Stefan Kanev: Clojure, ClojureScript and Why They're Awesome at I T.A.K.E. Unconference 2015

http://xkcd.com/{224,297}

Clojure & ClojureScript
Stefan Kanev
http://skanev.com/
@skanev
I.T.A.K.E. Unconf
26 May 2015
Soﬁa

twitter: @skanev
github: skanev
blog: http://skanev.com/

A modern LISP, hosted
in the JVM, with a
focus on concurrency

LISP:“that language with
the parentheses” 
also: a subculture

“LISP is too
hip, even for
me”
– a hipster

a * b + c * d
(+ (* a b) (* c d))

homoiconicity
data is code, code is
data

Sans the antiquities:
car cdr lambda

parallelism
Breaking a problem down to
smaller parts that can be
computed at the same time

concurrency
Synchronising a number of
independent processes that are
ﬁghting for the same resources

(+ 1 2 3 4 5 6)
(< u v w x y z)

(+ (* a b) (* c d))
(+ (* a b)
(* c d))

(defn say-hello [who]
(println "Hello" who "!"))
(say-hello "Doctor")

OMG Parentheses!
Or should I say:
((o) ((m)) (g) ( (( (( (( )) )) )) ))

Parentheses in LISP are
not unlike metric time

(defn classify [age]
(if (<= 13 age 19)
"Teenager"
"A normal person"))
(classify 18) ; "Teenager"

(defn factorial [n]
(if (= n 1)
1
(* n (factorial (- n 1)))))

(defn fib [n]
(cond (= n 0) 1
(= n 1) 1
:else (+ (fib (- n 1))
(fib (- n 2)))))

(map (fn [n] (str "Mr. " n))
["Anderson"
"Bond"
"Bean"])
; ("Mr. Anderson”
; "Mr. Bond"
; "Mr. Bean")

(filter prime? (range 2 100))
; (2 3 5 7 11 13 17 19 23 29 31 37 41
; 43 47 53 59 61 67 71 73 79 83 89 97)

(defn prime? [n]
(not-any? (fn [x] (zero? (rem n x)))
(range 2 (inc (Math/sqrt n)))))

maps (hashes)
vectors (arrays)
sets

“Modifying” a structure creates
a copy containing the change.
The original remains unmodiﬁed.

The “originals” are
maximally reused

a ! (3 2 1)
b ! (conj a 4)
(4 3 2 1)

123
a
a ! (3 2 1)
b ! (4 3 2 1)
4b
5c
c ! (conj a 5)

Hash Table
1020394597
1020205863
{:foo first, :bar second}
ﬁrst
second

Hash Table
1020394597
1020205863
Hash Table
1020394597
1020205863
1021027443
(conj table :qux 1024)

Vectors
(“arrays” in other langs)

Vectors are represented by trees
Each node has 32 children

325 = 33 554 432
326 = 1 073 741 824

“essentially constant
time”

100 € +50 € ⨯2
How much money will the account have?

100 €
+50 € = 150 €
x2 = 300 €

100 €
x2 = 200 €
+50 € = 250 €

100 €
200 €
150 €
x2
+50
100 €
100 €

100 €
150 €
200 €
x2
+50
100 €
100 €

state mutation is
modelled as a
transaction

if two transactions
“get in each others’
way”, one of them will
restart

(def account (ref 100))
; Thread 1 - Uncle Scrooge
(dosync
(alter account (fn [n] (* n 2)))
(println "Scrooge: set to " @account))
; Thread 2 - Donald Duck
(dosync
(alter account (fn [n] (+ n 50)))
(println "Donald: set to " @account))

100 €
300 €
150 €
x2
+50
100 €
100 €
X
x2
150 €

Powerful instrument
allowing expressive code

Also known as:
the mother of all
metaprogramming

Outputs on STDOUT.
We want to have the
result in a string
instead.

PrintStream original = System.out;
ByteArrayOutputStream output = new ByteArrayOutputStream();
try {
PrintStream fileStream = new PrintStream(output);
orgStream = System.out;
System.setOut(fileStream);
doStuff();
} finally {
System.setOut(original);
}
String output = output.toString();

String output = withOutStr {
doStuff();
}

(unless (hungry?)
(sleep)
(eat))
(if (not (hungry?))
(sleep)
(eat))

(defmacro unless [condition consequent alternative]
`(if (not ~condition)
~consequent
~alternative))

unless (isHungry()) {
sleep();
} else {
eat();
}

(cond (= n 0) 1
(= n 1) 1
:else (+ (fib (- n 2))
(fib (- n 1))))
(if (= n 0)
1
(if (= n 1)
1
(+ (fib (- n 2))
(fib (- n 1)))))

(source if-let)
(source await)

Stefan Kanev: Clojure, ClojureScript and Why They're Awesome at I T.A.K.E. Unconference 2015

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Stefan Kanev: Clojure, ClojureScript and Why They're Awesome at I T.A.K.E. Unconference 2015