1-notes.md

Functional programming isn't scary: Notes

• These notes accompany a talk I gave on the 15th of May at the Computer Science Dept of UCT.

• @RobStuttaford

• Slides: https://speakerdeck.com/robertstuttaford/functional-programming-star-isnt-star-scary

About Clojure

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Videos by Rich Hickey (creator of Clojure and designer of Datomic)

• Are we there yet?
• Simple made easy
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2-demo.clj

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; FUNCTIONAL COMPOSITION DEMO ;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; define a vector [] of maps {key value} at the symbol 'the-watch'
(def the-watch
  [{:name "Samuel Vimes"              :species "Human"                   :rank "Commander"}
   {:name "Carrot Ironfoundersson"    :species "Human, Dwarf (honorary)" :rank "Captain"}
   {:name "Fred Colon"                :species "Human"                   :rank "Sergeant"}
   {:name "C. W. St J. (Nobby) Nobbs" :species "Human (licensed)"        :rank "Lance-Corporal"}
   {:name "Angua von Überwald"        :species "Werewolf"                :rank "Lance-Corporal"}
   {:name "Detritus"                  :species "Troll"                   :rank "Lance-Corporal"}])

;; all expressions return a value. this one, a call to the 'count'
;; function, returns the count of the elemnts in 'the-watch'.
(count the-watch)

;; 

;; defines a function at the symbol 'has-species? with two arguments.
(defn has-species?
  [species character]
  ;; re-find matches regular expressions to strings.
  (re-find species
           ;; keywords are also functions when given a map!
           ;; will return the value at ':species' key.
           (:species character)))

;; partial composes a function with an argument 'baked' in.
;; gives us a new function to use directly.
;; regular expressions are written as strings with a # prefix.
(def human? (partial has-species? #"Human"))

;; filter composes a predicate - a true/false function - with a
;; collection. returns only those which match the predicate.
(filter human? the-watch)

;;

;; compose a string representation of a character
(defn stringify
  [character]
  (str (:rank character) " "
       (:name character) ", "
       (:species character)))

;; run each character through the stringify function
(map stringify the-watch)

;; 

(comment

  ;; putting it all together to do something meaninful:
  ;; println - our first 'impure' function!
  (println (clojure.string/join "\n"
                                (concat (list "The Watch:"
                                              "-----------------------")
                                        (map stringify the-watch))))

  ;; 

  ;; make it readable by using '->>', a 'threading' macro, which
  ;; reorganises code during compilation.
  ;; semantically identical to the previous version.
  (->> the-watch
       (map stringify)
       (concat (list "The Ankh-Morpork Watch:"
                     "-----------------------"))
       (clojure.string/join "\n")
       (println))

  ;; 

  ;; filter non-humans!
  (->> the-watch
       ;; complement makes a function that returns the opposite boolean
       ;; value to the function it is passed
       (filter (complement human?))
       (map stringify)
       (concat (list "The Ankh-Morpork Watch:"
                     "-----------------------"))
       (clojure.string/join "\n")
       (println))

  ;; 
  )















;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; IMMUTABLE DATA DEMO ;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;

(def a "this is a")

(println a)

;; 

;; bind [1 2 3] to 'a' for the duration of this let form's contents
(let [a [1 2 3]]
  (println a)

  ;; rebind a
  (let [a [4 5 6]]
    (println a))
  
  (println a))

;; 

(println a)

;; 

;; an example that actually does something:

(let [content (slurp "/Users/robert/quote.edn")]
  (println content)
  (let [content (read-string content)]
    (println (str "\n\t'" (:quote content) "' - " (:author content) "\n")))
  (println content))

;; 

(println content)

;;

3-quote.edn

{:quote "Always be wary of any helpful item that weighs less than its operating manual."
 :author "Terry Pratchett"}

gistfile1.txt

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