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

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)
     
Martin Maechler
ETH Zurich and R Core Team