Introduction to FP

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Paweł Kapała

in practice

Feedback loop

http://imgs.xkcd.com/comics/compiling.png

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Read

Eval

Print

Loop

Interactively with REPL!

All while the app is running!

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Part 1: ABC

nil

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false

true

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0 1 2 3

0.2

2/3

2

5302542730

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"str"

"Wawel"

"Kraków"

"hello world"

"John"

"Mścigniew"

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:key

:title

:id

:chupacabra

:name

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(list)

("a" "b" "c" "d" "e" "f")

(0 1 2 3 4 5 6 7)

(:a :b :c :d :e)

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()

[vec]

[0 1 2 3 4 5 6]

["John" "Jane" "Lucy"]

[:title :name :id]

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{map}

{:id 1 :name "John" :surname "Doe"}

{:subject "this" :predicate "is"
 :preposition "a" :object "map"}

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and they were all immutable!

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immutablility leads to clean code

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(def name binding)

a-symbol

bar

namespace/example

iamfoo

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(foo arg1 arg2)

(str "a" "b" "c")

(conj [] 1 2 3 4)

 

(+ 2 4 6 7)

(first [1 2 3 4])

 

(assoc {:id 1 :name "John"} :surname "Doe")

(= {:id 1 :name "Joe"} {:id 1 :name "Joe"})

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(defn fname [arg1 arg2]
body
)

(defn greeter [arg1 arg2]
    (str "Hello " arg1 " " arg2 "!"))

(defn plus [arg1 arg2]
    (+ arg1 arg2))

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#(body %1 %2)

#(+ %1 %2)

#(str "Hello " %1 " " %2 "!")

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and they were pure*!

*at least for the most part

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Part 2: The story

1 404 years ago...

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There lived...

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...Krakus

{:name "Krakus" :health 100}
(defn citizen [name]
  {:name name :health 100})

(citizen "Krakus")

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and he was king!

{:name "Krakus" :health 100 :title :king}
(defn promote [citizen title]
  (assoc citizen :title title))

(defn king [name] 
  (promote (citizen name) :king))

(king "Krakus")

assoc returns new map with key value added to it

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He had the greatest castle!

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{:name "Wawel"}
(defn castle [name]
  {:name name})

(castle "Wawel")
{:name "Wawel" :citizens [
  {:name "Krakus" :health 100 :title :king}]}
(defn occupy [what whom]
   (update what :citizens 
     (fnil conj []) whom))

(occupy (castle "Wawel") (king "Krakus"))

update returns map with a specified key changed by given function

conj returns collection with element appended

fnil executes specified function passing argument in case of nil

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He was rich!

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He had lots and lots of goods

{:energy 10 :size 10}
(defn food [energy]
  {:energy energy :size 10})

(defn ram[] (food 10))
(defn cow[] (food 20))

(ram)

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... stored at the castle

{:name "Wawel" :goods [{:energy 10 :size 10}]}
(defn store [structure food]
  (update structure :goods 
    (fnil conj []) food))

(store (castle "Wawel") (ram))

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... I said a lot!

({:energy 20 :size 10}
 {:energy 20 :size 10}
 {:energy 20 :size 10}
;... 15 more
 {:energy 20 :size 10}
 {:energy 20 :size 10})
(repeatedly 20 cow)

repeatadly returns (lazy) collection containing function result times the number specified, or infinite if size is not specified

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He had bravest knights

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(defn knight [name]
  (assoc 
    (promote (citizen name) :knight)
    :power 100))

(knight "Małowuj")
{:name "Małowuj" :title :knight :power 100}

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... a lot of them!

{:name "Wawel" :citizens [
  {:name "Małowuj" :title :knight :power 100 :health 100}
  {:name "Minigniew" :title :knight :power 100 :health 100}
  {:name "Gromisław" :title :knight :power 100 :health 100}
  {:name "Nowosiodł" :title :knight :power 100 :health 100}
  {:name "Twardomir" :title :knight :power 100 :health 100}
  {:name "Włościobyt" :title :knight :power 100 :health 100}]}
(reduce occupy (castle "Wawel") 
  (map knight ["Małowuj" "Minigniew" "Gromisław" 
  "Nowosiodł" "Twardomir" "Włościobyt"]))

map executes function for each collection element and returns a collection of results

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reduce calls two-arg function on a first element of the collection, then passes the return back to the function as a first argument and takes next element of the collection...

... but one day around 1:31pm

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Dragon appeared!

(defn dragon [name]
  {:name name :health 1000 :power 1000})

(dargon "Smok")
{:name "Smok" :health 1000 :power 1000}

... and it flew right onto Wawel!

(occupy (castle "Wawel") (dragon "Smok"))

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{:name "Wawel" :citizens [{:name "Smok" 
  :health 1000 :power 1000}]

... and it was ever hungry!

(defn eat [who what]
  (merge-with + who what))

(eat (dragon "Smok") (ram))
{:name "Smok" :health 1000 :power 1000 
 :energy 10 :size 10}

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merge-with combines two map into one using provided function (+ in this case) on the values of matching keys

King ordered: "kill the beast!"

(defn attack [target who]
  (update target :health 
          (fnil - 0) (:power who 0)))

(attack (dragon "Smok") (knight "Minigniew"))
{:name "Smok" :health 900 :power 1000}

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but the dragon struck back!

; {:name "Minigniew" :health -900
;  :title :knight :power 100}
true
(defn dead? [who]
  (<= (:health who 0) 0))

(dead? 
  (attack (knight "Minigniew") (dragon "Smok")))

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Skuba had an idea and took ram and some sulfur and tar

(defn sulfur []
  {:thirst 500})

(defn tar []
  {:size 500))

(defn ram-with-sulfur-and-tar []
  (merge-with + (ram) (sulfur) (tar)))

(ram-with-sulfur-and-tar)
{:energy 10 :size 510 :thirst 500}

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Trojan Ram!

... and the dragon ate it

(eat (dragon "Smok") 
     (ram-with-sulfur-and-tar))
{:name "Smok" :health 1000 :power 1000
 :energy 10 :size 510 :thirst 500}

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with a headache...

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...and after drinking 100 galons

(defn water []
  {:thirst -1 :size 10})

(defn vistula []
  (repeatedly water))

(take 100 (vistula))

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({:thirst -1 :size 10}
 {:thirst -1 :size 10}
 {:thirst -1 :size 10}
 {:thirst -1 :size 10}
 ;... 96x more
)

the dragon blew out to pieces!

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({:name "Smok-0" :health -1000 :power 1000} 
{:name "Smok-1" :health -1000 :power 1000} 
{:name "Smok-2" :health -1000 :power 1000} 
{:name "Smok-3" :health -1000 :power 1000}
;... 96x more
)
(defn pieceof [who no]
  (-> who
      (update :name str "-" no)
      (assoc :health -1000)))

(map #(pieceof (dragon "Smok") %1) (range 100))

-> Is called "a threading macro" that passes result of previous function call as first parameter to next call in block, the above is equivalent to:

(assoc (update who :name str "-" no) :health -1000)

He did it for Kraków !

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The end

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Simple? Perfect!

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https://github.com/kapware/clj-krakus/

So you want to learn clojure?

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4clojure.com

Clojure for the Brave and True

clojure.org

Structure and interpretation of computer programs

Mastering Clojure

clojurekoans.com

Find your mentor!

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Thank you!

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Introduction to functional programming in practice

By kapware

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