R7 dispatch and S4 inheritance

[nbenn]

Is it not possible for an R7 generic to understand S4 inheritance? Is this an intentional limitation or a bug? (Or am I simply confused for expecting this to work?)

methods::setClass("general")
methods::setClass("specific", contains = "general")

methods::is(methods::new("specific"), "general")
#> [1] TRUE

gen <- methods::getClass("general")

fun <- R7::new_generic("fun", "x")

R7::method(fun, gen) <- function(x) "ok"

fun(methods::new("specific"))
#> Error: Can't find method for generic `fun()` with dispatch classes:
#> - x: S4/specific

(I'm at RConsortium/OOP-WG@879da2a)

[hadley]

Probably a bug.

[nbenn]

Having a closer look at S4_class_dispatch (), my general class is explicitly being excluded from dispatch at

https://github.com/RConsortium/OOP-WG/blob/879da2af57e24243f509e61778a480c9c5dc3ef0/R/S4.R#L81-L83

as it is a "Virtual Class".

[mmaechler]

Hmm, I've not studied your code nor the details of S4_class_dispatch() but a virtual class in S4 is a class typically meant as a mother class of others or a class to define methods for dispatch.
However there can't be any objects of that S4 class, but just inheriting from that class.
E.g., in the Matrix package there's the virtual class "sparseMatrix" but sparse matrix objects are e.g. of class "dgCMatrix", "lsTMatrix" or ... all inheriting from (or extending) "sparseMatrix".

However, there are many methods defined to dispatch for "sparseMatrix" so maybe there is a bug in S4_class_dispatch() ?

[hadley]

Updated reprex:

library(methods)
library(S7)

setClass("general")
setClass("specific", contains = "general")

fun <- new_generic("fun", "x")
method(fun, getClass("general")) <- function(x) "general"

fun(new("specific"))
#> Error: Can't find method for generic `fun()` with dispatch classes:
#> - x: S4/specific

^{Created on 2023-04-14 with reprex v2.0.2}

[hadley]

Or to give a bit more info about what S7 is actually trying:

library(methods)
library(S7)

setClass("general")
setClass("specific", contains = "general")

fun <- new_generic("fun", "x")
method(fun, getClass("general")) <- function(x) "general"

method_explain(fun, object = new("specific"))
#>    fun([S4/specific])
#>    fun([ANY])

^{Created on 2023-04-15 with reprex v2.0.2}

[hadley]

It looks like S4_class_dispatch() is filtering out virtual classes to avoid dispatching on unions (which are handled elsewhere). It looks like I'll need to use a different method to filter out undesired classes from dispatch.