tuturial on high-order functions

(def test-vec 
  [{:age 14 :gender :female}
   {:age 15 :gender :male}
   {:age 17 :gender :female}
   {:age 13 :gender :male}])

(def test-vec-2
  [{:age 12 :gender :female}
   {:age 18 :gender :male}
   {:age 23 :gender :female}
   {:age 9 :gender :male}])

(def test-vec-3
  [{:age 9 :gender :female}
   {:age 22 :gender :male}
   {:age 20 :gender :female}
   {:age 5 :gender :male}])

(map (partial map :age) [test-vec test-vec-2 test-vec-3])
;=> ((14 15 17 13) (12 18 23 9) (9 22 20 5))

(map (comp (partial apply hash-map) 
           (partial mapcat (fn [[k v]] [k (count v)])) 
           (partial group-by identity))
     ((juxt (partial map :age) (partial map :gender)) 
        test-vec))
;=> ({15 1, 13 1, 17 1, 14 1} {:female 2, :male 2})


(def generate-count
  (comp (partial apply hash-map)
        (partial mapcat (fn [[k v]] [k (count v)])) 
        (partial group-by identity)))

(generate-count [:apple :pear :apple :berry :pear :berry])
;=> {:apple 2, :pear 2, :berry 2}

(def generate-counts 
  (partial map generate-count))

(generate-counts [[:apple :pear :apple :berry :pear :berry]
                  [:tom :jerry :tom :tom :jerry :alice]
                  [1 3 2 4 3 1 1 4 2]])
;=> ({:apple 2, :pear 2, :berry 2} {:alice 1, :tom 3, :jerry 2} {1 3, 4 2, 3 2, 2 2})