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

[Bryan Hanson]

> 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, “…”) .

Thanks, Bryan