[R-pkg-devel] Conditionally register method with generic in other package

Hadley Wickham h.wickham at gmail.com
Thu Dec 7 20:57:19 CET 2017

We use this technique in a couple of places:

Basic idea is to call registerS3method() manually, when needed - it's
just a little tricky because the suggested package may be loaded
either before or after your package.


On Thu, Dec 7, 2017 at 1:45 AM, Joshua Ulrich <josh.m.ulrich at gmail.com> wrote:
> I have a feeling I know the answer to this question, but I'm asking
> with the hope that I learn something new.
> Package A defines a S3 generic function, "foo()".  Package B defines a
> S3 class "bar", and a foo.bar() method.  This method is not strictly
> necessary for the package to function, and package A has many
> dependencies.
> To avoid excessive dependencies, I would like to only register
> foo.bar() if package A is installed at the time package B is
> installed.  If package A is installed after package B, then warn the
> user when package B is loaded and/or attached, so they can re-install
> A and have foo.bar() registered correctly.
> I could figure out a way to do this manually, but doubt it would be
> CRAN-approved.  What is the recommended way to handle this situation?
> I've received one suggestion to split B into 2 packages: one without
> the method, and one with.  Are there other alternatives?
> Thanks,
> Josh
> --
> Joshua Ulrich  |  about.me/joshuaulrich
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