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

• ← Macros
• Java Interop →

18. Datatypes - Koans Walkthrough

This video introduces datatypes, namely deftype, defrecord. These allow you to easily generate a class with named fields. defrecord only allows immutable fields, and it provides a full implementation of a Clojure map, so you can use it as a map. If you're trying to implement a data-structure that is not map-like (e.g. a vector), then you may want to use deftype instead.

Here is the 18_datatypes.clj source code:

(defrecord Nobel [prize])
(deftype Pulitzer [prize])

(defprotocol Award
  (present [this recipient]))

(defrecord Oscar [category]
  Award
  (present [this recipient]
    (print (str "Congratulations on your "
                (:category this) " Oscar, "
                recipient
                "!"))))

(deftype Razzie [category]
  Award
  (present [this recipient]
    __))

(meditations
  "Holding records is meaningful only when the record is worthy of you"
  (= __ (.prize (Nobel. "peace")))

  "Types are quite similar"
  (= __ (.prize (Pulitzer. "literature")))

  "Records may be treated like maps"
  (= __ (:prize (Nobel. "physics")))

  "While types may not"
  (= __ (:prize (Pulitzer. "poetry")))

  "Further study reveals why"
  (= __
     (map map? [(Nobel. "chemistry")
                (Pulitzer. "music")]))

  "Either sort of datatype can define methods in a protocol"
  (= __
     (with-out-str (present (Oscar. "Best Picture") "Evil Alien Conquerors")))

  "Surely we can implement our own by now"
  (= "You're really the Worst Picture, Final Destination 5... sorry."
     (with-out-str (present (Razzie. "Worst Picture") "Final Destination 5"))))