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

• ← Higher Order Functions
• Lazy Sequences →

9. Runtime Polymorphism - Koans Walkthrough

This video covers runtime polymorphism. You can dispatch functions based on arity (number of parameters), or anything else through the power of multi-methods.

Here is the 09_runtime_polymorphism.clj source code:

(defn hello
  ([] "Hello World!")
  ([a] (str "Hello, you silly " a "."))
  ([a & more] (str "Hello to this group: "
                   (apply str
                          (interpose ", " (concat (list a) more)))
                   "!")))

(defmulti diet (fn [x] (:eater x)))
(defmethod diet :herbivore [a] __)
(defmethod diet :carnivore [a] __)
(defmethod diet :default [a] __)

(meditations
  "Some functions can be used in different ways - with no arguments"
  (= __ (hello))

  "With one argument"
  (= __ (hello "world"))

  "Or with many arguments"
  (= __
     (hello "Peter" "Paul" "Mary"))

  "Multimethods allow more complex dispatching"
  (= "Bambi eats veggies."
     (diet {:species "deer" :name "Bambi" :age 1 :eater :herbivore}))

  "Different methods are used depending on the dispatch function result"
  (= "Simba eats animals."
     (diet {:species "lion" :name "Simba" :age 1 :eater :carnivore}))

  "You may use a default method when no others match"
  (= "I don't know what Rich Hickey eats."
     (diet {:name "Rich Hickey"})))