Marcel Klinzing - RWTH Aachen Universityhpac.rwth-aachen.de/teaching/sem-lsc-12/Clojure.pdf ·...

IntroductionBasics

ConcurrencyConclusion

Clojure

Marcel Klinzing

December 13, 2012

M. Klinzing Clojure 1/18

IntroductionBasics


Overview/History

Functional programminglanguage

Lisp dialect

Compiles to Java Bytecode

Implemented in Java

Created by Rich Hickey

Version 1.0.0 released in2009

Current Version: 1.4.0 Rich Hickey


IntroductionBasics


Features

“Program as data”

First-class functions

Dynamically typed

Java interoperability

Immutable, persistent data

Mutable references to data

Support for concurrent programming

and more. . .


IntroductionBasics


Forms

Part of Clojure called Reader reads chunks of the program calledforms

Reader translates forms into Clojure data structures

Clojure compiles data structures and executes them

Forms (excerpt):

Form Example

Boolean true, falseList (+ 1 2), (map + [1 2] [3 4])

Number 1, 3.142, 22/7String “hello world”

Symbol foo, foo-?, java.lang.ThreadVector [1 2 3]


IntroductionBasics


Variables and Functions

Defining a (global) variable: (def identifier value)

Defining a function: (defn identifier (parameter-vec body)+)

Calling a function: (identifier param1 param2 ...)

1 (defn f

2 ([] (println "hello, world!"))

3 ([x] (println x)))

45 ; brackets can be omitted if6 ; there is only one parameter−vec and body7 (defn g [] (println "bar"))

89 (f) ; Output: hello, world!

10 (f "foo") ; Output: foo11 (g) ; Output: bar


IntroductionBasics


Destructuring

Destructuring used to bind single values in a collection to variables

1 (defn f [ [x y] ]

2 (+ x y))

34 (println (f [1 1])) ; Output: 25 (println (f [4 5 6 7])) ; Output: 9


IntroductionBasics


Java Interoperability

Package java.lang imported automatically

Create new Object: (new classname & args)

Call of variables/methods:(. class-or-instance member-symbol & args)

Note that every Clojure function is an Object which implementsjava.lang.Runnable and java.lang.Callable

1 (defn delay-print []

2 (. Thread sleep 100)

3 (println "new Thread"))

45 (. (new Thread delay-print) start)

6 (print "start ")

7 ; Output: start new Thread


IntroductionBasics


Reference Types

Reference types = mutable references to immutable data

Reference types and their purpose:

Refs: Synchronous, coordinated updates of dataAgents: Asynchronous update of a single valueAtoms: Synchronous update of a single valueVars: Synchronous update of a thread-local value


IntroductionBasics


Software Transactional Memory

Software Transactional Memory (STM) is a model/concept to viewmemory as a database

STM used for coordinated updates (refs)

Updates of values are done in transactions

Properties of STM transactions:

Atomic updatesConsistent updatesIsolated updates


IntroductionBasics


STM - Multiversion Concurrency Control

STM uses Multiversion Concurrency Control (MVCC):

Start of a Transaction: Unique ID (called point) is generatedTransaction works against private copies of references (associatedwith the point)Changes to refs will be visible only after the transactioncommits/endsIf a transaction notices that a ref has been already set/altered byanother transaction, the transaction will restart

To start a transaction: (dosync & exprs)


IntroductionBasics


Refs

Creating a ref: (ref initial-value)

Dereferencing a ref: (deref ref-inst)

or shorter: @ref-inst

To change the reference: (ref-set new-value)

Note: ref-set can only be called in a transaction; otherwise aIllegalStateException is thrown


IntroductionBasics


Refs – Example

1 (def x (ref 0))

23 (defn increase []


5 (dosync (ref-set x (+ @x 1))))

67 (defn decrease []


9 (dosync (ref-set x (- @x 1))))

1011 (def inc-thread (new Thread increase))

12 (def dec-thread (new Thread decrease))

1314 (. inc-thread start) (. dec-thread start)

15 (. inc-thread join) (. dec-thread join)

16 (println @x) ; Output: 0


IntroductionBasics


Agents

Creating an agent: (agent initial-value)

Dereferencing for agents as for refs: @agent-inst

Changing of agents per update function:(send agent-inst update-fn & args)

(update-fn @agent-inst arg1 arg2 ...) is run later on a thread ina thread pool and the return value is “assigned” to the agent


IntroductionBasics


Agents – Example

1 (def ag (agent 0))

23 (defn increase [x]

4 (+ x 1))

56 (println @(send ag increase)) ; Output: 07 (. Thread sleep 100)

8 (println @ag) ; Output: 1


IntroductionBasics


Benchmark

Benchmark taken from the “Computer Language BenchmarksGame” ( http://shootout.alioth.debian.org )

“thread-ring” program:

Create 503 linked pre-emptive threadsThread 503 should be linked to thread 1, forming an unbroken ringPass a token to thread 1Pass the token from thread to thread N timesPrint the name of the last thread (1 to 503) to take the token


http://shootout.alioth.debian.org

IntroductionBasics


Benchmarks Results (excerpt)

N = 50,000,000Quad-core 2.4Ghz Intel R© Q6600 R©; 4GB RAM; 250GB SATA II diskdrivex64 Ubuntu

TM


IntroductionBasics


Conclusion

Pros:

Functional programming (first-class functions, small code, . . . )Java interoperabilitySupport for concurrent programming

Cons:

Speed (since it runs on the JVM)


IntroductionBasics


Other Clojure Implementations

ClojureScript

Compiles Clojure code to JavaScriptimplemented in Clojure

ClojureCLR

Native implementation of Clojure on the Common LanguageRuntime (CLR; virtual machine of Microsoft’s .NET framework)Implemented in C# (and Clojure itself)


References

Official site:http://clojure.org

Book:Stuart Halloway and Aaron Bedra. Programming Clojure.2nd ed. The Pragmatic Programmers, 2012.

Tutorial:R. Mark Volkmann. Clojure - Functional Programming for the JVMhttp://java.ociweb.com/mark/clojure/article.html

Benchmark:http://shootout.alioth.debian.org/

Pictures:

Slide 2: http://en.wikipedia.org/wiki/File:Rich_Hickey.jpg

M. Klinzing Clojure

http://clojure.org

http://java.ociweb.com/mark/clojure/article.html

http://shootout.alioth.debian.org/

http://en.wikipedia.org/wiki/File:Rich_Hickey.jpg

Thank you for your attention.

M. Klinzing Clojure

Backup Slides – Multimethods

Declaring a multimethod: (defmulti identifier choosing-fn)

Considering a function call (identifier arg1 arg2 ...);(choosing-fn arg1 arg2 ...) is called and the return valuedetermines the implementation to be called

Defining a multimethod:(defmethod identifier choosing-val (parameter-vec body)+)

M. Klinzing Clojure

Backup Slides – Metadata

Metadata = Data about data that doesn’t change its logical valuesWrite ^metadata-map before symbols to attach metadataExample:

1 (def x 1)

2 (defn ^{:doc "Returns its first argument"} f [x y] x)

34 (println (meta #’x))

5 ; Output: {:ns #<Namespace user>, :name x, :line 1,6 ; : file D:\Clojure\metadata. clj }7 (println (meta #’f))

8 ;Output: {: arglists ([ x y ]), :ns #<Namespace user>, :name f,9 ; :doc Returns its first argument, : line 3, : file ...}

10 (println (doc f))

11 ; Output: −−−−−−−−−−−−−−−−−−−−−−−−−12 ; user/f13 ; ([ x y ])14 ; Returns its first argument

M. Klinzing Clojure

Marcel Klinzing - RWTH Aachen Universityhpac.rwth-aachen.de/teaching/sem-lsc-12/Clojure.pdf ·...

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