extending protocol with protocol

extend-protocol-with-protocol.clj

; inspired by example from "Clojure Applied"
; book says that it's not possible

(ns sales.core)

(defprotocol Cost 
  (cost [this]))

(defrecord Product [price quantity]
  Cost
  (cost [this] 
        (* (.price this) (.quantity this))))

(defprotocol SaleCost 
  (sale-cost [this]))

(extend-protocol SaleCost
  sales.core.Cost
  (sale-cost [this] 
             (* (cost this) 0.75)))

(cost (->Product 100 2))      ; => 200
(sale-cost (->Product 100 2)) ; => 150