I am trying to make sense why the following does *not* result in
ambiguous method selection and thus a warning:
> setClass("A", slots=c(a = "numeric"))
> setClass("B", slots=c(b = "numeric"))
> setClass("AB", contains=c("A", "B"))
> setGeneric("myg", function(object) standardGeneric("myg"))
[1] "myg"
>
> setMethod("myg", "A", function(object) 1)
> setMethod("myg", "B", function(object) 2)
> ab <- new("AB", a=1, b=2)
> myg(ab)
[1] 1
On the other hand, the following code gives me a warning
> setMethod("+", c("A", "B"), function(e1, e2) 1)
> setMethod("+", c("B", "A"), function(e1, e2) 2)
> ab+ab
Note: method with signature ‘A#B’ chosen for function ‘+’,
target signature ‘AB#AB’.
"B#A" would also be valid
[1] 1
It appears that S4 is using the order of the superclasses A and B for
dispatching, and that this is not regarded as an ambiguity. Is this the
expected behavior? I am using R 4.2.1.
It seems this is contradictory to what the documentation of
methods::setMethod says: "The first possible source of ambiguity arises
if the class has several direct superclasses and methods have been
defined for more than one of those; R will consider these equally valid
and report an ambiguous choice."
Best,
Xiongtao
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