Clojure Koans 19 Datatypes Notebook

{:hello (clojure-version)}

(defrecord Nobel [prize])

;; question 19001: What is the function/role of `prize` here? It doesn't appear to be defined as per syntax I have seen before...

;; question 19002: Is prize an attribute/property or something else?

(deftype Pulitzer [prize]) ;; question: What is `deftype`?

;; question 19003: What exactly is `defrecord`, and what is its role and function?

;; question 19004: Why use a record or a type in Clojure?

(defprotocol Award

    (present [this recipient]))

;; description: An protocol named "Award" is created/defined (?) here with

;;              a present "method"/function (?)

;;              which takes (two parameters?)

;;                   a this instance (?)

;;                   and a recipient string (?).

;; question 19005: What is `defprotocol`?

;; answer 19005a: A protocol is similar to an interface. (?)

;; question 19006: What can you do with protocols?

;; answer 19006a: With protocols, you can define interfaces for records and for types.

(defrecord Oscar [category]

    Award

    (present [this recipient]

             (print (str "Congratulations on your "

                         (:category this) " Oscar, "

                         recipient

                         "!"))))

;; description: The Oscar record implements the Award protocol. The `this` refers to the instance of the Oscar.

;; skill 19001: Create a record which implements a protocol.

;; question 19007: What are typical/common/useful example protocols?

(deftype Razzie [category]

    Award

    (present [this recipient]

             (print (str "You're really the " (.category this) ", " recipient "... sorry."))))

;; skill 19002: Create a type (?) which implements a protocol

;; question 19008: How are types typically (hah) referred to in Clojure? "deftypes", types, or something else?

;; "Holding records is meaningful only when the record is worthy of you"

(= "peace" (.prize (Nobel. "peace")))

;; description: Create an instance of a Nobel record with one argument of "peace", then access its "prize" attribute

;; question 19009: What is a record in Clojure?

;; question 19010: What is the dot "." prefix meaning in Clojure?

;; question 19011: What is the dot "." suffix meaning in Clojure?

;; answer 19011a: The dot suffix indicates a constructor function which creates an instance here of records.

;; skill 19003: Construct an "instance" of a type by using `defrecord` (?)

;; "Types are quite similar"

(= "literature" (.prize (Pulitzer. "literature")))

;; question 19012: How are types and records different in Clojure? Why have both?

;; question 19013: What is `.prize` here functioning as? Is "accessor" or "getter" the preferred way to label it, or is there a better/more accepted/correct term? 

;; "Records may be treated like maps"

(= "physics" (:prize (Nobel. "physics")))

;; "While types may not"

(= nil (:prize (Pulitzer. "poetry")))

;; question 19018: What is the utility of this info, "Records may be treated like maps by using keywords to access properties/attributes, but types do not allow for this map-like access." ?

;; question 19014: What is the implementation behind-the-scenes of types in Clojure?

;; "Further study reveals why"

(= [true false]

   (map map? [(Nobel. "chemistry")

              (Pulitzer. "music")]))

;; question 19015: If records are maps (in implementation, functionality, or both?), then what are the differences between records and maps?

;; "Either sort of datatype can define methods in a protocol"

(= "Congratulations on your Best Picture Oscar, Evil Alien Conquerors!"

   (with-out-str (present (Oscar. "Best Picture") "Evil Alien Conquerors")))

;; question 19016: What is a protocol in Clojure?

;; "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")))

;; question 19017: Are deftype and defrecord used often by Clojure developers?