Clojure Metaprogramming (inspired by http://eyeofthesquid.com/blog/2013/11/17/metaprogramming-with-lists-in-clojure/ )

macro-fun.clj

(ns space-age)

(defn on-earth [seconds]
  (/ 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})
   
(defmacro def-planet [planet period]
  `(defn ~(symbol (str "on-" (name planet))) [seconds#]
      (/ (on-earth seconds#) ~period)))

(doseq [[planet period] orbital-periods]
  (def-planet planet period))

(on-mercury 32000000) ;=> 4.21022...

meta.clj

(ns space-age)

(defn on-earth [seconds]
  (/ 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})
   
(doseq [[planet period] orbital-periods]
          (eval `(defn ~(symbol (str "on-" (name planet))) [seconds#]
                   (/ (on-earth seconds#) ~period))))

(on-mercury 32000000) ;=> 4.21022...