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

• ← Destructuring
• Atoms →

15. Refs - Koans Walkthrough

This video introduces refs which let us store values, and mutate them in a coordinated, thread-safe way. Refs can only be mutated in dosync transaction.

Here is the 15_refs.clj source code:

(def the-world (ref "hello"))
(def bizarro-world (ref {}))

(meditations
  "In the beginning, there was a word"
  (= __ (deref the-world))

  "You can get the word more succinctly, but it's the same"
  (= __ @the-world)

  "You can be the change you wish to see in the world."
  (= __ (do
          (dosync (ref-set the-world "better"))
          @the-world))

  "Alter where you need not replace"
  (= __ (let [exclamator (fn [x] (str x "!"))]
          (dosync
           (alter the-world exclamator)
           (alter the-world exclamator)
           (alter the-world exclamator))
          @the-world))

  "Don't forget to do your work in a transaction!"
  (= 0 (do __
           @the-world))

  "Functions passed to alter may depend on the data in the ref"
  (= 20 (do
          (dosync (alter the-world ___))))

  "Two worlds are better than one"
  (= ["Real Jerry" "Bizarro Jerry"]
       (do
         (dosync
          (ref-set the-world {})
          (alter the-world assoc :jerry "Real Jerry")
          (alter bizarro-world assoc :jerry "Bizarro Jerry")
          __))))