[R-pkg-devel] Two packages with the same generic function

[Mark Leeds]

Hi Duncan: I maintain dynlm and your example is the exact reason I've been
getting emails from people regarding
it not working correctly. I've been telling them to load dplyr by using

library(dplyr, exclude = c("filter", "lag"))





On Mon, Jun 22, 2020 at 7:57 PM Duncan Murdoch <murdoch.duncan using gmail.com>
wrote:

> On 22/06/2020 3:48 p.m., Tom Wainwright wrote:
> > Yet another alternative is simply to prevent your second package from
> > overriding the previously defined generic. The basic problem is the ease
> > with which R allows overriding prior generic definitions (one of those
> bits
> > of bad behavior we in the USA used to call "a Bozo No-No"), which hides
> all
> > the previous methods, as demonstrated by the following code:
> >
> >> plot(1:3)
> >>> plot <- function(x, ...) UseMethod("plot")
> >>> plot(1:3)
> >> Error in UseMethod("plot") :
> >>    no applicable method for 'plot' applied to an object of class
> >> "c('integer', 'numeric')"
> >>> rm(plot)
> >>> plot(1:3)
> >
> >
> > (Despite Murdoch's suggestion that overriding the generic SHOULD issue a
> > warning, it doesn't seem to in R 4.0.1.)
>
> Sure it does, if pkgA and pkgB both export the same name, then you get a
> warning when you attach the second one.  For example,
>
>  > library(MASS)
>  > library(dplyr)
>
> Attaching package: ‘dplyr’
>
> The following object is masked from ‘package:MASS’:
>
>      select
>
> The following objects are masked from ‘package:stats’:
>
>      filter, lag
>
> The following objects are masked from ‘package:base’:
>
>      intersect, setdiff, setequal, union
>
> Users don't get warned about overriding names in packages they've
> loaded, because that would just be irritating.
>
> Duncan Murdoch
>
> >
> > So, we might try protecting the generic definitions of "foo" in both
> > packages by enclosing them in something like:
> >
> > tryCatch(invisible(methods("foo")), error = {foo <- function(x,...)
> >> UseMethod("foo")}, finally=NULL)
> >
> >
> > There's probably a more elegant way to accomplish this. This relies on
> > "methods" returning an error if "foo" has no defined methods, so it is
> not
> > redefined if their are previous methods. I haven't had time to try this
> in
> > the two-package example, but it might work, although I'm not sure how to
> > handle the Namespace declarations.
> >
> >    Tom Wainwright
> >
> > On Mon, Jun 22, 2020 at 10:41 AM Bert Gunter <bgunter.4567 using gmail.com>
> wrote:
> >
> >> ...
> >> and just to add to the query, assume the author of pkg B did (does) not
> >> know of pkg A and so, for example, could (did) not import any of pkg A's
> >> content into B. Given that there are at the moment ~20,000 packages out
> >> there, this does not seem to be an unreasonable assumption. One may even
> >> further assume that the user may not know that (s)he has package B
> loaded,
> >> as it may be a dependency of another package that (s)he uses. I
> certainly
> >> don't keep track of all the dependencies of packages I use.
> >>
> >> Under these assumptions, is there any more convenient alternative to
> >> Wolfgang's pkgA:foo(x) explicit call under such assumptions? If pkgA
> has a
> >> long name, what might one do?
> >>
> >> Bert Gunter
> >>
> >> "The trouble with having an open mind is that people keep coming along
> and
> >> sticking things into it."
> >> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
> >>
> >>
> >> On Mon, Jun 22, 2020 at 10:00 AM Viechtbauer, Wolfgang (SP) <
> >> wolfgang.viechtbauer using maastrichtuniversity.nl> wrote:
> >>
> >>> Hi All,
> >>>
> >>> Let's say there are two packages pkgA and pkgB, both of which have a
> >>> generic function
> >>>
> >>> foo <- function(x, ...)
> >>>     UseMethod("foo")
> >>>
> >>> and pkgA has a method for objects of class "A":
> >>>
> >>> foo.A <- function(x, ...)
> >>>     print(x)
> >>>
> >>> and pkgB has a method for objects of class "B":
> >>>
> >>> foo.B <- function(x, ...)
> >>>     plot(x)
> >>>
> >>> Both packages export foo and their method and declare their respective
> S3
> >>> methods, so:
> >>>
> >>> export(foo)
> >>> export(foo.A)
> >>> S3method(foo, A)
> >>>
> >>> in NAMESPACE of pkgA and
> >>>
> >>> export(foo)
> >>> export(foo.B)
> >>> S3method(foo, B)
> >>>
> >>> in NAMESPACE of pkgB.
> >>>
> >>> If a user loads pkgA first and then pkgB, this fails:
> >>>
> >>> library(pkgA)
> >>> library(pkgB)
> >>> x <- 1:4
> >>> class(x) <- "A"
> >>> foo(x)
> >>>
> >>> Error in UseMethod("foo") :
> >>>    no applicable method for 'foo' applied to an object of class "A"
> >>>
> >>> and vice-versa. Of course, pkgA::foo(x) works. Aside from pkgA
> importing
> >>> foo() or vice-versa, is there some other clever way to make this work?
> In
> >>> earlier versions of R (at least in 3.6.3), this used to work (i.e., the
> >>> generic foo() from pkgB would find method foo.A() and vice-versa), but
> >> not
> >>> since 4.0.0.
> >>>
> >>> Best,
> >>> Wolfgang
> >>>
> >>> ______________________________________________
> >>> R-package-devel using r-project.org mailing list
> >>> https://stat.ethz.ch/mailman/listinfo/r-package-devel
> >>>
> >>
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