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

5 messages · Janko Thyson, Gabriel Becker

Original
Janko Thyson
# 
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
Gabriel Becker
# 
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 <http://127.0.0.1:11942/library/methods/help/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:

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Janko Thyson
    

    

      
      #
    

  


  

      
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:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      Gabriel Becker
    

    

      
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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.

~G

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

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Janko Thyson
    

    

      
      #
    

  


  

      
Well, the benefit lies in the ability to pass along arguments via `...` to
more than one recipient that use *identical argument names* and/or when
these recipients are not necessarily located on the same calling stack
layer.

I'm *not* after a *general* change in the way arguments are
dispatched/functions are called as I'm actually a big friend of keepings
things quite explicit (thus declaring explicitly what's passed on to
subsequent functions by defining respective formal arguments).
Nevertheless, sometimes it's quite handy to use `...`.

Consider the implementation of `plot()`. It uses `...` quite extensively to
pass things along to `par()` which makes perfect sense: declaring formal
arguments for things that are merely passed along to `par()` in *all*
functions that depend on `par()` would probably be a developer's nightmare
w.r.t. refactoring should `par()` ever change.

But let's say that at one point in time, developers decide that `par()` can
also call something like `parShiny()` if `shiny = TRUE` in order
encapsulate shiny-specific graphical parameters in a own function (sorry, I
couldn't come up with a better example just now).  I'm using a simplified
example where `cex` is indeed a formal parameter (which is not the case in
the actual `par()`):

myPlot <- function(x, ...) {
  myPar(...)
}
myPar <- function (cex = 1.0, shiny = FALSE, ...) {
  if (!shiny) {
    message("myPar/cex:")
    print(cex)
  } else {
    parShiny(...)
  }
}
parShiny <- function (cex = 1.0) {
  message("parShiny/cex:")
  print(cex)
}

            

      
      
      
    

        
myPar/cex:
[1] 1.25

            

      
      
      
    

        
parShiny/cex:
[1] 1

So: due to the fact that `myPar()` has a formal argument `cex`, `...` is
out of the question for passing along `cex` to `parShiny()`. You'd have to
change things to `parShiny(cex = cex)` in the implementation of `myPar()`
in order for this to work as expected - which you might or might not feel
is (too) cumbersome.

While it probably makes a lot of sense to pass things along explicitly in
95 % of cases, there might be situations where you'd prefer to being able
to use `...`.

But I don't want to overstress the (current) purpose/use case behind my
request. I just wondered if the limitation of not being able to mix `...`
with other formal arguments could be lifted soon as the possiblity is
already stated at `?dotsMethods` :-)

On Fri, Nov 28, 2014 at 2:40 AM, Gabriel Becker <gmbecker at ucdavis.edu>
wrote: