[R-pkg-devel] Courtesy methods and explosive dependencies

[Lenth, Russell V]

> There can't really be an "ImportGenerics", because S3 is so informal.  A generic function is a function that calls UseMethod, but it can do anything else as well.  So R would need some fancy code analysis to know whether it was safe to import the generic but not all the dependencies of that package, and that could change when the package holding the generic was updated.
> 
> Examples of generics that do more than simply call UseMethod are rare, but they exist:  as.data.frame() and sort() are a couple.
> 
> Duncan Murdoch

Right -- ImportGenerics was a dumb idea. However, Something like ImportNoDeps makes sense as an additional category in the DESCRIPTION file. In my example, if such existed, I could put in the DESCRIPTION file:

    ImportNoDeps: multcomp

This would signal that multcomp -- but not necessarily any of its dependencies -- is required to install emmeans.  And in the NAMESPACE file:

    importFrom(multcomp, cld)
    S3method(cld, emmGrid)

... would import the needed generic and register my new S3 method.

I think this kind of construct could significantly reduce dependencies for packages that extend other packages' methods.

Russ