Generic functions on objects with multiple classes

Question

I am interested to write a generic method that behaves based on multiple class type of the same object. For example, say we have the following:

redApple <- function(){
 structure(list(), class = c("apple", "red"))
}

greenApple <- function(){
 structure(list(), class = c("apple", "green"))
}

eat <- function(x)UseMethod("eat")
eat.apple <- function(x)  print("Eating apple")

color <- function(x) UseMethod("color")
color.red <- function(x) print("my food is red")
color.green <- function(x) print("my food is green")

Now we create the following objects:

obj1 <- redApple()
obj2 <- greenApple()

Here color(obj1) prints my food is red and color(obj2) prints my food is green , while both eat(obj1) and eat(obj2) print Eating apple .

Now I'd like my generic function behave differently for obj1 and obj2 . For example I'd like eat(obj1) to print Eating red apple and eat(obj2) to print Eating green apple

Answer 1

You can check for the class of the object passed to the function and make cases accordingly:

eat.apple <- function(x)  
{
  if("green" %in% class(x)) print("Eating green apple")
  else if("red" %in% class(x)) print("Eating red apple")
  else print("Eating generic apple!")
}

##addition

eat.green <- function(x)  
{
  print("Eating green apple")
}

eat.red <- function(x)  
{
  print("Eating red apple")
}