R devel - Jun 2021 - S3 dispatch does not work for generics defined inside an environment

On 30/06/2021 5:22 a.m., Taras Zakharko wrote:
> Dear all,
> 
> I have a generic function and a bunch of methods defined in a separate
environment. Here is a reduced example:
> 
>     env <- local({
>       # define the generic function and the method
>       myfun <- function(x) UseMethod("myfun")
>       myfun.myclass <- function(x) print("called myfun.myclass?)
> 
>       # register the method
>       .S3method("myfun", "myclass", myfun.myclass)
> 
>       environment()
>    })
> 
> Since the method has been registered, I hoped that invocation like this
would work:
> 
>     env$myfun(structure(0, class = "myclass?))
> 
> However, this results in a ?no applicable method" error.
> 
> It is my understanding that registerS3method (called by .S3method) will
install the method string in the .__S3MethodsTable__. table of the environment
where the generic function is defined, and this table is subsequently used by
usemethod() inside R, so I am puzzled that the dispatch does not work. I checked
and the  .__S3MethodsTable__. of env is indeed setup correctly. I also tried
manually adding the method string to the global .__S3MethodsTable__. inside
.BaseNamespaceEnv to no effect.
> 
> In fact, the only way to make it work is to define either myfun or 
myfun.myclas in the global environment, which is something I would like to
avoid.
> 
> Thank you in advance for any pointers!
> 
registerS3method has an additional parameter "envir" which I believe 
would end up set to env in your code.  So this works:

 > eval(expression(myfun(structure(0, class = "myclass"))), envir =
env)
[1] "called myfun.myclass"

You could probably also call registerS3method with envir specified 
appropriately and get your original expression to work.

Duncan Murdoch

On Wed, Jun 30, 2021 at 5:17 AM Duncan Murdoch <murdoch.duncan at
gmail.com> wrote:
>
> On 30/06/2021 5:22 a.m., Taras Zakharko wrote:
> > Dear all,
> >
> > I have a generic function and a bunch of methods defined in a separate
environment. Here is a reduced example:
> >
> >     env <- local({
> >       # define the generic function and the method
> >       myfun <- function(x) UseMethod("myfun")
> >       myfun.myclass <- function(x) print("called
myfun.myclass?)
> >
> >       # register the method
> >       .S3method("myfun", "myclass", myfun.myclass)
> >
> >       environment()
> >    })
> >
> > Since the method has been registered, I hoped that invocation like
this would work:
> >
> >     env$myfun(structure(0, class = "myclass?))
> >
> > However, this results in a ?no applicable method" error.
> >
> > It is my understanding that registerS3method (called by .S3method)
will install the method string in the .__S3MethodsTable__. table of the
environment where the generic function is defined, and this table is
subsequently used by usemethod() inside R, so I am puzzled that the dispatch
does not work. I checked and the  .__S3MethodsTable__. of env is indeed setup
correctly. I also tried manually adding the method string to the global
.__S3MethodsTable__. inside .BaseNamespaceEnv to no effect.
> >
> > In fact, the only way to make it work is to define either myfun or 
myfun.myclas in the global environment, which is something I would like to
avoid.
> >
> > Thank you in advance for any pointers!
> >
>
> registerS3method has an additional parameter "envir" which I
believe
> would end up set to env in your code.  So this works:
>
>  > eval(expression(myfun(structure(0, class = "myclass"))),
envir = env)
> [1] "called myfun.myclass"
>
> You could probably also call registerS3method with envir specified
> appropriately and get your original expression to work.
>
That doesn't seem to work on 4.1.0 for me. The code below worked for
me in Oct-2020, though I'm not sure what version of R I was using at
the time. I was slow to upgrade to 4.0, so it was probably the latest
3.x version.

env <- new.env()
local({
   # define the generic function and the method
   myfun <- function(x) { UseMethod("myfun", x) }

   # register the method
   registerS3method("myfun", "myclass",
       function(x) { print("called myfun.myclass") },
       envir = env)
}, envir = env)
attach(env)
myfun(structure(0, class = "myclass"))

> Duncan Murdoch
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel


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