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

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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

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

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

IntroductionBasics

Benchmarks Results (excerpt)

IntroductionBasics

Conclusion

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

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:

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

M. Klinzing Clojure

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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