On Wed, 14 Nov 2007, William Valdar wrote:
> Dear All,
>
> I want to test whether a method exists for given object. For example,
> whether a function "deviance" is defined for an object of the "lm" class.
For an S3 generic 'f' and with an S3 object or an S3 class 'x', try
hasS3method <- function(f, x)
{
if(is.object(x)) x <- oldClass(x)
m <- methods(f)
cl <- sub(paste("^", f, ".", sep=""), "", m)
any(c("default", x) %in% cl)
}
(You can break this, e.g. by f="resid" or using implicit classes: it needs
inside knowledge to know if the latter would be invoked. Also, the set
of available methods is in principle scope-specific.)
For S4 generics and classes, look at selectMethod(optional=TRUE): this is
documented to return NULL if and only if there is no applicable method.
> My imperfect understanding leads me to think something like
>
> hasMethod("deviance", object)
> hasMethod("deviance", "lm")
> existsMethod("deviance", signature(class="lm"))
>
> or similar might work (I don't fully understand how to manipulate
> signatures), but all the variations on this I have tried return FALSE.
> (Except, interestingly, when I first load library lme4, after which all
> return TRUE even for non-existant classes and functions).
>
> I realize there are several ways in which R implements function
> polymorphism and that this is all documented somewhere but a hint would
> save me considerable time. I would also prefer not to resort to the hack
> solution of try()ing the function with the object and then catching the
> error to determine whether it was defined.
>
> Thanks,
>
> Will
>
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
> Dr William Valdar ++44 (0)1865 287 589
> Wellcome Trust Centre [EMAIL PROTECTED]
> for Human Genetics, Oxford www.well.ox.ac.uk/~valdar
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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