[Rd] Spurious warning in as.data.frame.factor()

[Martin Maechler]

>>>>> Ivan Krylov 
>>>>>     on Thu, 14 Mar 2024 14:17:38 +0300 writes:

    > On Thu, 14 Mar 2024 10:41:54 +0100
    > Martin Maechler <maechler using stat.math.ethz.ch> wrote:

    >> Anybody trying S7 examples and see if they work w/o producing
    >> wrong warnings?

    > It looks like this is not applicable to S7. If I overwrite
    > as.data.frame with a newly created S7 generic, it fails to dispatch on
    > existing S3 classes:

    > new_generic('as.data.frame', 'x')(factor(1))
    > # Error: Can't find method for `as.data.frame(S3<factor>)`.

    > But there is no need to overwrite the generic, because S7 classes
    > should work with existing S3 generics:

    > foo <- new_class('foo', parent = class_double)
    > method(as.data.frame, foo) <- function(x) structure(
    >   # this is probably not generally correct
    >   list(x),
    >   names = deparse1(substitute(x)),
    >   row.names = seq_len(length(x)),
    >   class = 'data.frame'
    > )
    > str(as.data.frame(foo(pi)))
    > # 'data.frame':   1 obs. of  1 variable:
    > #  $ x: <foo> num 3.14

    > So I think that is nothing to break because S7 methods for
    > as.data.frame will rely on S3 for dispatch.

Yes, as it should be.  Thank you for checking..


    >> > The patch passes make check-devel, but I'm not sure how to safely
    >> > put setGeneric('as.data.frame'); as.data.frame(factor(1:10)) in a
    >> > regression test.  
    >> 
    >> {What's the danger/problem?  we do have "similar" tests in both
    >> src/library/methods/tests/*.R
    >> tests/reg-S4.R
    >> 
    >> -- maybe we can discuss bi-laterally  (or here, as you prefer)
    >> }

    > This might be educational for other people wanting to add a regression
    > test to their patch. I see that tests/reg-tests-1e.R is already running
    > under options(warn = 2), so if I add the following near line 750
    > ("Deprecation of *direct* calls to as.data.frame.<someVector>")...

    > # Should not warn for a call from a derivedDefaultMethod to the raw
    > # S3 method -- implementation detail of S4 dispatch
    > setGeneric('as.data.frame')
    > as.data.frame(factor(1))

    > ...then as.data.frame will remain an S4 generic. Should the test then
    > rm(as.data.frame) and keep going? (Or even keep the S4 generic?) Is
    > there any hidden state I may be breaking for the rest of the test this
    > way? 
    > The test does pass like this, so this may be worrying about nothing.

Indeed, this could be educational;  I think just adding

     removeGeneric('as.data.frame')

is appropriate here as it is self-explaining and should not leave
much traces.

I'm about to test this in reg-tests-1e.R and with make check-all
and commit later today,
thanking you, Ivan!

Martin