[R-pkg-devel] [External] Re: S3 generic/method consistency warning for formula method

[Joris Meys]

Hi Brian,

I get what you want to do, but S3 isn't suited for that at all. If all elements in ... have to be the same class, you can use S4 (see ?dotsMethods). If not, then using S3 makes even less sense. Take following example:

x <- list()
y <- c(1,2)

foo(x,y) will dispatch to foo.list()
foo(y,x) will dispatch to foo.character()

You'll be in a world of hurt if you want to maintain or update that code. If you insist on using S3, you'll have to fix the position (and hence the name) of the argument you want to dispatch on.

my 2 cents
Cheers
Joris

--
Joris Meys
Statistical consultant

Department of Data Analysis and Mathematical Modelling
Ghent University
Coupure Links 653, B-9000 Gent (Belgium)
------------------------------

Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php


________________________________________
From: R-package-devel <r-package-devel-bounces using r-project.org> on behalf of Smith, Brian J <brian-j-smith using uiowa.edu>
Sent: Friday, January 17, 2020 4:59 PM
To: Duncan Murdoch; r-package-devel using r-project.org
Subject: Re: [R-pkg-devel]  [External] Re: S3 generic/method consistency warning for formula method

Thanks Duncan.

I was surprised too when first realizing this was possible.  I believe the reason it works is that UseMethod dispatches on the first supplied argument by default.  So, really the dispatch is on the first element of the ellipsis.  The 'c' function works like this, albeit as a primitive function.

I would like to dispatch on the ellipsis to allow a user to name values being passed in.  Defining an initial x argument would indeed eliminate the warning, but would not allow for user-naming of the value.  I could have the user pass a list of values to x and do dispatching manually, but that seems like more steps for the user and more complication for the implementation than ought to be necessary.  There are other downsides to that approach in my particularly application...

The issue mentioned below about calling the function with no arguments can be solved by defining the following default method.

foo.default <- function(...) list(...)