[Rd] Discrepancy between is.list() and is(x, "list")

William Dunlap wdun|@p @end|ng |rom t|bco@com
Tue Mar 26 23:00:29 CET 2019

I think this goes back to SV4 (c. late 1990's).  The is.<type>  functions
are much older (c. mid 1970's) , from before any class system was in S.
is() and inherits() were introduced with the S4 class system and were meant
to escape from the prison made by ancient design choices.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Tue, Mar 26, 2019 at 2:11 PM Abs Spurdle <spurdle.a using gmail.com> wrote:

> If I can merge this thread with the one I started yesterday...
> > "If the object does not have a class attribute, it has an implicit
> class..."
> > which I take to mean that if an object does have a class attribute it
> does not also have an implicit class.
> > I think this is reasonable behavior. Consider the "Date" class, which
> stores values as "numeric":
> > > class(Sys.Date())
> > [1] "Date"
> > > inherits(Sys.Date(),"numeric")
> > [1] FALSE
> > > class(unclass(Sys.Date()))
> > [1] "numeric"
> > > Sys.Date()%%2
> > Error in Ops.Date(Sys.Date(), 2) : %% not defined for "Date" objects
> > Letting the modulus operator (as one example) inherit the numeric class
> here could create problems.
> I disagree.
> A date object should probably extend integers rather than numerics, in the
> first place.
> However, if it extends numeric, then it extends numeric, otherwise it's a
> contradiction.
> So, inherits(Sys.Date(),"numeric") should return true.
> Modulo operators should be defined for both dates and numerics.
> However, the application of modulo operators to dates, is perhaps unclear,
> at least in the general case, anyway.
> > so instead of hitting utils:::head.function, it hits utils:::head.default
> > I also see this behavior at least as far aback as 3.5.1, so its not new
> to 3.5.3.
> These seem like significant design flaws.
> Implicit classes or whatever you want to call them, are clearly part of the
> class hierarchy.
> They should be included in inherits(), is() and standard method dispatch,
> regardless of whether they are part of the class vector or not.
> Also, is this something that was introduced in R 3.5.1?
> The only thing worse than a design flaw is a design flaw that isn't
> backward compatible.
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