[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(...)

-----Original Message-----
From: Duncan Murdoch <murdoch.duncan at gmail.com>
Sent: Friday, January 17, 2020 9:24 AM
To: Smith, Brian J <brian-j-smith at uiowa.edu>; 
r-package-devel at r-project.org
Subject: [External] Re: [R-pkg-devel] S3 generic/method consistency 
warning for formula method

On 17/01/2020 9:14 a.m., Smith, Brian J wrote:

Hello all,

I am getting an R CMD check warning about S3 consistency for a formula method of a generic function that dispatches on the ellipsis argument.  The formula method seems to function properly when called, and methods for other types tried do not trigger the warning.  Below is an example generic function 'foo' and methods that will reproduce the issue, followed by the check warning message produced.

foo <- function(...) UseMethod("foo") foo.factor <- function(...) 
list(...) foo.formula <- function(...) list(...) foo.logical <- 
function(...)
list(...) foo.matrix <- function(...) list(...) foo.numeric <-
function(...) list(...)

I'm surprised that you are allowed to have a generic with no arguments 
except "...".  I imagine the error will go away if you change the 
definitions to

foo <- function(x, ...) UseMethod("foo") foo.factor <- function(x, 
...) list(x, ...) foo.formula <- function(x, ...) list(x, ...) 
foo.logical <- function(x, ...) list(x, ...) foo.matrix <- function(x, 
...) list(x, ...) foo.numeric <- function(x, ...) list(x, ...)

With your original definition, it would be legal to call foo(), but 
the
UseMethod() would fail.

Duncan Murdoch

R CMD check warning:

W  checking S3 generic/method consistency (1.7s)
     foo:
       function(...)
     foo.formula:
       function(...)
     
     See section 'Generic functions and methods' in the 'Writing R
     Extensions' manual.

I would appreciate any help in understanding and addressing this check warning.

Thank you,
Brian

*****************************************************************
Brian J Smith, PhD
Professor, Department of Biostatistics Director, Biostatistics Core, 
Holden Comprehensive Cancer Center The University of Iowa
145 North Riverside Drive, N311 CPHB
Phone: 319-384-1587
Email: brian-j-smith at uiowa.edu