s3 r6

require(R6)

dog <- list(name = 'hundno', say = 'whhooo!')
class(dog) <- "animal"

cat <- list(name = 'kitty', say = 'meow!')
class(cat) <- "animal"

ipad <- list(name = "iPad mini")
class(ipad) <- "computer"

make_sound <- function(x, ...) {
    UseMethod("make_sound", x)
}

make_sound.animal <- function(x, ...) {
    print(x$say)
}


make_sound.animal <- function(x, ...) {
    print(x$say)
}

make_sound.computer <- function(x, ...) {
    print("...")
}

make_sound(cat)
make_sound(dog)
make_sound(ipad)


### The same thing in R6

animal_generator <- R6::R6Class(
    "animal",
    private = list(
        .dna = NULL
    ),
    public = list(
        name = NULL,
        say = NULL,
        initialize = function(name, say) {
            self$name <- name
            self$say <- say
            private$.dna <- paste0(sample(c("A", "T", "G", "C"), 20, replace = TRUE), collapse = "")
        },
        make_sound = function() {
            print(self$say)
        }
    ),
    active = list(
        dna = function(field) {
            if (missing(field)) {
                private$.dna
            } else {
                stop("You can't modify the info.")
            }
        })
)


computer_generator <- R6::R6Class(
    "computer",
    public = list(
        name = NULL,
        initialize = function(name) {
            self$name <- name
        },
        make_sound = function() {
            print("...")
        }
    )
)


dog <- animal_generator$new('hundno', 'whoooo!')
cat <- animal_generator$new('kitty', 'meow!')

dog$make_sound()
cat$make_sound()

### Active binding
dog$dna

dog$dna <- "I am modifying your DNA!"

###

ipad <- computer_generator$new('ipad')
ipad$make_sound()