Defining generic methods for suggested packages

[jtlandis]

Hello everyone,

When building a package that is exporting a generic function, what is the proper way to export a method for a class within a "Suggests" package?

For example, if package foo exports the foo() generic, but lists package bar as a Suggests, what would be the proper way to register methods for classes within bar?

Would it be something to the effect of:

# from the foo package
.onLoad <- function(lib, pkg) {

  if (isNamespaceLoaded("bar")) {
    method(foo, bar::class_bar) <- function(x) {cat("we dispatched on `bar::class_bar`")}
  } else {
    setHook(packageEvent("bar", "onLoad"), function(){
      method(foo, bar::class_bar) <- function(x) {cat("we dispatched on `bar::class_bar`")}
    })
  }
}

[teunbrand]

I think the advice for external generics is written up here: https://rconsortium.github.io/S7/reference/new_external_generic.html

[jtlandis]

This is not quite what I am looking for and a bit opposite for my use case.

The link describes how to conditionally register external generics for classes within your own package.

For my use case, the foo generic exists in my own package I want to conditionally provide a method for an external class bar depending on if it is loaded or may get loaded in the future.

[hadley]

I think your approach with onLoad is reasonable, until we come up with something better. Just one thing to note is that you should call the setHook() unconditionally; this protects from (the admittedly rare case) where the user unloads then reload the package. I'd also extract out a function to reduce the duplication:

register_bar <- function() {
  method(foo, bar::class_bar) <- function(x) {
    cat("we dispatched on `foo_bar`")
  }
}

.onLoad <- function(lib, pkg) {
  if (isNamespaceLoaded("bar")) {
    register_bar()
  }
  setHook(packageEvent("bar", "onLoad"), register_bar())
}

[jtlandis]

FWIW, This issue was inspired by the pattern from S7::methods_register which also evaluates setHook in the FALSE branch:

> S7::methods_register
function ()
{
    package <- packageName(parent.frame())
    ns <- topenv(parent.frame())
    tbl <- S7_methods_table(package)
    for (x in tbl) {
        register <- registrar(x$generic, x$signature, x$method,
            ns)
        if (isNamespaceLoaded(x$generic$package)) {
            register()
        }
        else {
            setHook(packageEvent(x$generic$package, "onLoad"),
                register)
        }
    }
    invisible()
}

[hadley]

We should fix that.