[R-pkg-devel] Getting two independent packages with identical S3 generics to dispatch each other's methods
Dear Duncan et al., Firstly, my apologies for the duplicated query. It seems that I had searched everywhere but the mailing list where I asked the question. Secondly, I was wondering if I could get some indication whether CRAN would accept a package with the following code and documentation (and only that): 1) A number of exported generics of the form summary_formula(object, ..., lhs), simulate_formula(object, nsim=1, seed=NULL, ..., lhs), etc., which expect a formula as their first argument, evaluate the LHS of the formula, and dispatch based on the class of the result, which can also be overridden by the lhs= argument. 2) Corresponding S3 methods summary.formula(), simulate.formula(), etc. methods, that call the corresponding *_formula() generic. I am familiar with the generics package, but I don't think it's a good place for this functionality, because this is not the typical dispatching behaviour, and because *.formula() exports are not technically generics but S3 methods. In particular, as far as I know, existing mechanisms make it easy to cherry-pick generics, but they don't make it easy to cherry-pick methods. Best Regards, Pavel
On Sat, 2020-07-11 at 07:29 -0400, Duncan Murdoch wrote:
If the semantics of the two generics must remain identical in the future, then there is an implicit dependency between the code in the packages. You should formalize this by using one of the dependency mechanisms that the language provides, i.e. the clean solution. Duncan Murdoch On 10/07/2020 7:51 p.m., Pavel N. Krivitsky wrote:
Dear All, I would like to have two packages that do not depend on each other that have an identical generic to be able to dispatch to each other's (non- conflicting) methods. If it is of interest, the background for why this is needed is given at the end of this e-mail. As it is, it looks like two packages that do not depend on each other both define a generic, they do not see each other's S3 methods. For example, in the two attached minimal packages, which define and export generic foo() (identical in both packages) and methods foo.character() and foo.numeric() that are exported via S3method(), we get,
library(test.character)
foo("a")
foo.character() called.
library(test.numeric)
Attaching package: ?test.numeric?
The following object is masked from ?package:test.character?:
foo
foo(1)
foo.numeric() called.
foo("a")
Error in UseMethod("foo") :
no applicable method for 'foo' applied to an object of class
"character"
That is, test.numeric::foo() doesn't "see"
test.character:::foo.character() and vice versa. Is there a way to
make
them see each other?
This issue has arisen before, e.g. at
https://stackoverflow.com/questions/25251136/how-to-conditionally-define-a-generic-function-in-r-namespace
.
The "clean" solution is, of course, to create a third package to
define
the generic that the two packages could import (and, if necessary,
reexport). However, that involves creating an almost-empty package
that
then has to be submitted to CRAN, maintained, and add some amount
of
storage and computational overhead. Is there another way to do this
that is transparent to the end user?
# Background
This arose as a result of two packages (lme4 and ergm) both wanting
to
implement a simulate.formula() method, causing conflicts when the
user
wanted to use both at the same time.
ergm has a mechanism for dispatching based on the class of the LHS
of
the formula. It does so by defining a generic, simulate_formula()
which
evaluates the formula's LHS and dispatches a method (e.g.,
simulate_formula.<CLASS>()) based on that.
Since lme4 and ergm generally use different LHSs, we are thinking
of
resolving the conflict by copying the LHS dispatching mechanism
from
ergm to lme4, and then defining our own summary_formula methods as
needed.
Thank you in advance,
Pavel
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