >>>> I agree that we can be careful and narrow and still see a
    >>>> nice improvement in behavior. While Herve's point is valid
    >>>> and I understand his frustration, I think staying within
    >>>> the matrix vs c(matrix, array) space is the right scope
    >>>> for this work in terms of fiddling with inheritance.
    >>> [.................]
    >>>>> Also, we seem to have a rule that inherits(x, c)  iff  c %in% class(x),
    >>>> good point, and that's why my usage of  inherits(.,.) was not
    >>>> quite to the point.  [OTOH, it was to the point, as indeed from
    >>>> the ?class / ?inherits docu, S3 method dispatch and inherits
    >>>> must be consistent ]
    >>>>> which would break -- unless we change class(x) to return the whole
    >>>> set of inherited classes, which I sense that we'd rather not do....
    >>> [................]
    >>>> Note again that both "matrix" and "array" are special [see ?class] as
    >>>> being of  __implicit class__  and I am considering that this
    >>>> implicit class behavior for these two should be slightly
    >>>> changed ....
    >>>> And indeed I think you are right on spot and this would mean
    >>>> that indeed the implicit class
    >>>> "matrix" should rather become c("matrix", "array").
    >>> I've made up my mind (and not been contradicted by my fellow R
    >>> corers) to try go there for  R 4.0.0   next April.
    >>> I've found the few places in base R that needed a change (to
    >>> pass 'make check-all' in the R sources) and found that indeed a
    >>> overzealous check in 'Matrix' needed also a change (a place
    >>> where the checking code assume  class(<matrix>) |--> "matrix" ).
    >>> There are certainly many more package (codes and checks) that
    >>> need adaption .. i.e., should be changed rather *before* the
    >>> above change is activated in R-devel (and then will affect all CRAN
    >>> and Bioconductor checks.)
    >>> To this end, I've published an  'R Blog' yesterday,
    >>> http://bit.ly/R_blog_class_think_2x
    >>> which translates to
    >>> https://developer.r-project.org/Blog/public/2019/11/09/when-you-think-class.-think-again/index.html
    >>> notably mentioning why using  class(x) == "...."  (or '!=')  or
    >>> switch(class(.) ...)  is quite unsafe and hence bad and you
    >>> should very often not replace  class(x)  by  class(x)[1]  but
    >>> really use the "only truly correct" ;-)
    >>> inherits(x,  "...")
    >>> or
    >>> is(x,  "....")   # if you're advanced/brave enough (:-) to
    >>> # use formal classes (S4)
    >> Thanks for the helpful blog post Martin. Is the following
    >> “test_class”  %in% class(some_object)
    >> which I think in your symbols would be
    >> “…” %in% class(x)
    >> safe as far as you see it? By safe, I mean equivalent to your suggestion of inherits(x, “…”) .

    > Those aren't equivalent if S4 gets involved.  You can see it if you run 
    > this code:

    > example("new") # Creates an object named t2 of class "trackcurve"
    >                # that contains "track"

    > inherits(t2, "track")  # TRUE
    > "track" %in% class(t2) # FALSE

    > I can't think of any examples not involving S4.

    > Duncan Murdoch

Thank you, Duncan.
That's definitely a strong reason for inherits(), because often
in such code, you don't know in advance what objects will be
passed to your function.

On Twitter, others have asked "the same",  arguing that 

	"<someclass>"  %in%  class(.)

> uses usual syntax, and thus looks less intimidating than
> inherit() and less cryptic than is()

I think you should all use -- and *teach* --
inherits(.) more often, and it would no longer be intimidating.

Also, for the speed fetishists:  inherits() will typically be 
slightly (but significantly) faster than  ` %in% class(.) `


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