Generic S3 function for multiple classes - r

Question

I have a generic function like this:
convert <- function(x) UseMethod("simplifyResultConvert")
convert.default <- function(x) {
x
}
convert.POSIXct <- function(x) {
as.character(x)
}
convert.factor <- function(x) {
as.character(x)
}
convert.Date <- function(x) {
as.character(x)
}
Is there any way to simplify it by making one generic for type: POSIXct, Date and factor?
To make it clear: I need something like:
convert.(POSIXct || factor || date) <- funciton(x) {as.character(x)}

Answer 1

Write it like this:
convert.default <- function(x) x
convert.Date <-
convert.factor <-
convert.POSIXct <- function(x) as.character(x)
A further simplification would be:
convert.default <- identity
convert.Date <-
convert.factor <-
convert.POSIXct <- as.character

Answer 2

You can put a test of the class in the default method, for example:
convert.default <- function(x) {
if (inherits(x, "POSIXct") ||
inherits(x, "factor") ||
inherits(x, "Date"))
as.character(x)
else
x
}
This is not quite the same as what you want, because an object could have class c("POSIXct", "other") and if there was a convert.other() method set, it would be called instead of the default.
If you really want the behaviour you asked for, you need the three functions, but you can save a bit of typing by using
convert.Date <- convert.factor <- convert.POSIXct <- function(x) {
as.character(x)
}

Answer 3

Since there are no super-classes in S3, this is not possible with S3. However, you can easily do this with S4 which allows defining super-classes:
setGeneric("convert", function(object) {
standardGeneric("convert")
})
setClassUnion("fooClasses", members = c("factor", "Date", "POSIXt")) #POSIXt is a super-class
setMethod("convert", signature(object = "fooClasses"), function(object) {
as.character(object)
})
class(convert(as.Date("2010-10-10")))
#[1] "character"
class(convert(as.POSIXct("2010-10-10")))
#[1] "character"