Higher Order Functions

ClojureScript Koans Higher Order Functions Answers
http://clojurescriptkoans.com/#higher-order-functions/1

1)The map function relates a sequence to another

(= '( 4 8 12 )(map (fn [x] (* 4 x)) [1 2 3]))

2)You may create that mapping

(= '(1 4 9 16 25) (map (fn [x]  (* x x) ) [1 2 3 4 5]))

3)Or use the names of existing functions

(= [false false true false false] (map nil? [:a :b nil :c :d]))

4)A filter can be strong

(=  '()  (filter (fn [x] false) '(:anything :goes :here)))

5)Or very weak

(=  '(:anything :goes :here)  (filter (fn [x] true) '(:anything :goes :here)))

6)Or somewhere in between

(= [10 20 30] (filter (fn [x]  (< x 40) ) [10 20 30 40 50 60 70 80]))

7)Maps and filters may be combined

(= [10 20 30] (map (fn [x]  (* x 10) ) (filter (fn [x]  (< x 4) ) [1 2 3 4 5 6 7 8])))

8)Reducing can increase the result

(=  24  (reduce (fn [a b] (* a b)) [1 2 3 4]))

9)You can start somewhere else

(= 2400 (reduce (fn [a b] (* a b))  100  [1 2 3 4]))

10)Numbers are not the only things one can reduce

(= "longest" (reduce (fn [a b] (if (<  (count a) (count b) ) b a)) ["which" "is" "the" "longest" "word"]))