Is it possible to dispatch S3 methods based on the class of an argument specified by its position only?

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Imagine we have the following S3 generic that we want to dispatch using the class of the first argument:

genfun <- function(x, ...) UseMethod("genfun", x)

Let's now consider a numeric method for this generic:

genfun.numeric <- function(x, y) {
  x[y]
}

The following call works just perfectly well:

genfun(1:5, 3)

Of note, it appears that the names of the arguments of the method do not need to match those of the generic. Indeed, if we define the numeric method this way:

genfun.numeric <- function(a, b) {
  a[b]
}

It also works:

genfun(1:5, 3)

However, the following call fails:

genfun(a = 1:5, b = 3)

whereas, if we return to the first definition of the method that match the arguments' names of the generic:

genfun.numeric <- function(x, y) {
  x[y]
}

a call with named arguments works just fine:

genfun(x = 1:5, y = 3)

or:

genfun(y = 3, x = 1:5)

Is there a way to make the method definition based on arguments a and b also work when called with named argument?

dispatch r
1 Answers

Yes, there is. Define the generic without arguments other than the dots argument.

genfun <- function(...) UseMethod("genfun")

Now both methods work with or without named arguments.

genfun.numeric <- function(x, y) {
  x[y]
}

genfun(1:5, 3)
#[1] 3

And the other one.

genfun.numeric <- function(a, b) {
  a[b]
}

genfun(1:5, 3)
#[1] 3

genfun(a = 1:5, b = 3)
#[1] 3
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