Because the signature is always (A,A) or (B,B). Then, as in AB we have A and B
and no relationship between A and B, R chooses the method lexicographically.
The result is as expected: f for A is chosen.
If you would do something like:
setClass("A", contains = "list")
setClass("B", contains = "list")
setClass("AB", contains = c("A", "B"))
setGeneric("f", function(x, y) standardGeneric("f"))
setMethod("f", signature("A", "ANY"), function(x, y) "A-ANY")
setMethod("f", signature("ANY", "A"), function(x, y) "ANY-A")
setMethod("f", signature("B", "B"), function(x, y) "B-B")
ab <- new("AB")
f(ab, ab)
You get an ambiguity, as there is no function with signature pair that can be
directly matched for A. But for B such a signature pair exists. So R chooses B.
If you would specify again a function for signature (A,A) we are back:
setMethod("f", signature("A", "A"), function(x, y) "A-A")
f(ab, ab)
Best
Simon
On Aug 14, 2013, at 5:25 PM, Hadley Wickham <[email protected]> wrote:
>> In my opinion the reason for the behavior lies in the specific multiple
>> inheritance structure between AB, B and A.
>
> So what if we don't make such a weird inheritance structure, and
> instead have A and B inherit from a common parent:
>
> setClass("A", contains = "list")
> setClass("B", contains = "list")
> setClass("AB", contains = c("A", "B"))
>
> setGeneric("f", function(x, y) standardGeneric("f"))
> setMethod("f", signature("A", "A"), function(x, y) "A-A")
> setMethod("f", signature("B", "B"), function(x, y) "B-B")
>
> ab <- new("AB")
> f(ab, ab)
>
> Why isn't there a warning about ambiguous dispatch?
>
> Hadley
>
> --
> Chief Scientist, RStudio
> http://had.co.nz/
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