hootsuite-lt-clojure-uji.md

Introduction to Clojure

Clojure

• imperative
• S-Expression
• anti object-oriented

programming language

• on JVM
• on JavaScript

Hello world

(prn "hello world!")

Hello world (2)

(def hello-world "Bonjour!")

(defn -main []
  (prn hello-world))

Use functions

• JavaScript

console.log((x + 10) * 2);

• Clojure

(prn (* (+ x 10) 2))

• Or

(prn (-> (+ x 10) (* 2)))

Lambda

(fn [x]
  (* (+ x 10) 2))

• Or with macro

#(* (+ % 10) 2)

Other data structures

• Symbol :abc

• Vector

[1 2 3 4 5]
["a" :b \c]

• Map (dictionary; hash)

{:a "hi" :b []}

Good

easy to

• read
  • macro / immutability / naming
• write
  • syntax
• make it concurrent
  • immutability / future / STM

Good naming

• defn
• prn
• cons/conj

(cons 1 [2 3]) ;=> [1 2 3]
(conj [2 3] 1) ;=> [2 3 1]

Good naming

(if a
  b
  c)

(if a
  (do
    b1
    b2)
  c)

(when a
  b1
  c2)

Comment

• comment
• #_

(prn #_(+ 2 (* 3 4)) 999)

JVM

• Clojure is good partly because it's on JVM

• Clojure is terribly bad because it's on JVM

JVM

• Java

x.m(a, b);
C.f(x);

• Clojure

(.m x a b)
(C/f x)

Performance

• slow to start, fast to run

• faster to start, slower to run compare to Scala

Performance

Ruby (2.1.0)

• hello world: 0.02sec
• fib(40): 15.58sec

Clojure (1.5.1)

• hello world: 1.19sec
• fib(40): 4.84sec

Practical?

Yes!

• Leiningen (build tool)

• clojars.org (library; ecosystem)

• tools that depend on libraries for java

tricky parts

• No continuations (call/cc)
• No automatic tail call optimization
• No alternative string literals
• (No user-defined reader macro)

(System/exit 0)

• Clojure

• made by Rich Hickey since 2007

• google "tryclojure" to run some code even without installing clojure