[Rd] method using several (and different) arguments in turn

[Martin Morgan]

On 02/14/2012 08:43 AM, james.foadi at diamond.ac.uk wrote:
> Dear R-developers community, I have the following generic:
>
> setGeneric(
>                     name="newsample",
>                     def=function(x,y,z,a,b,c,...){standardGeneric("newsample")}
>
> And I can build several methods for this generic. One useful thing is to use "newsample"
> with only one of the 6 arguments listed. At the moment this is what I do:
>
> setMethod(
>                     f="newsample",
>                     signature=c("missing","missing","numeric","missing","missing","missing"),
>                     function(x,y,z,a,b,c,...)
>                     {
>                         ..............................
>                         ..............................
>
>                      }
>                      )
>
> This would be used when the single argument is z:
>
> newsample(z=12.5)
>
> To use newsample with another argument (say x) I should implement the same as before,
> but with signature c("numeric","missing","missing","missing","missing","missing").
> Is there another shorter and easier way to do this?

Hi James --

A matter of opinion, but multiple dispatch like this can be very 
complicated, e.g., figuring out the 'next' method when dispatching on 
two or more arguments; I'd really discourage it.

A different approach, assuming that x, y, z, ... are all numeric() but 
that the sample to be drawn differs, is to define a small class 
hierarchy to be used for dispatch.

   setClass("TypeOfSample")

   setClass("XSample", contains="TypeOfSample")
   XSample <- new("XSample")  ## a 'singleton', used for dispatch

   setClass("YSample", contains="TypeOfSample")
   YSample <- new("YSample")

and then

   setGeneric("newsample",
       function(type, x=numeric(), ...) standardGeneric("newsample"),
       signature="type")

   setMethod("newsample", "XSample", function(type, x=numeric(), ...) {
       "XSample"
   })

   setMethod("newsample", "YSample", function(type, x=numeric(), ...) {
       "YSample"
   })

One could implement a default method on "TypeOfSample", and use 
callNextMethod() after initial transformation, if that were the pattern.

To use:

   newsample(XSample, x=1:100)

Martin

>
>
> J
>


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