Multiple dispatch for `subset` methods in R

问题

I am developing a package and want to write two subset methods for objects of a custom class, myclass, with dispatch on two arguments, first one being the object to subset, of class myclass, and the second being either logical of character vector, like so:

setMethod(
    f = "subset",
    signature = c(x = "myclass", subset = "logical"),
    definition = function(x, subset){
            # function body 
    }
)

setMethod(
    f = "subset",
    signature = c(x = "myclass", subset = "character"),
    definition = function(x, subset){ 
        # different function body 
    }
)

However, I cannot do this because the S3 generic dispatches on one argument only. And I don't want to create a new generic for subset because it would mask the existing generic when my package is loaded. One way around this issue, I think, would be to create a generic and methods of different name, but this would not be very intuitive for the users, right? So am I missing/misunderstanding something, and is there any witty way to have multiple dispatch for S3 generics?

回答1:

Normally in this situation you would set subset as an S4 generic but since you have reasons for not wanting to do that, you can get around this by defining a separate generic and calling it from within the S3 method, along the lines of

mySubset <- function(x,subset){
  stop("this is only a generic function: it should never be called!")
}
setGeneric("mySubset")
## methods for mySubset
setMethod(
    f = "mySubset",
    signature = c(x = "myclass", subset = "logical"),
    definition = function(x, subset){
            # function body 
    }
)

setMethod(
    f = "mySubset",
    signature = c(x = "myclass", subset = "character"),
    definition = function(x, subset){ 
        # different function body 
    }
)

## default method using "ANY" (lower priority)
setMethod(
    f = "mySubset",
    signature = c(x = "myclass", subset = "ANY"),
    definition = function(x, subset){ 
       ## insert default behaviour (might be an error),
       ## a call to subset.default or whatever
    }
)

## now write an S3 method for subset that calls the S4 generic if
## x is of class myclass

subset.myClass <- function(x,subset){
  mySubset(x,subset)
}

This preserves the S3-only behaviour of subset, but you now have S4 level control over method dispatch provided that x is of class myclass.

Your users don't need to appreciate this distinction; they can still call subset(x,class) in the same way they are accustomed to, when x has your new class.

来源：https://stackoverflow.com/questions/49346939/multiple-dispatch-for-subset-methods-in-r

标签

methods