Transitioning S3 generics to S7 generics

[teunbrand]

Hi there,

A problem I'm encountering is when I have a pkgA::foo() generic that I want to transition from S3 to S7.
All works fine within pkgA, that I can transition as a whole.
The issue arises when a second package has an pkgB::foo.their_class() S3 method for the pkgA::foo() generic.
It appears that we cannot register S3 methods for S7 generics, so the relationship between the generic and the external method becomes dysfunctional.
In an ideal world we would have pkgB transition from an S3 method to S7 method, but this isn't currently realistic for my case.

At the risk of advice seeking, I'll ask: are there ways to transition gracefully?
If there are, please consider this a request for clarification in the documentation.
If not, is this something that you may consider?

[teunbrand]

One consideration is that I can define my default method like so:

library(S7)
foo <- new_generic("foo", "x")

# Default method dispatches to S3
method(foo, class_any) <- function(x, ...) {
  UseMethod("foo")
}

# Define a 'true' default method
foo.default <- function(x) paste0(x, " + 10")

Which allows me to keep writing S7 methods to that generic:

method(foo, class_numeric) <- function(x) x + 10

But also allows others to define S3 methods:

foo.factor <- function(x) {
  levels(x) <- paste0(levels(x), " + 10")
  x
}

Which works to a superficial degree, but doesn't quite invoke the right NextMethod() and such.

[hadley]

Hmmmm, I'm reasonably certain our intent was for this to work automatically, so we may need to double check that S7 generics fall back to S3 dispatch.

[teunbrand]

That'd help a lot for packages that transition to S7 and have reverse dependencies. I have tried to write S3 methods for S7 generics and that failed, so I'm pretty sure that there isn't a fallback mechanism active.

[jimbrig]

im in the same boat as @teunbrand where I need interoperable S3 and S7 class systems. for example, say I am developing an API client wrapping httr2 and would like to define my own request and response classes that still leverage httr2_request and httr2_response S3 classes and print methods, etc.

[lawremi]

The solution described above is the unofficially supported one for now. It's not clear whether would ever have S7 generics fall back to UseMethod() automatically. S7 and S3 generics are fundamentally different things.

[hadley]

@lawremi what if we made it opt-in? Like foo <- new_generic("foo", "x", fallback_s3 = TRUE)

[mmaechler]

@lawremi what if we made it opt-in? Like foo <- new_generic("foo", "x", fallback_s3 = TRUE)

Maybe that'd be the best we can do; it would allow more interoperability, and notably help
for gradual transition form S3 to S7 for existing code bases, allowing some (or even much) back compatibility for reverse dependent code
(and I agree that it should be opt-in rather than automatic).

[lawremi]

I'm somewhat partial to the "workaround" where one just defines a default method that calls UseMethod(). It's relatively simple and self-documenting. However, this does bring up whether we should behave like setGeneric() and adopt any existing function of the new generic name as the default method. For example, in this use case, there already was an S3 generic. In other cases it might be an S4 generic or just a plain function.