Does using package generics require the package to be in Depends or Imports?

Question

First of all, I have read around this topic a lot. I have studied the conversations here, here, here and here. The problem is, however, I still think one particular topic isn\'t

Answer 1

I'm not a complete expert on this, but here is something that works:

#' @importFrom ggplot2 autoplot
#' @export autoplot

autoplot <- autoplot

#' @export "autoplot.foo"

autoplot.foo <- function(obj, ...) {
  cat("bar\n")
}

Unfortunately we need to explicitly export the method because for some reason S3 dispatch from a package namespace (in this case ggplot2) doesn't find the method registered from a package it doesn't import from. This kind of makes sense except that it works for base generics so there is a subtlety of the S3 registration mechanism that I'm not privy to.

Re whether to use :: within your package or not, it's up for debate. You don't have to if you use @import or @importFrom, but some (including Hadley) suggest you should for clarity. I personally recommend against it because the R profiler doesn't resolve function names of the style package::method, which makes it harder to optimize code (there are workarounds that can meet both objectives).

Note you should absolutely use @import and not rely solely on :: (you will also have to update the DESCRIPTION file). Failing to do so will prevent R from being aware of the dependency. I'm pretty sure R CMD check yells about this.