A Taste of Clojure

Transcript

1. A taste of ! ! David Leung! ! Twitter: @davleung Email: david@davidslab.com GPG Fingerprint: 217E 1ECE 2349 D178 73E5 C194 1E2E C02A A74A A531
2. What is Clojure? Hosted Runs on multiple major platforms JVM CLR JS Runtime (ClojureScript)
3. 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))
4. 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 (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
5. 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"]  
6. 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???
7. 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
8. 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
9. 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
10. 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.
11. 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
12. What is Clojure? Hosted Functional Parallelism Parallel processing Watch out for the overhead involved in distributing work to the work! ! EXAMPLE 4 (DEMO) !
13. 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?
14. What is Clojure? Hosted Functional Parallelism Concurrency 3 flavors of Reference Types Coordinated Uncoordinated Synchronous Refs Atoms Asynchronous Agents
15. 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).
16. 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
17. 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
18. 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.
19. 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.
20. 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?
21. 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"))
22. 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   ! !
23. 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.
24. 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.
25. Working with Clojure Editors ! emacs Light Table VimClojure CounterClockWise (eclipse) IntelliJ
26. Working with Clojure Build Tools ! Leiningen for automating Clojure projects without setting your hair on fire http://leiningen.org ! ht
27. 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
28. Working with Clojure 6JCPMU ! ! David Leung! ! Twitter: @davleung Email: david@davidslab.com GPG Fingerprint: 217E 1ECE 2349 D178 73E5 C194 1E2E C02A A74A A531