This Talk is so Meta

Introduction to metaprogramming in ruby

Transcript

1. Jakarta.rb 25 September 2013
2. Xinuc (Ruby) Programmer @xinuc (github, twitter) Works at Bukalapak.com Intro
3. We're hiring www.bukalapak.com/career Intro
4. This Talk is so Meta
5. Metaprogramming, because F*** You, That's why!
6. Metaprogramming: the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do potato potato potato... Metaprogramming
7. Write Program that writes program (that writes program) (that writes program) Metaprogramming
8. - Meet your first Meta - attr_accessor - The basic - block - ruby class structure - method lookup - eigenclass - The technique - define_method - method_missing - method cache - class_eval - instance_eval - eval Agenda
9. Meet Your first meta
10. class Person { private String name; public String getName() { return this.name; } public void setName(String name) { this.name = name; } } attr_accessor
11. class Person attr_accessor :name end attr_accessor
12. class Person def name @name end def name=(name) @name = name end end attr_accessor
13. The Basic Some groundwork we need to lay
14. block
15. A block is a just chunk of code that can be called 3.times do puts “hello” end block
16. A block is just a special form of a proc def show_me_the_block(&block) block end block vs proc
17. $ b = show_me_the_block do puts “hello” end => #<Proc:0x00000000d8d600:4> $ b.call => Hello $ b.class => Proc block vs proc
18. my_proc = proc { puts “hello” } my_proc.call => hello 3.times &my_proc => hello hello hello block vs proc
19. A block is a closure because it can access its environment name = “xinuc” 3.times do puts “hello #{name}” end block & closure
20. Ruby method lookup
21. Inside a ruby object klass iv_tbl flags klass iv_tbl flags m_tbl super object class
22. Class structure MyClass Object Class super klass klass klass
23. Class structure Object BasicObject super MyClassA super MyClassB super
24. Class structure Object BasicObject super MyClassA super MyClassB super MyModule mixin
25. eigenclass
26. o = Object.new o.hello => NoMethodError: undefined method `hello' def o.hello puts “hello” end o.hello => hello eigen_class
27. o = Object.new eigen = o.singleton_class #1.9 eigen = class << o; self; end eigen_class
28. Class structure Object BasicObject super MyClassA super eigen_class <virtual> super MyModule object klass mixin
29. The Techniques
30. Define method
31. # nested def def learn_to_shout puts “learning....” def shout puts “Wazzzaaaap!!” end end Define method
32. $ shout => undefined local variable or method `shout' $ learn_to_shout => learning.... $ shout => Wazzzaaaap!! Define method
33. def learn_to_shout puts “learning....” def shout puts “Wazzzaaaap!!” end end Define method
34. def who_are_you puts “I'm batman” def who_are_you puts “I'm superman” def who_are_you puts “I'm ironman” end end end Define method
35. def who_are_you puts “I'm batman” def who_are_you puts “I'm superman” def who_are_you puts “I'm ironman” end end end Hypocrite method
36. def who_are_you puts “I'm batman” def who_are_you puts “I'm superman” def who_are_you puts “I'm ironman” end end end Yo dawg method
37. define_method
38. define_method method takes a method name and a block, then creates a method with block as the method body define_method :name do |args| # method body end define_method method
39. def attr_accessor(*attrs) attrs.each do |attr| define_method attr do instance_variable_get “@#{attr}” end define_method “#{attr}=” do |val| instance_variable_set “@#{attr}”, val end end end attr_accessor
40. method_missing
41. class Person def method_missing(name, *args, &block) puts “method #{name}” end end p = Person.new p.omg_lol => method omg_lol method_missing
42. require 'builder' xml = Builder::XmlMarkup.new xml.coder do xml.name 'xinuc' xml.language 'Ruby' end method_missing
43. Class structure Object BasicObject super MyClass super eigen_class <virtual> super MyModule my_object klass mixin
44. Lazy method definition
45. class Person def method_missing(name, *args, &block) self.class.class_eval do define_method(name) do puts ”method #{defined} defined” end end self.send name, *args end end Lazy method definition
46. p = Person.new p.respond_to? :hello => false p.hello => method hello defined p.respond_to? :hello => true Lazy method definition
47. class User < ActiveRecord::Base end User.find_by_username “xinuc” Lazy method definition
48. class_eval
49. class Person end Person.class_eval do def hello puts “hello” end end p = Person.new p.hello class_eval
50. instance_eval
51. class Person end p = Person.new p.instance_eval do def hello puts “hello” end end instance_eval
52. class Person end p = Person.new p.singleton_class.class_eval do def hello puts “hello” end end p.hello instance_eval
53. instance_eval change the “self” inside a block s = “Hello” s.instance_eval do puts self.upcase end instance_eval
54. # sunspot class User < ActiveRecord::Base # method text & integer # not defined here searchable do # but we can call method text & integer here text :name integer :age end end instance_eval
55. Here comes the devil
56. Class.new Create classes dynamically
57. 1000.times do Class.new do end end Class.new
58. “The evil” eval
59. eval <<-EOF def do_evil puts “do evil things” end EOF do_evil eval
60. Warning!
61. Metaprogramming is a chainsaw warning
62. You don't slice your bread with a chainsaw! warning
63. Unless you're a bear with chainsaw hands warning
64. Thank you