S3 example

s3.Rmd

```{r}
area <- function(x) UseMethod("area", x)

rectangle <- function(a, b) {
  structure(list(a = a, b = b), class = "rectangle")
}
area.rectangle <- function(x) {
  x$a * x$b
}

circle <- function(r) {
  structure(list(r = r), class = "circle")
}
area.circle <- function(x) {
  pi * x$r * x$r
}
```

Now calculating area:
```
area(rectangle(2, 4))
area(circle(3))
```

We can also define print
```
as.character.rectangle <- function(x) {
  "rectangle"
}
```