Clojure: An Introduction

From FinJUG meeting March 2013. Introduction to Clojure.
http://finjug.org/

Transcript

1. Clojure
2. @teropa
3. Clojure is a dynamic, functional, general purpose programming language for the JVM.
4. Clojure is a dynamic, functional, general purpose programming language for the JVM. dynamically typed, extensible, interactive
5. Clojure is a dynamic, functional, general purpose programming language for the JVM. dynamically typed, extensible, interactive programming with functions, emphasis on immutability
6. Clojure is a dynamic, functional, general purpose programming language for the JVM. dynamically typed, extensible, interactive programming with functions, emphasis on immutability “Use it anywhere you would use Java”
7. Clojure is a dynamic, functional, general purpose programming language for the JVM. dynamically typed, extensible, interactive programming with functions, emphasis on immutability “Use it anywhere you would use Java” runs on the JVM, designed for the JVM
8. 1991 Python 1995 Java Ruby JavaScript PHP 2003 Scala Groovy 2007 Clojure
9. imperative functional static dynamic
10. imperative functional static dynamic Java
11. imperative functional static dynamic Java Scala
12. imperative functional static dynamic Java Scala Python Ruby Groovy
13. imperative functional static dynamic Java Scala Python Ruby Groovy Clojure
14. Six Mindwarps ... so far.
15. 1/6: Data & Behavior
16. OOP talk() String name int age Animal fetchBall() String breed Dog purr() String breed boolean grumpy Cat
17. The “Anemic Domain Model” String number String owner String bank int balance Account String firstName String lastName String number String ssn int yearStarted Employee String name String code int inventory Product String name String code String address String tel Customer DateTime timestamp String clerk int totalAmount int vatAmount Receipt
18. VerbEscorters action() Controller calculate() Calculator build() Builder validate() Validator makeImageTag() makeForm() Helper load() save() Service create() delete() Manager formatTime() formatNumber() getVatClasses() Util
19. Steve Yegge Verbs in Javaland are responsible for all the work, but as they are held in contempt by all, no Verb is ever permitted to wander about freely. If a Verb is seen in public at all, it must be escorted at all times by a Noun. Of course “escort”, being a Verb itself, is hardly allowed to run around naked; one must procure a VerbEscorter to facilitate the escorting. http://steve-yegge.blogspot.fi/2006/03/ execution-in-kingdom-of-nouns.html
20. Rich Hickey Object-orientation is overrated. [It was] born out of simulation, [but is] now used for everything, even when inappropriate http://clojure.org/rationale
21. Data List Vector Map Set (1 2 3 4) [1 2 3 4] {:name “Joe”, :parents [:jane :bob]} #{:one :two :three :four}
22. Functions conj assoc dissoc first rest nth peek pop keys vals empty? every? some cycle map reduce filter remove concat mapcat interleave interpose group-by partition split-at zipmap take drop
23. Data is data. Functions are functions. That’s OK.
24. Steve Yegge Classes are really the only modeling tool Java provides you. So whenever a new idea occurs to you, you have to sculpt it or wrap it or smash at it until it becomes a thing, even if it began life as an action, a process, or any other non-”thing” concept. I’ve really come around to what Perl folks were telling me 8 or 9 years ago: “Dude, not everything is an object.” http://steve-yegge.blogspot.fi/2006/03/ execution-in-kingdom-of-nouns.html
25. 1/6: Data & Behavior Separate
26. 2/6: Syntax
27. Syntactic Patterns InputStream in = file.openStream(); try { // ... } catch (IOException ioe) { // ... } finally { try { in.close(); } catch (IOException ioe) { // ... } }
28. Language Extensions try(InputStream in = file.openStream()) { // ... } catch (IOException ioe) { // ... }
29. ...after 15 years.
30. Parentheses! (defn all-valid? [records] (every? empty? (map :errors records)))
31. Parentheses! record.getErrors() (errors record)
32. Parentheses! record.getErrors() ( ) errors record
33. Code Function definition Arguments Body (defn all-valid? [records] (every? empty? (map :errors records)))
34. (defn all-valid? [records] (every? empty? (map :errors records))) Data
35. ( defn all-valid? [records] ( every? empty? ( map :errors records ) ) ) Data
36. ( defn all-valid? [records] ( every? empty? ( map :errors records ) ) ) Data List Symbol Vector List List Keyword
37. Code is data. Functions transform data... ...functions transform code!
38. Macros (let [in (.openStream file)] (try (.read in) (finally (.close in))))
39. Macros (defmacro with-open [bindings & body] `(let ~bindings (try ~@body (finally (.close ~(first bindings))))))
40. Macros (with-open [in (.openStream file)] (.read in))
41. Paul Graham If you understand how compilers work, what’s really going on is not so much that Lisp has a strange syntax as that Lisp has no syntax. You write programs in the parse trees that get generated within the compiler when other languages are parsed. But these parse trees are fully accessible to your programs. You can write programs that manipulate them. In Lisp, these programs are called macros. They are programs that write programs. http://www.paulgraham.com/avg.html
42. 2/6: Syntax Extensible
43. 3/6: State
44. org.hibernate.LazyInitializationException: could not initialize proxy - no Session at org.hibernate.proxy.AbstractLazyInitializer.initialize (AbstractLazyInitializer.java:86) at org.hibernate.proxy.AbstractLazyInitializer.getImplementation (AbstractLazyInitializer.java:140) at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke (JavassistLazyInitializer.java:190)
45. Mutable Stateful Object Graphs Portfolio Customer User Permission Currency CalculationPolicy
46. Stateless Code Database Library Results Query
47. Stateless Code Function Results Arguments
48. Stateless Code Results Arguments
49. Stateless Functions conj assoc dissoc first rest nth peek pop keys vals empty? every? some cycle map reduce filter remove concat mapcat interleave interpose group-by partition split-at zipmap take drop
50. 3/6: State Mostly Unnecessary
51. 4/6: State & Concurrency
52. ["joe", "bob", "jane", "engelbert", "phyllis"]
53. ["joe", "bob", "jane", "engelbert", "phyllis"]
54. ["joe", "bob", "jane", "engelbert", "phyllis"]
55. ["joe", "bob", "jane", "engelbert", "phyllis"]
56. ["joe", "bob", "jane", “jill”, "engelbert", "phyllis"]
58. ["joe", "bob", "jane", "engelbert", "phyllis"] Locking
59. ["joe", "bob", "jane", "engelbert", "phyllis"] Locking
60. ["joe", "bob", "jane", "engelbert", "phyllis"] Locking
61. ["joe", "bob", "jane", "engelbert", "phyllis"] Locking
62. ["joe", "bob", "jane", "engelbert", "phyllis"] Locking
63. ["joe", "bob", "jane", "engelbert", "phyllis"] Locking
64. Rich Hickey One could argue that systems with manual locking are usually broken, and therefore their behavior itself, never mind their performance, is unpredictable. That’s been my experience with manual locking in the hands of mere mortal developers. http://www.azulsystems.com/blog/cliff/ 2008-05-27-clojure-stms-vs-locks
65. Immutable Data Structures List Vector Map Set (1 2 3 4) [1 2 3 4] {:name “Joe”, :parents [:jane :bob]} #{:one :two :three :four}
66. ...And Stateless Functions conj assoc dissoc first rest nth peek pop keys vals empty? every? some cycle map reduce filter remove concat mapcat interleave interpose group-by partition split-at zipmap take drop
67. ...And Managed References (def bob (ref 10)) (def joe (ref 50)) (defn transfer [from to amount] (dosync (alter from - amount) (alter to + amount))) (transfer bob joe 5)
68. 4/6: State & Concurrency Managed
69. 5/6: Workflow
70. Edit Compile Run
71. REPL Read- Eval- Print- Loop
72. Ron Garret The Remote Agent software, running on a custom port of Harlequin Common Lisp, flew aboard Deep Space 1, the first mission of NASA’s New Millennium program. Remote agent controlled DS1 for two days in 1999. During that time we were able to debug and fix a race condition that had not shown up during ground testing. Debugging a program on a $100M piece of hardware that is 100 million miles away is an interesting experience. http://www.flownet.com/gat/jpl-lisp.html
73. 5/6: Workflow Continuous
74. 6/6: Java
75. Java Ruby Python Platform Libraries Language Platform Libraries Language Platform Libraries Language
76. Java JRuby Jython Platform Libraries Language Libraries Language Libraries Language
77. The JVM is Badass
78. Language Ports
79. Clojure
80. (defn checknum [n] (if (zero? (rem n 2)) "even" "odd")) public interface IFn extends Callable, Runnable java.lang.String long
81. (doto (Executors/newCachedThreadPool) (.submit work1) (.submit work2) (.shutdown) (.awaitTermination 20 TimeUnit/SECONDS)) Java static method call Java static field reference Java instance method call
82. Stuart Halloway Clojure is a better Java than Java. http://www.infoq.com/presentations/Clojure- Java-Interop
83. 6/6: Java As platform
84. Data & Behavior: Separate Syntax: Extensible State: Mostly Unnecessary State & Concurrency: Managed Workflow: Continuous Java: As Platform
85. @teropa