A short introduction to Clojure, a LISP variant for the Java Virtual Machine.

1. Clojure
Clojure
A practical LISP for the JVM
¨
Matthias Nußler
m.nuessler@web.de
February 1, 2011
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2. Clojure
Outline
Introduction
LISP
LISP Quotes
The Clojure Language
Java Interoperability
Calling Java from Clojure
Calling Clojure from Java
Concurrency
IDE Support
Questions & Answers
Bibliography
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3. Clojure
Introduction
What is Clojure?
Clojure is
A fresh approach to LISP
A functional programming language
Created by Rich Hickey
Dynamically typed
Hosted on the Java Virtual Machine
Java interoperability is a design goal, rather than an
implementation detail
Strong concurrency semantics
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4. Clojure
LISP
LISP
LISt Processing (lists are a major data structure)
Originally designed in 1958 by John McCarthy
One of the oldest programming languages in existence
(only Fortran is older)
Once the favored programming language for AI research
Based on lambda calculus
Numerous LIPS dialects exist such as
Scheme
Common LISP
“Lots of Irritating Superfluous Parentheses”?
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5. Clojure
LISP
LISP Quotes
LISP Quotes (1)
“Lisp is worth learning for the profound enlightenment
experience you will have when you finally get it; that experience
will make you a better programmer for the rest of your days,
even if you never actually use Lisp itself a lot.”
–Eric Raymond: How to Become a Hacker
“Lisp is a programmable programming language.”
–John Foderaro: CACM, September 1991
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6. Clojure
LISP
LISP Quotes
LISP Quotes (2)
“Greenspun’s Tenth Rule of Programming: any sufficiently
complicated C or Fortran program contains an ad hoc
informally-specified bug-ridden slow implementation of half of
Common Lisp.”
–Philip Greenspun
“the greatest single programming language ever designed”
–Alan Kay
More quotes: http://www.paulgraham.com/quotes.html
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7. Clojure
The Clojure Language
Clojure Language Basics
“No syntax” :-)
Prefix notation
Lists are a core data structure in Clojure/LISP
Code is written in the language’s own data-structures:
(function param1 param2)
Code as data philosophy
Constants: Evaluate to themselves
Symbols: Evaluate to the value bound to them (unless
quoted)
Bind a value to a symbol with def:
(def a-symbol 42) ; => #’user/a-symbol
a-symbol ; => 42
’a-symbol ; => a-symbol
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8. Clojure
The Clojure Language
Function definition
fn creates an (anonymous) function object
Similar to lambda in Scheme (and Ruby)
(fn [x] (* x x))
Or even shorter using a macro: #(* %1 %1)
Function object can be bound to a symbol
(def square (fn square [x] (* x x)))
(square 3) ; => 9
Clojure provides a convenient shortcut
def plus fn → defn
(defn square [x] (* x x))
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9. Clojure
The Clojure Language
Clojure Data Structures
Data type Example
Numbers 1, 3.14, 3/4, ...
Characters a b c
Strings "a string" → Clojure strings are Java strings
Regexps #""
Symbols a b c
Keywords :default :key
Lists (1 2 3)
Vectors [1 2 3]
Maps {:a 1, :b 2, :c 3}
Sets #{"red" "green" "blue"}
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10. Clojure
The Clojure Language
Higher Order Functions
Higher order functions functions use functions as parameters or
return values. Examples:
every?
filter
map
reduce
Factorial using reduce
1 (defn factorial [n]
2 (let [ numbers ( range 1 (+ 1 n))]
3 ( reduce * numbers )))
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11. Clojure
Java Interoperability
Interoperability Overview
With Clojure it is possible to
Create instances of Java classes
Invoke instance methods
Invoke static methods on Java classes
Implement Java interfaces, extend classes
Generate Java classes and interfaces
Compile Clojure code to Java byte code
Call Clojure from Java
→ Access to thousands of Java libraries and frameworks
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12. Clojure
Java Interoperability
Calling Java from Clojure
Creating Instances
Using the new special form
1 ( import ’(java.text SimpleDateFormat ))
2 (def sdf (new SimpleDateFormat " yyyy-MM-dd "))
Shorter with dot notation
3 (def sdf ( SimpleDateFormat . " yyyy-MM-dd "))
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13. Clojure
Java Interoperability
Calling Java from Clojure
Member Access
Invoking an instance method
1 (defn date-from-string [ date-string ]
2 (let [sdf ( SimpleDateFormat . " yyyy-MM-dd ")]
3 (. parse sdf date-string )))
Accessing a field
4 (def point (java.awt. Point . 10 20))
5 (.x point ) ; => 10
Invoking static methods
6 (Long/ parseLong " 12321 ")
7 ( Thread / sleep 3000)
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14. Clojure
Java Interoperability
Calling Java from Clojure
Method chaining
Method chaining using dot-dot notation
1 (.. ( Calendar / getInstance ) getTimeZone getDisplayName )
That is equivalent to the following Java code
2 Calendar . getInstance (). getTimezone ()
3 . getDisplayName ();
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15. Clojure
Java Interoperability
Calling Java from Clojure
Calling multiple methods on a Java object
Long version
1 ( import ’(java.util Calendar ))
2 (defn the-past-midnight-1 []
3 (let [ calendar-obj ( Calendar / getInstance )]
4 (. set calendar-obj Calendar / AM_PM Calendar /AM)
5 (. set calendar-obj Calendar /HOUR 0)
6 (. set calendar-obj Calendar / MINUTE 0)
7 (. set calendar-obj Calendar / SECOND 0)
8 (. set calendar-obj Calendar / MILLISECOND 0)
9 (. getTime calendar-obj )))
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16. Clojure
Java Interoperability
Calling Java from Clojure
Calling multiple methods on a Java object
Shorter using the doto macro
1 (defn the-past-midnight-2 []
2 (let [ calendar-obj ( Calendar / getInstance )]
3 (doto calendar-obj
4 (. set Calendar / AM_PM Calendar /AM)
5 (. set Calendar /HOUR 0)
6 (. set Calendar / MINUTE 0)
7 (. set Calendar / SECOND 0)
8 (. set Calendar / MILLISECOND 0))
9 (. getTime calendar-obj )))
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17. Clojure
Java Interoperability
Calling Java from Clojure
Implementing interfaces / extending classes
With proxy
1 (defn action-listener [idx frei buttons ]
2 (proxy [java.awt. event . ActionListener ] []
3 ( actionPerformed [e]
4 (...))))
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18. Clojure
Java Interoperability
Calling Clojure from Java
Calling Clojure from Java
1 import clojure .lang.RT;
2 import clojure .lang.Var;
3
4 public class Driver {
5 public static void main( String [] args) throws Exceptio
6 RT. loadResourceScript (" clojure_script .clj");
7 Var report = RT.var("clj. script . examples ", " print-re
8 Integer result = ( Integer ) report . invoke ("Siva");
9 System .out. println ("[Java] Result : " + result );
10 }
11 }
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19. Clojure
Concurrency
Clojure’s Approach to Concurrency
Immutable objects
Persistent data-structures (preserve the previous version
of themselves when modfied)
Managed references (4 different kinds)
Clojure programs are guaranteed not to deadlock
Software Transactional Memory (STM) system
has ACI properties: atomicity, consistency, isolation (no
durability)
implements multiversion concurrency control (MVCC)
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20. Clojure
Concurrency
Managed References
Ref. type Useful for
ref shared changes, synchronous, coordinated changes
agent shared changes, asynchronous, independent changes
atom shared changes, synchronous, independent changes
var isolated changes
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21. Clojure
Concurrency
Refs, Agents, Atoms, Vars (1)
Refs
A ref holds a value that can be changed in a synchronous
and coordinated manner
Creating a ref: (def all-users (ref {}))
Dereferencing a ref: (deref all-users)
Mutating a ref using the ref-set, alter or commute
functions
(ref-set all-users {})
STM transaction required!
Correct way: (dosync (ref-set all-users {}))
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22. Clojure
Concurrency
Refs, Agents, Atoms, Vars (2)
Agents
Allow for asynchronous and independent changes to
shared mutable data
Hold values that can be changed using special functions
(asynchronously, at some point in the future)
Atoms
Allow for synchronous and independent changes to
mutable data
Atoms are updated synchronously (immediately).
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23. Clojure
Concurrency
Refs, Agents, Atoms, Vars (2)
Agents
Allow for asynchronous and independent changes to
shared mutable data
Hold values that can be changed using special functions
(asynchronously, at some point in the future)
Atoms
Allow for synchronous and independent changes to
mutable data
Atoms are updated synchronously (immediately).
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24. Clojure
Concurrency
Refs, Agents, Atoms, Vars (3)
Vars
Can be thought of as pointers to mutable storage locations,
which can be updated on a per- thread basis
Similar to java.lang.ThreadLocal
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25. Clojure
IDE Support
IDE Support
Eclipse: Counterclockwise
http://code.google.com/p/counterclockwise/
IntelliJ IDEA: La Clojure
http://plugins.intellij.net/plugin/?id=4050
Netbeans: Enclojure
http://www.enclojure.org/
Emacs
Vim
Textmate
...
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26. Clojure
Questions & Answers
Questions?
Source: http://xkcd.com/224/
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27. Clojure
Bibliography
Bibliography
Rathore, Amit: Clojure in Action
MEAP Edition, Manning Publications, 2010
Neppert, Burkhard: Clojure unter der Lupe
Javamagazin 2/2011, Software & Support
Media GmbH
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