clojure-koans/09_higher_order_functions.clj

  ; "The map function relates a sequence to another"
  (= [__ __ __] (map (fn [x] (* 4 x)) [1 2 3]))

  ; "You may create that mapping"
  (= [1 4 9 16 25] (map (fn [x] __) [1 2 3 4 5]))

  ; "Or use the names of existing functions"
  (= __ (map nil? [:a :b nil :c :d]))

  ; "A filter can be strong"
  (= __ (filter (fn [x] false) '(:anything :goes :here)))

  ; "Or very weak"
  (= __ (filter (fn [x] true) '(:anything :goes :here)))

  ; "Or somewhere in between"
  (= [10 20 30] (filter (fn [x] __) [10 20 30 40 50 60 70 80]))

  ; "Maps and filters may be combined"
  (= [10 20 30] (map (fn [x] __) (filter (fn [x] __) [1 2 3 4 5 6 7 8])))

  ; "Reducing can increase the result"
  (= __ (reduce (fn [a b] (* a b)) [1 2 3 4]))

  ; "You can start somewhere else"
  (= 2400 (reduce (fn [a b] (* a b)) __ [1 2 3 4]))

  ; "Numbers are not the only things one can reduce"
  (= "longest" (reduce (fn [a b]
                         (if (< __ __) b a))
                       ["which" "word" "is" "longest"]))