Clojure Koans Walkthrough

1. Introduction
2. Lists
3. Vectors
4. Sets
5. Maps
6. Functions
7. Conditionals
8. Higher Order Functions
9. Runtime Polymorphism
10. Lazy Sequences
11. Sequence Comprehensions
12. Creating Functions
13. Recursion
14. Destructuring
15. Refs
16. Atoms
17. Macros
18. Datatypes
19. Java Interop
20. Partition

Recommended Books

• ← Sequence Comprehensions
• Recursion →

12. Creating Functions - Koans Walkthrough

This video introduces some sleek ways to create functions, with tools such as "complement", function composition and partial application. Clojure really does let you write code in a "point free" style.

Here is the 12_creating_functions.clj source code:

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

(meditations
  "One may know what they seek by knowing what they do not seek"
  (= [__ __ __] (let [not-a-symbol? (complement symbol?)]
                  (map not-a-symbol? [:a 'b "c"])))

  "Praise and 'complement' may help you separate the wheat from the chaff"
  (= [:wheat "wheat" 'wheat]
       (let [not-nil? ___]
         (filter not-nil? [nil :wheat nil "wheat" nil 'wheat nil])))

  "Partial functions allow procrastination"
  (= 20 (let [multiply-by-5 (partial * 5)]
          (___ __)))

  "Don't forget: first things first"
  (= [__ __ __ __]
       (let [ab-adder (partial concat [:a :b])]
         (ab-adder [__ __])))

  "Functions can join forces as one 'composed' function"
  (= 25 (let [inc-and-square (comp square inc)]
          (inc-and-square __)))

  "Have a go on a double dec-er"
  (= __ (let [double-dec (comp dec dec)]
          (double-dec 10)))

  "Be careful about the order in which you mix your functions"
  (= 99 (let [square-and-dec ___]
          (square-and-dec 10))))