A taste of

!!

David Leung!!

Twitter: @davleung Email: david@davidslab.com

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What is Clojure?

Hosted Runs on multiple major

platforms

JVM

CLR

JS Runtime (ClojureScript)

What is Clojure?

Hosted Excellent Interops

EXAMPLE 1 Retrieve a webpage using Java library !!;;  Imports  the  java.net.URL  class  into  current  namespace  (import  ‘java.net.URL)      ;;  Creating  an  instance  of  URL.  ;;  In  Java:  ;;      URL  con  =  new  URL(“http://www.reddit.com”)  (def  con  (URL.  “http://www.reddit.com”))  !!;;  GET  the  page  and  print  it!  (println  (slurp  con))

What is Clojure?

Hosted Functional

First class functions

Build software by composing over functions

EXAMPLE 2 Applying arbitrary function to a collection of data

!;;  Name  a  collection  of  numbers  (def  data  [1  2  3  4  5])  !;;  Creates  a  function  called  process-­‐data  that  accepts  a  collection  and  an  operation  to  be  perofrmed  on  data  (defn  process-­‐data  [collection  operation]          (apply  operation  collection))  !;;  Addition  (process-­‐data  data  +)  !;;  Multiplication  (process-­‐data  data  *)  !

Relax if you aren’t following the syntax :-)

You’ll learn this in the workshop

What is Clojure?

Hosted Functional

Immutability

Easy Reasoning and Debugging

In the following Ruby code, can you be sure that x is not modified?

!x  =  {problems:  99}  !dodgyMethod(x);  !#  what  is  the  value  of  x?  x[":problems"]  

What is Clojure?

Hosted Functional

Immutability

Separation of Identity & State

!

Identity is a reference to something.

State is the aggregated values of an identity at a particular point in time.

In Clojure, an identity’s state is updated by creating a new value and assign it back to the identify.

!

Whaaaaat? Isn’t that inefficient???

What is Clojure?

Hosted Functional

Immutability

Efficient Structure Sharing !Let’s create a hash: !(def  a  {:a  5  :b  6  :c  7  :d  8})  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Credit: Image from “Clojure Programming” by Chas Emerick, Brian Carper & Christophe Grand

What is Clojure?

Hosted Functional

Immutability

Efficient Structure Sharing !!(def  a  {:a  5  :b  6  :c  7  :d  8})  !(def  b  (assoc  a  :c  0))  !!!!!!!!!!!!!!!!!!

!

“Copying” data is cheap! !!!Credit: Image from “Clojure Programming” by Chas Emerick, Brian Carper & Christophe Grand

What is Clojure?

Hosted Functional

Immutability

Fosters Concurrent Programming

!“Immutable objects are always

thread safe.” —Brian Goetz

(Author of Java Concurrency in Practice) !

!!More about Concurrency later

What is Clojure?

Hosted Functional

Parallelism Parallel processing

Trivial Parallel Processing Problem

Problem The Boss just asked you to parse 100K serialized JSON objects into a hash. Each string is ~300KB.

The collection/array of strings are stored in the variable problemsCollection. !Write a snippet that makes use of all cores to process this collection. Don’t worry about storing the result. !Do this within a minute, or you are fired. !Time starts now.

What is Clojure?

Hosted Functional

Parallelism Parallel processing

Trivial Parallel Processing Problem

Solution

!!;;  non-­‐parallelized  version  that  will  get  you  fired  (dorun  (map  json/read-­‐str  problemsCollection))  !;;  parallelized  version  (dorun  (pmap  json/read-­‐str  problemsCollection))  !

!

EXAMPLE 3 (DEMO)

!

Note pmap is a parallelized version of map

What is Clojure?

Hosted Functional

Parallelism Parallel processing

Watch out for the overhead involved in distributing work to the work!

!

EXAMPLE 4 (DEMO)

!

What is Clojure?

Hosted Functional Parallelism

Concurrency Two types of

Concurrent Operations

Coordinated Concurrency

One or more actors (threads) must coordinate to produce the right computation.

e.g. Bank transactions

!

Synchronous Concurrency

Should the thread that wants to update a reference be blocked until their return?

What is Clojure?

Hosted Functional Parallelism

Concurrency 3 flavors of Reference

Types

Coordinated Uncoordinated

Synchronous Refs Atoms

Asynchronous Agents

What is Clojure?

Hosted Functional Parallelism

Concurrency Atoms

Create an atom

(def  counter          (atom  0))  

Access an atom’s value (dereferencing)

(deref  counter)  !;;  syntactical  sugar  @counter  

Compare and Swap

(swap!  counter  +  2)  !

Note: Clojure uses ! in a function’s name to denote that the function mutates state (memory/IO).

What is Clojure?

Hosted Functional Parallelism

Concurrency Software Transactional

Memory (STM)

STM: Database-like Transactions for Concurrency

!

Clojure controls updates to references via a system called STM. It removes the need for manual locking.

!

!

!

!

!

Fulfills the Atomicity, Consistency, and Isolation aspects of the Database Transaction Properties ACID.

STM is to concurrency what

Garbage Collector is to memory management

What is Clojure?

Hosted Functional Parallelism

Concurrency Software Transactional

Memory (STM)

There are more to concurrency

Clojure has “data flow variables” too!

We’ll cover concurrency in the workshops later. Things we will look at:

!

Future

Delay

Promise

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp

What is a Lisp?

!

!

!

!

!

!

Lisp is invented by John McCarthy in 1958.

Lisp pioneered ideas like tree data structures, dynamic typing, garbage collection, higher-order functions, recursion.

John McCarthy is quite the hero in Computer Science.

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp

Lisp: LISt Processing

Lisp is written in symbolic expressions, or S-expressions.

S-expressions are written as a pair of parenthesis surrounding a sequence of symbols:

(f a b c) !

The first position in the list is also known as the functional position. When evaluated, the function f will be called with the rest of the list as arguments.

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp

Collection Types

Collection types in Clojure

!

List (a b c d e) Essentially a linked list

Vector [a b c d e] Like “arrays” in Ruby/Python

!

Wait a second…

Didn’t we already see those square brackets and parentheses when we define a function?

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp

Homoiconicity

Homoiconicity

Lisp is a language where the structure of the code is represented by standard data structures (list, vectors, hash).

Repeat after me

!

Code as Data. Data as Code.

!

!(defn  wont-­‐make-­‐you-­‐blind  [name]      (println  name  ",  parentheses  won't  make  you  blind"))

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp

Homoiconicity

Construct a function call with collection manipulation functions

!;;  the  quotation  mark  stops  Clojure  treating  the  ;;  list  as  a  function  call  (def  arguments  '(1  2  3))  !;;  cons  is  a  traditional  lisp  function  ;;  that  prepends  an  object  to  the  head  of  a  list  (def  function-­‐call  (cons  +  arguments))  !(eval  function-­‐call)  ;;  =>  6  !!

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp Macros

Building Malleable Systems

!

As well as being able to write code that “writes code” (metaprogramming), you can even change the evaluation semantics by writing macros!

We’ll explore this in the workshops.

What is Clojure?

Hosted Functional Parallelism

Concurrency Lisp

Language Extensions

The flexible of Clojure allows Clojure and your codebase to stretch and bend to your needs.

Notable Clojure extensions:

!

core.async Implementation of channels and blocks from the Go programming language.

core.logic Implementation of miniKanren for doing relational/constraint logic programming.

core.typed Optional typing for Clojure.

Working with Clojure

Editors !emacs Light Table VimClojure CounterClockWise (eclipse) IntelliJ

Working with Clojure

Build Tools !

Leiningen for automating Clojure projects without setting your hair on fire

http://leiningen.org !ht

Working with Clojure

Learning Resources

Test Driven Learning 4Clojure 4clojure.com Clojure Koans clojurekoans.com !Books Clojure Programming Joy of Clojure Second Edition Clojure Cookbook

Working with Clojure

!!

David Leung!!

Twitter: @davleung Email: david@davidslab.com

GPG Fingerprint: 217E 1ECE 2349 D178 73E5 C194 1E2E C02A A74A A531