Clojure Koans Answers and Explanations - 12 - Creating Functions

Let's look at the solutions for the next of Clojure Koans Creating Functions.

(defn square [x] (* x x))

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;1

(= [__ __ __] (let [not-a-symbol? (complement symbol?)]

                  (map not-a-symbol? [:a 'b "c"])))

; As usual, for reading such expressions, start by

; reducing them

; the combination of let and application of the newly

; created function acts on the vector [:a 'b "c"]

; but only 'b is a symbol. After complement

; we get (true false true)

(= [true false true] (let [not-a-symbol? (complement symbol?)]

                  (map not-a-symbol? [:a 'b "c"])))

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;2

(= [:wheat "wheat" 'wheat]

       (let [not-nil? ___]

         (filter not-nil? [nil :wheat nil "wheat" nil 'wheat nil])))

; if you followed the previous example closely then

(= [:wheat "wheat" 'wheat]

       (let [not-nil? (complement nil?)]

         (filter not-nil? [nil :wheat nil "wheat" nil 'wheat nil])))

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;3

(= 20 (let [multiply-by-5 (partial * 5)]

          (___ __)))

; the partial fn multiply-by-5 multiplies by 5

; we need and additonal 4

(= 20 (let [multiply-by-5 (partial * 5)]

          (multiply-by-5 4)))

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;4

(= [__ __ __ __]

       (let [ab-adder (partial concat [:a :b])]

         (ab-adder [__ __])))

(= [:a :b :c :d]

       (let [ab-adder (partial concat [:a :b])]

         (ab-adder [:c :d])))

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;5

(= 25 (let [inc-and-square (comp square inc)]

          (inc-and-square __)))

; what, but 4?

; because inc is to be applied along with square

(= 25 (let [inc-and-square (comp square inc)]

          (inc-and-square 5)))

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;6

(= __ (let [double-dec (comp dec dec)]

          (double-dec 10)))

; since dec is applied twice

(= 8 (let [double-dec (comp dec dec)]

          (double-dec 10)))

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;7

(= 99 (let [square-and-dec ___]

          (square-and-dec 10))))

; the ordering of functions is critcial

; those coming from imperative languages can vouch for this

(= 99 (let [square-and-dec (comp dec square)]

          (square-and-dec 10)))