;;  A pizza is either a circular pizza or a rectangular pizza

;; c_pizza -- circular pizza with radius
(define-struct c_pizza (radius))

;; r_pizza -- rectangular pizza with a width and length
(define-struct r_pizza (w l))


;; lop -- list of pizzas

;; lop_area: lop --> num 
;;total area of a lop
(define (lop_area a-lop)
  (cond
    [(empty? a-lop) 0]
    [(cons? a-lop) (+ (pizza_area (first a-lop)) (lop_area (rest a-lop)))]))

;; pizza_area:  pizza --> area
;; area of a pizza
(define (pizza_area p)
  (cond
    [(c_pizza? p) (* pi (sqr (c_pizza-radius p)))]
    [(r_pizza? p) (* (r_pizza-w p)(r_pizza-l p))]))

(define myPizzas (list (make-c_pizza 10) (make-r_pizza 5 8) (make-r_pizza 10 7) (make-c_pizza 12)))

"lop_area test:"
(lop_area empty)
(lop_area (list (make-c_pizza 1)))
(lop_area (list (make-r_pizza 2 3)))
(lop_area myPizzas)