# Constructors
circle <- function(r) structure(list(r=r), class="circle")
rectangle <- function(a, b) {
    # equivalent to > structure(list(a=a, b=b), class="rectangle")
    x <- list(a=a, b=b)
    class(x) <- "rectangle"
    x # return
}

# generics (custom)
area <- function(shape) UseMethod("area")
diam <- function(shape) UseMethod("diam")

# methods (implementation)
print.circle <- function(circle, ...) with(circle, cat("< circle r =", r, ">\n"))
#' Computes area of a circle object
#'
#' @param circle Instance of a circle.
#' 
#' @returns Area of the given \code{circle}.
#' @export
area.circle <- function(circle) with(circle, pi * r^2)
diam.circle <- function(circle) with(circle, 2 * r)

print.rectangle <- function(rect, ...) {
    cat("<rectangle a =", rect$a, ", b =", rect$b, ">\n", set="")
}
area.rectangle <- function(rect) rect$a * rect$b
diam.rectangle <- function(rect) sqrt(rect$a^2 + rect$b^2)

# usage
circ <- circle(2)
rect <- rectangle(a = 1, b = 2)

print(area(circ))
print(diam(rect))

print(circ)
print(rect)