units-test.clj

(ns distance.test.miles
  (:use [clojure.test])
  (:require [distance.units])
  (:import [distance.units Miles])
  (:import [distance.units Kilometers])
  (:import [distance.units Meters])
  (:use [distance.units]))

(deftest miles-to-miles
  (is
   (= (to-miles (Miles. 5)) (Miles. 5))
   "Converting miles to miles returned a different length."))

(deftest miles-to-km
  (is
   (= (to-km (Miles. 1)) (Kilometers. 1.609))
   "Converting miles to kilometers failed."))

(deftest miles-to-meters
  (is
   (= (to-meters (Miles. 1)) (Meters. 1609))
   "Converting miles to meters failed."))

(deftest km-to-km
  (is
   (= (to-km (Kilometers. 5)) (Kilometers. 5))
   "Converting kilometers to kilometers returned a different length."))

(deftest km-to-miles
  (is
   (= (to-miles (Kilometers. 1.609)) (Miles. 1))
   "Converting kilometers to miles failed."))

(deftest km-to-meters
  (is
   (= (to-meters (Kilometers. 1)) (Meters. 1000))
   "Converting kilometers to meters failed."))

(deftest meters-to-meters
  (is
   (= (to-meters (Meters. 500)) (Meters. 500))
   "Converting meters to meters failed."))

(deftest meters-to-kilometers
  (is
   (= (to-km (Meters. 1000)) (Kilometers. 1))
   "Converting meters to kilometers failed."))

(deftest meters-to-miles
  (is
   (= (to-miles (Meters. 1609)) (Miles. 1))
   "Converting meters to miles failed."))

units.clj

(ns distance.units)

(def *miles-meters-ratio* 1.609)
(def *metric-ratio* 1000)

(defrecord Miles [length])
(defrecord Kilometers [length])
(defrecord Meters [length])

(defprotocol Convertable
  (to-miles [this])
  (to-km [this])
  (to-meters [this]))

(extend-type Miles
  Convertable
  (to-miles [this] (Miles. (:length this)))
  (to-km [this] (Kilometers. (* (:length this) *miles-meters-ratio*)))
  (to-meters [this] (to-meters (to-km this))))

(extend-type Kilometers
  Convertable
  (to-miles [this] (Miles. (/ (:length this) *miles-meters-ratio*)))
  (to-km [this] (Kilometers. (:length this)))
  (to-meters [this] (Meters. (* (:length this) *metric-ratio*))))

(extend-type Meters
  Convertable
  (to-miles [this] (to-miles (to-km this)))
  (to-km [this] (Kilometers. (/ (:length this) *metric-ratio*)))
  (to-meters [this] this))