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

[Duncan Murdoch]

On 22/06/2020 1:40 p.m., Bert Gunter 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?

It's always possible to make a new name, e.g.

fooA <- pkgA::foo
fooB <- pkgB::foo

If you are writing a package, this can be done in the NAMESPACE file, e.g.

importFrom(pkgA, fooA = foo)

though this doesn't appear to be documented in the usual places.

Duncan Murdoch


> 
> 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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