Overloading methods in R

f <- function(...) UseMethod("f", NULL)

f.NULL <- function(...) {
args <- list(...)
classes <- sapply(args, class)
.Class <- paste(classes, collapse = ".")
NextMethod("f", ...)
}

f.numeric <- function(...) 2 * ..1
f.numeric.numeric <- function(...) ..1 + ..2
f.character.numeric.Date <- function(...) {
args <- list(...)
paste(args[[1]], args[[2]], format(args[[3]], "%Y-%m-%d"))
}
f.default <- function(...) print(list(...))


f(1) # 2
f(1,2) # 3
f("a", 23, Sys.Date()) # "a 23 2005-04-21"
f() # list()

Thanks Gabor! This answers a big part of my question. I am just curious 
why
something like this doesn't work in S4:

-------------------------
setGeneric("foo", function(object, ...) standardGeneric("foo"))

foo.NULL <- function(object, ...) {
args <- list(...)
classes <- sapply(args, class)
.Class <- paste(classes, collapse = ".")
}

foo.default <- function(object, ...) paste("wrong args!")

foo.numeric <- function(object, ...) 2 * ..1
foo.numeric.numeric <- function(object, ...) ..1 + ..2
--------------------------


On 4/21/05, Ali - <saveez@hotmail.com> wrote: 

f <- function(...) UseMethod("f", NULL)

f.NULL <- function(...) {
args <- list(...)
classes <- sapply(args, class)
.Class <- paste(classes, collapse = ".")
NextMethod("f", ...)
}

f.numeric <- function(...) 2 * ..1
f.numeric.numeric <- function(...) ..1 + ..2
f.character.numeric.Date <- function(...) {
args <- list(...)
paste(args[[1]], args[[2]], format(args[[3]], "%Y-%m-%d"))
}
f.default <- function(...) print(list(...))


f(1) # 2
f(1,2) # 3
f("a", 23, Sys.Date()) # "a 23 2005-04-21"
f() # list()

Thanks Gabor! This answers a big part of my question. I am just curious 
why
something like this doesn't work in S4:

-------------------------
setGeneric("foo", function(object, ...) standardGeneric("foo"))

foo.NULL <- function(object, ...) {
args <- list(...)
classes <- sapply(args, class)
.Class <- paste(classes, collapse = ".")
}

foo.default <- function(object, ...) paste("wrong args!")

foo.numeric <- function(object, ...) 2 * ..1
foo.numeric.numeric <- function(object, ...) ..1 + ..2
--------------------------

 I am not 100% sure I understand what the question is but I think what
you are looking for is the fact that ... cannot be part of the signature 
in
S4. That is ... can be among the method arguments but you can't 
dispatch on those arguments in S4.