Programming with Freedom & Joy

Confessions of an ExJavaEE enthusiast that found enlightenment through Clojure

Hildeberto Mendonça@htmfilho

100 Years Anniversary of General Relativity Theory

E = mc2

Philosophy Behind Relativity

Object Oriented and Imperative

● Object oriented– Created for simulation systems (Simula 67)

– Object represent state and behavior which is a requirement for simulations

– Very good for user interface development (a simulation of concrete objects. Ex. button)

– But, is it good for everything else?

– By putting in a distributed environment you have clones and reports of gang rapes

● Imperative– The art of programming in sequential steps to accomplish tasks

– Sometimes the list of steps is so large that frameworks should be used to make the work practical

– By mixing it with object-oriented programming you have nitroglycerin (steps to manage states)

Java to Clojure

Public class StringUtils {

Public static boolean isBlank(String str) {

Int strlen;

if(str == null || (strlen = str.length()) == 0) {

Return true;

}

For (int i = 0; I < strlen; i++ ) {

if((Character.isWhitespace(str.charAt(i)) == false)) {

Return false;

}

}

Return true;

}

}

(defn blank? [s] (every? #(Character/isWhitespace %) s))

HDD (Handicraft Driven Development)

Java doesn't leave time for the essential in live

What Happened Hildeberto?!

1.Oracle stopped supporting Glassfish

2.Pushed to look for alternatives

3.Started migrating to Jboss (Nightmare!!!)

4.Realized that Java EE is a broken standard

5.Pushed to look for alternatives once again

6.Found Scala + Akka + Play (Wonderfull!!!)

7.But Scala is pretty damn complex :-(

THE WORLD IS SOOO COMPLEX!!!

I've met Clojure at Devoxx 2013DJs mixing with it!! :o

The Turning Point Towards Clojure

Clojure

● Dialect of LISP (List Processing)● Dynamically typed but strongly typed● Functional (not purely functional, a bit more)● Host interoperability● Software transactional memory● Everything is an expression● Compiled● Macros● Lazy sequences

By Rich Hickey

LISP?

● Created by John McCarthy● He coined the term “Artificial Intelligence”● Exceptionally intelligent● Created the “if” structure.

John McCarthy

Learn Once Code Everywhere

Java Clojure Bytecode JVM

C# Clojure CIL code CLR

Clojure ClojureScript JavaScript JS Engine

Python Clojure Python Cpython

● Hosted languagehttp://clojure.org

http://clojure.org/clojureclr

http://clojure.org/clojurescript

https://github.com/halgari/clojure-py

Syntax in One Slide

● Numbers – 3454.23● Ratios – 3/56● Strings - “Carnaval”● Characters - \a \b \return● Symbols – alex b c● Keywords - :a :b :c● Booleans – true false● Null – nil● Regex patterns #”a*b”

● Lists (1 2 3 4 5 4)● Vectors [1 2 3 3 5]● Maps {:a 1 :b 2 :c 2}● Sets #{1 2 3 4 5}● Prefixes ' #() @ ~ ~@● Macros● ; (documentation)

It is simple because it is all about interpretation of data structures, not text

REPL

Basic Side-by-Side with Java

Clojure

(not k)

(inc a)

(/ (+ x y) 2)

(instance? Java.utils.List al)

(if (not a) (inc b) (dec b))

(Math/pow 2 10)

(.someMethod obj “foo” (.otherMethod otherObj o))

Java

!k

a++, ++a, a += 1, a + 1

(x + y) / 2

al instanceof java.util.List

!a ? b + 1 : b – 1;

Math.pow(2, 10);

obj.someMethod(“foo”, otherObj.otherMethod(o));

Homoiconicity

● Code as data

(if (odd? 2) “odd” “even”)● Significant simplification (less memorization)

(Symbol arg1 arg2 … argN)You already learned 80% of what it takes to program in Clojure!

● Metaprogramming facilities (macros)

(def n (fn […] …)) => (defn n […] ...)● Represents the abstract syntax tree directly

Abstract Syntax Tree Example

● RequiresRole && userHasRole(“MODIFY”, assetId)

AND

requiresRole Call

userHasHole assetId

“MODIFY”

● (and requiresRole (userHasRole “MODIFY” assetId))

Data Structures

● Persistent (versioned)● Immutable

T1 T2

Structural Sharing

(conj [1 2 3 4] 5)[1 2 3 4]

Traditional Evaluation

CodeText

Compiler

Executable.class/.jar

Run java JVM

Effect

Not happy with the result? Start over.

Characters

Bytecode

Clojure Evaluation

Codefile

Reader

Evaluator / compiler

JVM

Effect

Not happy with the result? Send more code to the reader.

Characters

Data structures

Bytecode

Once started, it keeps running while you work.

Local / RemoteTCP/IP

Connection

Program

Characters

Data structures

Macro

Data structures

Macros

Data StructuresCode Data

You Macro

Tool SupportCursive (IntelliJ)Counterclockwise (Eclipse)Visual StudioLightTableEmacsVim

LightTable

Written in ClojureScript

Community

● Everything is on GitHub: https://github.com/clojure

● 70 user groups; only one in Brazil (São Paulo)● Mailing list with ~10k users (everybody is nice)● Position on Tiobe: 62

http://www.clojure-toolbox.com

http://www.braveclojure.com

http://www.4clojure.com

Books

Thanks!!

Hildeberto.com

@htmfilho