[Rd] Feature request: mixing `...` (three dots) with other formal arguments in S4 methods

Gabriel Becker gmbecker at ucdavis.edu
Fri Nov 28 02:40:52 CET 2014

I think I understand what you're saying now, but I'm still kind of missing
the benefit from the approach.

As far as I can tell just giving foo formals for the arguments you want it
to catch gives you the end result you want, doesn't it?

And if the generic has ... in it, you can (if you're very careful) add
formals to specific methods that would capture arguments not meant for
other methods of the same generic.


On Thu, Nov 27, 2014 at 11:26 AM, Janko Thyson <janko.thyson at gmail.com>

> Hi Gabriel,
> and thanks for answering. I'm basically just trying to find a way to use
> the power of `...` in more complex scenarios and I'm well aware that this
> might not be the best approach ;-)
> Regarding your actual question:
> "Are you suggesting methods be dispatched based on the *contents* of ...
> [...]?"
> Yes, I guess currently I kind of do - but not on the argument *names*
> I'm not expecting functions to detect the argument *names*  from `...`,
> but the relevant "argument containers" from which then the actual arguments
> should be extracted and used:
> I thought the *actual* arguments to be passed via `...` to subsequent
> functions/methods could be put into an "arguments container" (as a list so
> you could easily use them with `do.call(foo)`) that has a class that `foo`
> expects for its `...` argument (e.g. `ThreedotsForFoo`). What I would like
> to accomplish is that `foo` auto-detects those parts coming in via `...`
> that are *relevant* for itself (e.g. instances of the argument container
> `ThreedotsForFoo`), that it handles them in a proper way (i.e. extracting
> the *actual* arguments from the container) and that it passes `...` along
> to subsequently called functions.
> That's why I would need methods that use mix of regular formal arguments
> and `...`.
> Best regards,
> Janko
> On Thu, Nov 27, 2014 at 7:48 PM, Gabriel Becker <gmbecker at ucdavis.edu>
> wrote:
>> Janko,
>> I'm not entirely sure I understand your proposal. Are you suggesting
>> methods be dispatched based on the *contents* of ... (ie which arguments
>> are in there)? This seems like it would be pretty different from how
>> dispatch behaves now, which is entirely class based.
>> Even the dispatching based on ... via dots methods is class based, having
>> nothing to do AFAIK with the argument names. From ?dotsMethods
>> A method selecting on “...” is specified by a single class in the call to
>> setMethod <>. If
>> all the actual arguments corresponding to “...” have this class, the
>> corresponding method is selected directly.
>> Otherwise, the class of each argument and that class' superclasses are
>> computed, beginning with the first “...” argument. For the first argument,
>> eligible methods are those for any of the classes. For each succeeding
>> argument that introduces a class not considered previously, the eligible
>> methods are further restricted to those matching the argument's class or
>> superclasses. If no further eligible classes exist, the iteration breaks
>> out and the default method, if any, is selected.
>> No mention of argument name there.
>> ~G
>> On Thu, Nov 27, 2014 at 9:45 AM, Janko Thyson <janko.thyson at gmail.com>
>> wrote:
>>> Dear List,
>>> I'm currently investigating if the argument dispatch mechanism based on
>>> `...` could somehow be "generalized" to scenarios that involve `r`
>>> recipients located across `c` calling stack layers *and* combined with
>>> the
>>> S4 method mechanism (for those interested see
>>> http://stackoverflow.com/questions/26963900/generalizing-three-dots-argument-dispatch-s4-methods-for-argument-set-i
>>> for an (conceptual) approach of how this could be realized).
>>> AFAICT, this would require that `...` can be *mixed* with other signature
>>> arguments, which is currently not supported as stated in `?dotsMethods`:
>>> Quote {
>>> Using "..." in a Signature
>>> Beginning with version 2.8.0 of R, S4 methods can be dispatched (selected
>>> and called) corresponding to the special argument “...”. Currently, “...”
>>> cannot be mixed with other formal arguments: either the signature of the
>>> generic function is “...” only, or it does not contain “...”. (This
>>> restriction may be lifted in a future version.)
>>> }
>>> Would it be possible to consider lifting this limitation soon?
>>> Thanks a lot to everyone maintaining R!!
>>> Janko
>>>         [[alternative HTML version deleted]]
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>> --
>> Gabriel Becker
>> Graduate Student
>> Statistics Department
>> University of California, Davis

Gabriel Becker
Graduate Student
Statistics Department
University of California, Davis

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