gistfile1.clj

;; WARNING: the code here wasn't tested in REPL

(ns space-age)

(def earth-year-in-seconds 31557600.0)

(def orbital-periods
  {:mercury 0.2408467
   :venus   0.61519726
   :mars    1.8808158
   :jupiter 11.862615
   :saturn  29.447498
   :uranus  84.016846
   :neptune 164.79132})

;; Instead of implicitly creating root bindings for functions I create a "factory" function.

(defn solar-age-calculation-for
  "Returns a function which, when given an age in seconds, calculates age in given planet's solar years."
  [planet]
  (if-let [period (orbital-periods planet)]
    (fn [seconds]
      (/ seconds (* period earth-year-in-seconds)))
    (throw (Exception. (str "Invalid planet " planet)))))

;; This way, I can easily extend the API with specialized functions for planets.
;; Yes, it's a couple more keystrokes to create these than generating them with
;; a macro, but the bindings are explicit and easily discoverable when going through
;; the code.

(def solar-age-on-mercury (solar-age-calculation-for :mercury))

;; If the space of possibilities would be too big to create all possible bindings
;; by hand, I would either use the factory function in contexts where there are
;; many values to be calculated for a given planet or use a generalized version:

(defn solar-age-on [planet period]
  (partial solar-age-calculation-for planet))

;; The generalized function has additional benefit of allowing for calculation of the value
;; for multiple planets.

(defn solar-age-for-all-planets [seconds]
  (into {} (map (juxt identity solar-age-on) (keys orbital-periods) (repeat seconds))))