Abstract types in R

Question

I want to create class in R, let's say it is an S4 class for a person. E.g.

setClass("Person", slots = list(name = "character", mood = "myMoodType"))

Now I want to create myMoodType to be an abstract type that can only take the three values "Happy", "Sad" and "Unknown".

I know that I could do this using the validity for S4 classes and have the mood as a character type and check the validity by verifying that the character string provided is one of the three options I list. But I would like to know if I can define an abstract type, like in julia, e.g.

abstract myMoodType
type Happy   <: myMoodType             end
type Sad     <: myMoodType             end
type Unknown <: myMoodType             end

What would be the correct way to approach this in R?

Answer 1

This might not be one of R's strongest and most smooth feature, but you could solve it in the following way. For more information see the documentation or the Advanced R chapter on S4.

First set up the Person class with the mood represented as a factor, and link it to a validation function that checks its levels.

check_person <- function(object) {
    if(identical(levels(object@mood), c("Happy", "Sad", "Unknown"))){
        return(TRUE)
    } else {
        return("Invalid mood.")
    }
}

setClass("Person",
         representation(name = "character", mood = "factor"),
         prototype = list(name = NA_character_,
                          mood = factor(NA, c("Happy", "Sad", "Unknown"))),
         validity = check_person)

Creating new instances with new is however a bit messy since we have to write out all the levels each time:

john <- new("Person", name="John", mood=factor("Happy", levels=c("Happy", "Sad", "Unknown")))
lucy <- new("Person", name="Lucy", mood=factor("Sad", levels=c("Happy", "Sad", "Unknown")))

Otherwise we'll get an error:

new("Person", name="Eve", mood="Unknown")

Error in validObject(.Object) : 
  invalid class “Person” object: invalid object for slot "mood" in class "Person":
  got class "character", should be or extend class "factor"

To get around that you could make your own constructor:

new_person <- function(name, mood){
    new("Person", name = name, mood = factor(mood, levels = c("Happy", "Sad", "Unknown")))
}
new_person("Eve", "Unknown")

An object of class "Person"
Slot "name":
[1] "Eve"

Slot "mood":
[1] Unknown
Levels: Happy Sad Unknown