[Rd] Using missing() in a S4 method with extra arguments

[Martin Morgan]

On 03/11/2011 02:07 AM, Andreas Borg wrote:
> Hi all,
> 
> I have a function which makes use of missing() to determine which
> arguments are provided in the call - basically, there are two sets of
> arguments that map to different strategies the function uses to fulfill
> its task. After conversion to an S4 generic I've run into the problem
> that if a method uses extra arguments that are not in the signature of
> the generic, usage of missing() fails. The following example exemplifies
> this:
> 
>    setGeneric("fun", function(x=0, y=0, ...) standardGeneric("fun"))
>    # both methods should output if the second argument is missing
>    setMethod("fun", "character", function(x=0, y=0, ...) missing(y))
>    setMethod("fun", "numeric", function(x=0, y=0, z=0, ...) missing(y))
> 
>    fun("a") # this works fine
>    fun(1) # this gives "FALSE

Hi Andreas --

if you're testing for the missing-ness of y, and y is in the function
signature, then use that for dispatch

   setMethod(fun, c("character", "missing"),
             function(x=0, y=0, z=0, ...) "missing")
   setMethod(fun, c("character", "ANY"),
             function(x=0, y=0, z=0, ...) "not missing")

Since you're dispatching on x and y, it doesn't really make sense (to me
;) to assign default values to them. Testing for missing-ness of z would
I think have to rely on NA / NULL or other sentinel.

Martin
> 
> I've understood so far that this is due to the fact that the "numeric"
> method in this example is rewritten to:
> 
>    function (x = 0, y = 0, ...)
>    {
>        .local <- function (x = 0, y = 0, z = 0, ...)
>        missing(y)
>        .local(x, y, ...)
>    }
> 
> The call to .local evaluates y and it is no more missing.
> 
> Is there any alternative that works in this case? Or is there a chance
> that missing() might be changed to work in this case in the near future?
> 
> Of course I know I could set NA or NULL as default values and check for
> these, but there are reasons I want to have legal default values for all
> arguments.
> 
> Best regards,
> 
> Andreas
> 
> Andreas Borg
> Medizinische Informatik
> 
> UNIVERSITÄTSMEDIZIN
> der Johannes Gutenberg-Universität
> Institut für Medizinische Biometrie, Epidemiologie und Informatik
> Obere Zahlbacher Straße 69, 55131 Mainz
> www.imbei.uni-mainz.de
> 
> Telefon +49 (0) 6131 175062
> E-Mail: borg at imbei.uni-mainz.de
> 
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