Money

money.clj

(ns ch1.money)

(declare validate-same-currency) 

(defrecord Currency [divisor sym desc])

(defrecord Money [amount ^Currency currency] 
  java.lang.Comparable
    (compareTo [m1 m2]
      (validate-same-currency m1 m2)
      (compare (:amount m1) (:amount m2))))

(def currencies {:usd (->Currency 100 "USD" "US Dollars")
                 :eur (->Currency 100 "EUR" "Euro")})

(defn- validate-same-currency
  [m1 m2]
  (or (= (:currency m1) (:currency m2))
      (throw
        (ex-info "Currencies do not match."
          {:m1 m1 :m2 m2}))))
(defn =$
  ([m1] true)
  ([m1 m2] (zero? (.compareTo m1 m2)))
  ([m1 m2 & monies]
    (every? zero? (map #(.compareTo m1 %) (conj monies m2)))))

(defn +$ 
  ([m1] m1)
  ([m1 m2]
    (validate-same-currency m1 m2)
    (->Money (+ (:amount m1) (:amount m2)) (:currency m1)))
  ([m1 m2 & monies]
    (reduce +$ m1 (conj monies m2))))

(defn *$ [m n] (->Money (* n (:amount m)) (:currency m)))

(defn make-money
  ([] (make-money 0))
  ([amount] (make-money amount :usd))
  ([amount currency] (->Money amount currency)))

(defn -$ [m1 m2 & monies] ,,,)

(defn allocate [m proportions] ,,,)

(def zero-dollars (make-money 0 :usd))