Embedding generics when defining methods

[hadley]

library(S7)
foo1 <- new_class("foo1", properties = list(x = class_integer))
foo2 <- new_class("foo2", foo1, properties = list(y = class_double))

method(convert, list(foo1, class_integer)) <- function(from, to) {
  from@x
}
ls()
#> [1] "convert" "foo1"    "foo2"

I think this is going to be a problem because defining a method for a generic defined in another package will end up embedding a copy of that generic in your package. Not sure what we can do here given that this is a fairly deep seated behaviour of <- functions.

[hadley]

Oh maybe method<- could just return NULL? — Nope that doesn't work because if the generic is defined in the current environment, it will reset it.

This almost works:

  if (identical(environment(generic), parent.frame())) {
    invisible(generic)
  } else {
    invisible(NULL)
  }

But isn't quite right either.

Maybe we need to do some post-hoc clean up?

[hadley]

Something like this would work:

#' .onLoad <- function(...) {
#'   S7::methods_register()
#' }
#' S7::package_clean_up()

[hadley]

Maybe if generic is an external generic, return that object instead of the actual generic. Do something to these external generic objects on load.

[hadley]

Maybe this isn't as hard as I thought — method_register() already has all the mechanics to return an external generic if needed, and there's no harm in that object continuing to live in the package namespace. (And in fact there's a small advantage in that we only need to create the external generic object once, rather than once each time).