Clojure Applied

chapter_1.clj

(ns clojure-applied.core)

; define a map - values can be looked up in near constant time.
(def earth {
  :name "Earth"
  :moons 4
  :volume 1.083
  :mass 5.972
  :type :Planet
})


; RESEARCH - Maps, Vectors and sets are implemented
; by Hash Array Mapped Tries

(get earth :moons)
(earth :moons)

; using a map to represent an entity type, but it isn't very useful
; if we wish to re-use it.
(defrecord Planet [
  name
  moons
  volume
  mass
])

; Two factory functions are automatically
; created to assist creating new instances.

; the 'positional factory function', which expects
; a value for each attribute.
; requires all attributes to be included in the
; specified order
(def earth (->Planet "Earth" 1 1.083 5.972))
(print earth)

; and the 'map factory function' that expects
; a map with keyed values.
; allows omission of optional attributes, provides
; more description, and continues to work if
; more attributes are added.
(def earth
  (map->Planet {:name "Earth"
                :moons 1
                :volume 1.083
                :mass 5.972
                }))

; Constructing entities with options
(declare validate-same-currency)

(defrecord Currency [divisor symbol description])

(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)

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