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Using missing() in a S4 method with extra arguments

2 messages · Andreas Borg, Martin Morgan

Original
Andreas Borg
# 
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

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

UNIVERSIT?TSMEDIZIN
der Johannes Gutenberg-Universit?t
Institut f?r Medizinische Biometrie, Epidemiologie und Informatik
Obere Zahlbacher Stra?e 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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Martin Morgan
#
On 03/11/2011 02:07 AM, Andreas Borg wrote:
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