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
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