The Curious Clojureist

Transcription

1 The Curious Clojureist NEAL FORD director / software architect meme wrangler ThoughtWorks nford@thoughtworks.com 2002 Summit Boulevard, Atlanta, GA nealford.com thoughtworks.com

2 what is this talk about? clojure s 4 elevators: 1. java interop 2. lisp 3. functional 4. state & concurrency the ecosystem the coolness

3 what is clojure? Clojure is a dynamic, strongly typed, functional, highperformance implementation of a lisp on the JVM.

4 isn t lisp the one with all the () s? yes

5 why all the () s? Lisp is a homoiconic language. Lisp programs consist of lisp data structures. all kinds of useful!

6 ecosystem

7 clojure.org/

8 code.google.com/p/counterclockwise/

9 github.com/technomancy/emacs-starter-kit

10 github.com/technomancy/leiningen

11 compojure (web framework) clj-record (pseudo-port of ActiveRecord) github.com/weavejester/ compojure github.com/duelinmarkers/ clj-record clojureql (relational algebra -> SQL) clojurescript (clojure on JavaScript) github.com/laujensen/ clojureql github.com/clojure/ clojurescript

12 what does clojure code look like?

13 data types? type example java equivalent string "foo" String character \f Character regex #"fo*" Pattern a. p. integer 42 Int/Long/BigInteger double Double a.p. double M BigDecimal boolean true Boolean nil nil null symbol foo, + N/A keyword :foo, ::foo N/A

14 data literals? type properties example list vector map singly-linked, insert at front (1 2 3) indexed, insert at rear [1 2 3] key/value {:a 100 :b 90} set key #{:a :b}

15 function calls? semantics: fn call arg (println "Hello World") structure: symbol list string

16 function definition? define a fn fn name docstring arguments (defn greet "Returns a friendly greeting" [your-name] (str "Hello, " your-name)) fn body

17 homoiconicity? symbol symbol string vector (defn greet "Returns a friendly greeting" [your-name] (str "Hello, " your-name)) it s all data list

18 function meta-data? prefix with ^ class name or arbitrary map (defn ^String greet "Returns a friendly greeting" [your-name] (str "Hello, " your-name))

19 what is the java interop story with clojure?

20 interop? syntax extensions to reach all of Java compiles to bytecode fast call Clojure from Java

21 construction? new Widget("foo") (Widget. "red")

22 static members? Math.PI Math/PI

23 instance members? rnd.nextint() (.nextint rnd)

24 chained access? person.getaddress().getzipcode() (.. person getaddress getzipcode)

25 () count? 8 6

26 how would you implement an interface? interface (reify Runnable (run [] (println "Hello"))) method bodies add more interfaces here

27 what would a typical method look like in clojure? isblank()

28 isblank()

29 isblank() types

30 isblank() class

31 isblank() + higher-order function

32 isblank() corner cases

33 isblank() lispify!

34 when does verbosity == obscurity?

35 what s so special about lisp? feature industry norm cool kids clojure conditionals variables garbage collection recursion function type symbol type whole language available everything s an expression homoiconicity

36 what are special forms? the syntactic scaffolding of your language imports scopes protection meta-data control flow anything using a keyword

37 how are special forms outside lisp different? μ limited to specific use μ look different μ may have special semantics unavailable to you μ hamper reuse

38 what s special about lisp s special forms? λ look just like everything else λ may have special semantics available to you λ can be augmented with macros

39 all forms are created equal? form syntax example function list (println "hello") operator list (+ 1 2) method call list (.trim " hello ") import list (require 'mylib) metadata list (with-meta obj m) control flow list (when valid? (proceed)) scope list (dosync (alter...))

40 why is this important? special forms are easier to understand individually but create combinatorial complexity in aggregate to deal with complexity, you categorize & you end up

41 what s the alternative to patterns in lisp? m a c r o s

42 how can macros reduce repetition? (.add frame panel) (.pack frame) (.setvisible frame true) (doto frame (.add panel) (.pack) (.setvisible true) say it only once doto returns frame now we have an expression

43 Many of the features of Clojure are implemented with macros. How can you tell? you can t!

44 an example of a simple macro? (defmacro when [test & body] (list 'if test (cons 'do body))) (when x (println "x is true"))) macroexpansion it s all data (if x (do (println "x is true")))

45 what are some types of macros? type control flow vars java interop examples when when-not and or defn defmacro defmulti.. doto deftype proxy rearranging -> ->> -?> scopes special form dosync time with-open fn lazy-seq let

46 how does functional make my life better?

47 how does function change the structure of my code? indexofany

48 indexofany StringUtils.indexOfAny(null, *) = -1 StringUtils.indexOfAny("", *) = -1 StringUtils.indexOfAny(*, null) = -1 StringUtils.indexOfAny(*, []) = -1 StringUtils.indexOfAny("zzabyycdxx",['z','a']) = 0 StringUtils.indexOfAny("zzabyycdxx",['b','y']) = 3 StringUtils.indexOfAny("aba", ['z']) = -1

49 indexofany

50 indexofany simplify corner cases

51 indexofany type decls

52 indexofany + when clause

53 indexofany + comprehension

54 indexofany lispify

55 which version is simpler? imperative functional functions 1 1 classes 1 0 internal exit points 2 0 variables 3 0 branches 4 0 boolean ops 1 0 function calls* 6 3 total 18 4

56 which is more general? ; idxs of heads in stream of coin flips (index-filter #{:h} [:t :t :h :t :h :t :t :t :h :h]) -> ( ) ; Fibonaccis pass 1000 at n=17 (first (index-filter #(> % 1000) (fibo))) -> 17

57 which is more general? imperative functional searches strings searches any sequence matches characters matches any predicate returns first match returns lazy seq of all matches

58 what s clojure s unique take on state & concurrency?

59 what about persistent data structures? immutable change by function application maintain performance guarantees full fidelity old versions

60 how would that work with a linked list? your list my list newt tern rabbit

61 and more complex data structures? your trie bit-partitioned tries my trie log2 n: too slow

62 how many tries to make it fast enough? 32-way tries log32 n: fast enough!

63 what s wrong with variables?!?? variable??? identity state variable??? time

64 what s clojure s view of identity identity explicit semantic value value identity value state

65 compare identity, state, & time? term value identity state time meaning immutable data in a persistent data structure series of causally related values over time identity at a point in time relative: before/ simultaneous/after ordering of causal values

66 what is clojure s epochal time model? F F F Process events (pure functions) v1 v2 v3 v4 Observers/ Identity (succession of states) perception/ memory States (immutable values)

67 what s the unified update model? return becomes next state of ref (change-state ref fn [args*]) snapshot always available no user locking, no deadlocks writes never impede readers gets current state of ref

68 software transactional memory

69 how does STM work? v1 v2 v3 v4 v1 v2 v3 v4 v1 v2 v3 v4 v1 v2 v3 v4 F F F F transactions F F F F F F F F

70 what s the syntax for STM? identity (def messages (ref [])) initial value

71 how do you read a value? (deref messages) => => []

72 how do you alter a message? (alter r update-fn & args) oldval r (apply update-fn oldval args) newval

73 how do you update a message? apply an... (defn add-message [msg] (dosync (alter messages conj msg))) scope a transaction...update fn

74 what s cool about clojure? highly expressive language seamless java interop functional advanced concurrency

75 ? s please fill out the session evaluations NEAL FORD director / software architect meme wrangler ThoughtWorks This work is licensed under the Creative Commons Attribution-Share Alike 3.0 License. nford@thoughtworks.com 2002 Summit Boulevard, Atlanta, GA nealford.com thoughtworks.com