Clojure for Java developers

Assumptions
Some experience with Java development
Little or no experience with Clojure
You want to try something different!
Haskell, Erlang and
Scala seem hairy!

Why get functional ?
4 cores in a typical developers Mac book Pro
How long until 128 cores
in a laptop is common?

Parallelism made simpler
Provides tools to create massively parallel
applications
Not a panacea

Think differently
Functional concepts
Changing state

One of many JVM languages
Java
Groovy
JRuby
Jython
Scala
Clojure
Jaskell
Erjalang

A little more ubiquitous
.Net framework – DLR/CLR
In browsers > ClojureScript
Native compilers

Very small syntax
( )
[ ], { }
def, defn, defproject
list, map, vector, set
let
.
_
atom

Mathematical concepts
Functions have a value
For a given set of arguments, you
should always get the same
result
(Referential transparency)

Clojure concepts
Encourages Pure Functional approach
- use STM to change state
Functions as first class citizens
- functions as arguments as they return a value
Make JVM interoperation simple
- easy to use your existing Java applications

A better Lisp !
Sensible () usage
Sensible macro names
JVM Interoperability
Lisp is the 2nd oldest programming language still in use

Which LISP is your wingman ?
Common Lisp Clojure

The dark side of Clojure
( x )( x )

( ( x ) )( ( x ) )

( ( ( x ) ) )( ( ( x ) ) )

( ( ( ( x ) ) ) )( ( ( ( x ) ) ) )

( ( ( ( ( x ) ) ) ) )( ( ( ( ( x ) ) ) ) )

()
verses
{ () };
Clojure verses Java syntax

([] (([])))
verses
{ ({([])}) };
Well actually more like....

Its all byte code in the end..
Any object in Clojure is just a regular java
object
Types inherit from:
java.lang.object

What class is that...
(class "Jr0cket")
=> java.lang.String
(class
(defn hello-world [name]
(str "Hello cruel world")))
=> clojure.lang.Var
Using (type …) you can discover either the metadata or class of something

Prefix notation
3 + 4 + 5 – 6 / 3
(+ 3 4 5 (- (/ 6 3)))

Java made simpler
// Java
import java.util.Date;
{
Date currentDate = new Date();
}
; Clojure
(import java.util.Date)
(new Date)

Ratio
Unique data type
Allow lazy evaluation
Avoid loss of
precision
(/ 2 4)
(/ 2.0 4)
(/ 1 3)
(/ 1.0 3)
(class (/ 1 3)

Prefix notation everywhere
(defn square-the-number [x]
(* x x))

Defining a data structure
( def my-data-structure [ data ] )
( def days-of-the-week
[“Monday” “Tuesday” “Wednesday”])
You can dynamically redefine what the name binds to, but the vector is immutable

Really simple data structure
(def jr0cket
{:first-name "John",
:last-name "Stevenson"})
Using a map to hold key-value pairs.
A good way to group data into a meaningful concept

Defining simple data
(def name data)

Calling Behaviour
(function-name data data)

List – Ordered collection
(list 1 3 5 7)
'(1 3 5 7)
(quote (1 3 5 7))
(1 2 3) ; 1 is not a function

Vectors – hashed ordered list
[:matrix-characters
[:neo :morpheus :trinity :smith]]
(first
[:neo :morpheus :trinity :smith])
(nth
[:matrix :babylon5 :firefly] 2)
(concat [:neo] [:trinity])

Maps – unordered key/values
{:a 1 :b 2}
{:a 1, :b 2}
{ :a 1 :b }
java.lang.ArrayIndexOutOfBounds
Exception: 3
{ :a 1 :b 2}
{:a 1, :b 2}
{:a {:a 1}}
{:a {:a 1}}
{{:a 1} :a}
{{:a 1} :a}
; idiom - put :a on the left

Lists are for code
and data
Vectors are for data
and arguments
Its not that black and white, but its a useful starting point when learning

Joda Time
(use 'clj-time.core)
(date-time 1986 10 14)
(hour (date-time 1986 10 14 22))
(from-time-zone (date-time 2012 10 17)
(time-zone-for-offset -8))
(after? (date-time 1986 10)
(date-time 1986 9))
(show-formatters)

Importing Java into Clojure
(ns drawing-demo
(:import [javax.swing Jpanel JFrame]
[java.awt Dimension]))

Calling Java GUI
(javax.swing.JOptionPane/showMessageDialog nil
"Hello Java Developers" )

Simplifying with doto
doto evaluates the first expression in a chain of expressions and saves it in a
temporary variable. It then inserts that variable as the first argument in each of the
following expressions. Finally, doto returns the value of the temporary variable.

Working with Java
Java Classes
● fullstop after class name
(JFrame. )
(Math/cos 3) ; static method call
Java methods
● fullstop before method name
(.getContentPane frame) ;;method name first
(. frame getContentPane) ;;object first

All hail the REPL
An interactive shell for
clojure
Fast feedback loop
for clojure

Maven
Just like any other Java project
Step 1)
Add Clojure library dependency to your
POM
Step 2)
Download the Internet !!!

Leiningen
lein new
lein deps
lein repl
lein jack-in
● Create a new Clojure project
● Download all dependencies
● Start the interactive shell (repl)
● Start a REPL server
leiningen.org

Eclipse & Clojure
= Counter Clockwise

Light table
Kickstarter project to build a development environment,
focused on the developer experience

Clojure calling Java web stuff
(let [conn]
(doto (HttpUrlConnection. Url)
(.setRequestMethod “POST”)
(.setDoOutput true)
(.setInstaneFollowRedirects
true))])

Deploy Clojure to the Cloud
lein new heroku my-web-app

Getting a little more functional

Recursive functions
● Functions that call
themselves
● Fractal coding
● Tail recursion
● Avoids blowing the
stack
● A trick as the JVM
does not support tail
recursion directly :-(

Recursion – managing memory
(defn recursive-counter [value]
(print value)
(if (< value 1000)
(recur (+ value 4))))
(recursive-counter 100)

Software Transactional Memory
Provides safe,
concurrent access to
memory
Agents allow
encapsulated access
to mutable resources
http://www.studiotonne.com/illustration/software-transactional-memory/

Atoms - Coding state changes
; Clojure atom code
(def mouseposition (atom [0 0]))
(let [[mx my] @mouseposition])
(reset! mouseposition [x y])

Database access
Clojure-contrib has sql
library
Korma - "Tasty SQL for
Clojure"
CongoMongo for
MongoDB
https://devcenter.heroku.com/articles/clojure-web-application

Where to find out more...
clojure.org/
cheatsheet

Thank you
@jr0cket
London Clojurians
Google Group

Clojure for Java developers

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