The Spooky Science of Ruby

Grab a scalpel as we put Ruby on the table to look at this lovely language's internals. We'll start with class inheritance and method lookup, and then explore the mysterious singleton class and how it fits in. We'll use our newfound knowledge to turn children into zombies and find a cure to turn them back again. Okay, so maybe it won't be too spooky, but you'll come away having a better understanding of Ruby objects and their internals (and braaaains!).

Transcript

1. The Spooky Science of Ruby a.k.a. Zombies like to Metaprogram Zoe Kay June 25, 2015 Open Source Bridge
2. Hi!
6. Topics
7. • Class Inheritance Topics
8. • Class Inheritance • Method Lookup Topics
9. • Class Inheritance • Method Lookup • Dynamic Methods Topics
10. • Class Inheritance • Method Lookup • Dynamic Methods • Singleton Class Topics
11. Disclaimer • You won’t use this daily • Metaprogramming is a dangerous tool • This talk is not about judging Ruby good or bad, it’s about understanding it
12. Disclaimer of the disclaimer • Use this for debugging • Write better code • Figure out other people’s code • Fun!
13. class Child attr_accessor :intelligence, :creativity def initialize @intelligence = 1 @creativity = 1 end def play self.intelligence += 1 self.creativity += 1 end end
14. Class Hierarchy
15. >> Child.ancestors
16. >> Child.ancestors => [Child, Object, Kernel, BasicObject]

17. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
 >> Child.superclass
18. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
 >> Child.superclass => Object

19. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
 >> Child.superclass => Object
 >> Child.superclass.superclass
20. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
 >> Child.superclass => Object
 >> Child.superclass.superclass => BasicObject
21. “Object is the default root of all Ruby objects.” >> Child.ancestors => [Child, Object, Kernel, BasicObject] - Ruby Docs http://ruby-doc.org/core-2.2.2/Object.html
22. >> NilClass.ancestors => [NilClass, Object, Kernel, BasicObject]
23. >> NilClass.ancestors => [NilClass, Object, Kernel, BasicObject] >> Module.ancestors => [Module, Object, Kernel, BasicObject]
24. >> NilClass.ancestors => [NilClass, Object, Kernel, BasicObject] >> Class.ancestors => [Class, Module, Object, Kernel, BasicObject] >> Module.ancestors => [Module, Object, Kernel, BasicObject]
25. “Methods on Object are available to all classes unless explicitly overridden.” >> Child.ancestors => [Child, Object, Kernel, BasicObject] - Ruby Docs http://ruby-doc.org/core-2.2.2/Object.html
26. >> Object.methods => [:allocate, :new, :superclass]
27. >> Object.instance_methods
28. >> Object.instance_methods => [:==, :equal?, :!,
 :!=, :instance_eval, :instance_exec, :__send__, :__id__]
29. “Object mixes in the Kernel module, making the built-in kernel functions globally accessible.” >> Child.ancestors => [Child, Object, Kernel, BasicObject] - Ruby Docs http://ruby-doc.org/core-2.2.2/Object.html
30. >> Kernel.methods
31. >> Kernel.methods =>[:sprintf, :format, :Integer, :Float, :String, :Array, :Hash, :warn, :rai se, :fail, :global_variables, :__method__, :__callee__, :__dir__, :eval, :l ocal_variables, :iterator?, :block_given?, :catch, :throw, :loop, :trace_va r, :untrace_var, :at_exit, :syscall, :open, :printf, :print, :putc, :puts, :gets, :readline, :select, :readlines, :`, :p, :test, :srand, :rand, :trap, :exec, :fork, :exit!, :system, :spawn, :sleep, :exit, :abort, :load, :requi re, :require_relative, :autoload, :autoload?, :proc, :lambda, :binding, :ca ller, :caller_locations, :Rational, :Complex, :set_trace_func, :pp, :freeze , :===, :==, :<=>, :<, :<=, :>, :>=, :to_s, :inspect, :included_modules, :i nclude?, :name, :ancestors, :instance_methods, :public_instance_methods, :p rotected_instance_methods, :private_instance_methods, :constants, :const_ge t, :const_set, :const_defined?, :const_missing, :class_variables, :remove_c lass_variable, :class_variable_get, :class_variable_set, :class_variable_de fined?, :public_constant, :private_constant, :singleton_class?, :include, : prepend, :module_exec, :class_exec, :module_eval, :class_eval, :method_defi ned?, :public_method_defined?, :private_method_defined?, :protected_method_ defined?, :public_class_method, :private_class_method, :instance_method, :p ublic_instance_method, :pretty_print_cycle, :pretty_print, :pretty_print_in stance_variables, :pretty_print_inspect, :nil?, :=~, :! ~, :eql?, :hash, :class, :singleton_class, :clone, :dup, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :frozen?, :methods, :singleton_met hods, :protected_methods, :private_methods, :public_methods, :instance_vari ables, :instance_variable_get, :instance_variable_set, :instance_variable_d efined?, :remove_instance_variable, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :extend, :display, :method, :public_meth od, :singleton_method, :define_singleton_method, :object_id, :to_enum, :enu m_for, :pretty_inspect, :equal?, :!, :! =, :instance_eval, :instance_exec, :__send__, :__id__]
32. to_s puts inspect kind_of? send respond_to?
33. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
34. >> Object.ancestors
35. >> Object.ancestors => [Kernel, BasicObject]

36. >> Object.ancestors => [Kernel, BasicObject]
 >> Kernel.ancestors
37. >> Object.ancestors => [Kernel, BasicObject]
 >> Kernel.ancestors => [Kernel]
38. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
39. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
40. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
41. >> Child.ancestors => [Child, Object, Kernel, BasicObject]
42. “BasicObject is the parent class of all classes in Ruby. It's an explicit blank class.” >> Child.ancestors => [Child, Object, Kernel, BasicObject] - Ruby Docs http://ruby-doc.org/core-2.2.2/BasicObject.html
43. >> BasicObject.methods
44. >> BasicObject.methods => []

45. >> BasicObject.methods => []
 >> BasicObject.instance_methods
46. >> BasicObject.methods => []
 >> BasicObject.instance_methods => []
47. class Dog
 end
 
 >> Dog.ancestors
 => [Dog, Object, Kernel, BasicObject]
48. class Dog
 end
 
 >> Dog.ancestors
 => [Dog, Object, Kernel, BasicObject] 
 class Dog < BasicObject
 end
 
 >> Dog.ancestors
 => [Dog, BasicObject]
49. Method lookup zoe .play
50. Child Kernel Object zoe Basic Object
51. Child Kernel #play Object zoe Basic Object
52. Child Kernel #play #to_s Object zoe Basic Object
53. Child Kernel #play #to_s #to_s Object zoe Basic Object
54. class Human; end

55. class Human; end
 class Adult < Human
 end

56. class Human; end
 class Adult < Human
 end
 class Child < Human
 end
57. #read_book #read_book Child Kernel Object Human Adult #read_book class Human; end
 class Adult < Human
 end
 class Child < Human
 end B Ob
58. class Child < Human include module A include module B ... end
59. Child Object Human B A class Child < Human include module A include module B ... end zoe
60. Child Object Human B A class Child < Human include module A include module B ... end zoe
61. Child Object Human B A class Child < Human include module A include module B ... end zoe
62. >> Child.ancestors => [Child, B, A, Human, Object… ]
63. class Child < Human prepend module A include module B ... end
64. Child Object Human B A class Child < Human prepend module A include module B ... end zoe
65. Child Object Human B A class Child < Human prepend module A include module B ... end zoe
66. Child Object Human B A class Child < Human prepend module A include module B ... end zoe
67. >> Child.ancestors => [A, Child, B, Human, Object… ]
68. class Child < Human include SugarCrash include MysteryModule prepend MoreSugar include EvenMoreSugar end
69. class Child < Human include SugarCrash include MysteryModule prepend MoreSugar include EvenMoreSugar end >> Child.instance_method(:eat_sugar)
70. class Child < Human include SugarCrash include MysteryModule prepend MoreSugar include EvenMoreSugar end >> Child.instance_method(:eat_sugar) => #<UnboundMethod: Child(EvenMoreSugar)#eat_sugar>
71. class Child < Human include SugarCrash include MysteryModule prepend MoreSugar include EvenMoreSugar end >> Child.instance_method(:eat_sugar) => #<UnboundMethod: Child(EvenMoreSugar)#eat_sugar>
72. class Child < Human include SugarCrash include MysteryModule prepend MoreSugar include EvenMoreSugar end >> Child.instance_method(:eat_sugar) => #<UnboundMethod: Child(EvenMoreSugar)#eat_sugar>
73. class Child < Human include SugarCrash include MysteryModule prepend MoreSugar include EvenMoreSugar end >> Child.instance_method(:eat_sugar) => #<UnboundMethod: Child(EvenMoreSugar)#eat_sugar>
74. >> Child.ancestors => [Child, ChildModule, Human, HumanModule1, HumanModule2, Object, Kernel, BasicObject]
75. Zombies
77. >> Object.const_set('Zombie', Class.new) => Zombie
78. >> Object.const_set('Zombie', Class.new) => Zombie >> Object.const_set('zombie', Class.new)
79. >> Object.const_set('Zombie', Class.new) => Zombie >> Object.const_set('zombie', Class.new) => NameError: wrong constant name zombie
80. >> Zombie.const_set(‘FAVORITE_FOOD’, ‘brains’) => “brains” >> Zombie::FAVORITE_FOOD => "brains"
81. >> Zombie.const_set(‘FAVORITE_FOOD’, ‘brains’) => “brains” >> Zombie::FAVORITE_FOOD => "brains" >> Zombie::FAVORITE_FOOD = 'limbs'
82. >> Zombie.const_set(‘FAVORITE_FOOD’, ‘brains’) => “brains” >> Zombie::FAVORITE_FOOD => "brains" >> Zombie::FAVORITE_FOOD = 'limbs' => warning: already initialized constant Zombie::FAVORITE_FOOD
 => “limbs"

83. >> Zombie.const_set(‘FAVORITE_FOOD’, ‘brains’) => “brains” >> Zombie::FAVORITE_FOOD => "brains" >> Zombie::FAVORITE_FOOD = 'limbs' => warning: already initialized constant Zombie::FAVORITE_FOOD
 => “limbs"
 >> Zombie::FAVORITE_FOOD
 => "limbs"
84. >> Object.const_set('Zombie', Class.new) => Zombie
85. >> Object.const_set('Zombie', Class.new) => Zombie = >> Zombie = Class.new
86. >> Object.const_set('Zombie', Class.new) => Zombie = >> class Zombie >> end = >> Zombie = Class.new
87. >> Object.const_get(:Zombie) => Zombie
89. class Zombie def eat puts “Mm, brains” end end
90. class Zombie def eat puts “Mm, brains” end end
91. class Zombie def eat puts “Mm, brains” end end class Zombie define_method(:eat) { puts “Mm, brains” } end
92. class Zombie def eat puts “Mm, brains” end end class Zombie define_method(:eat) { puts “Mm, brains” } end
93. class Zombie def eat puts “Mm, brains” end end zombie_proc = Proc.new do
 puts “Mm, brains”
 end
 class Zombie define_method(:eat) { puts “Mm, brains” } end
94. class Zombie def eat puts “Mm, brains” end end zombie_proc = Proc.new do
 puts “Mm, brains”
 end
 Zombie.send(:define_method, :eat, zombie_proc ) class Zombie define_method(:eat) { puts “Mm, brains” } end
95. pamela = Zombie.new
96. pamela = Zombie.new
97. pamela.eat => “Mm, brains” pamela = Zombie.new
98. class Child < Human ... def get_bitten_by_zombie horrible_zombie_change end end
99. class Child < Human ... def get_bitten_by_zombie horrible_zombie_change end end
100. >> martin = Child.new
101. >> martin = Child.new >> martin.intelligence
102. >> martin = Child.new >> martin.intelligence => 1

103. >> martin = Child.new >> martin.intelligence => 1
 >> martin.get_bitten_by_zombie
104. >> martin = Child.new >> martin.intelligence => 1
 >> martin.get_bitten_by_zombie => “Aaaagh!!”

105. >> martin = Child.new >> martin.intelligence => 1
 >> martin.get_bitten_by_zombie => “Aaaagh!!”
 >> martin.intelligence
106. >> martin = Child.new >> martin.intelligence => 1
 >> martin.get_bitten_by_zombie => “Aaaagh!!”
 >> martin.intelligence => 0
107. Child Object Human B A Method lookup chain
108. Singleton Class
109. Singleton Class Eigenclass
110. Singleton Class Eigenclass Shadow class
111. Singleton Class Eigenclass Shadow class Anonymous class
112. Singleton Class Eigenclass Shadow class Anonymous class Virtual class
113. Singleton Class Eigenclass Shadow class Anonymous class Virtual class Metaclass
114. Singleton Class Eigenclass Shadow class Anonymous class Virtual class Metaclass Object-specific class
115. Singleton Class
116. singleton class != singleton design pattern
117. >> zoe.play
118. >> zoe.play >> zoe.intelligence
119. >> zoe.play >> zoe.intelligence => 2

120. >> zoe.play >> zoe.intelligence => 2
 >> def zoe.play
 >> @intelligence += 10
 >> end
121. >> zoe.play
122. >> zoe.play >> zoe.intelligence
123. >> zoe.play >> zoe.intelligence => 12
124. >> zoe.play >> zoe.intelligence => 12 >> zoe.play
125. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence
126. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22

127. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22
 >> martin.play
128. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22
 >> martin.play >> martin.intelligence
129. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22
 >> martin.play >> martin.intelligence => 1
130. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22
 >> martin.play >> martin.intelligence => 1 >> martin.play
131. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22
 >> martin.play >> martin.intelligence => 1 >> martin.play >> martin.intelligence
132. >> zoe.play >> zoe.intelligence => 12 >> zoe.play >> zoe.intelligence => 22
 >> martin.play >> martin.intelligence => 1 >> martin.play >> martin.intelligence => 2
133. Child Kernel Object Human zoe Bas Obje
134. Child Kernel Object Human zoe Singleton zoe Bas Obje
135. >> zoe.singleton_class
136. >> zoe.singleton_class => #<Class:#<Child:0x007fa404907e50>>
137. >> zoe.singleton_class => #<Class:#<Child:0x007fa404907e50>> 
 >> zoe.singleton_methods
138. >> zoe.singleton_class => #<Class:#<Child:0x007fa404907e50>> 
 >> zoe.singleton_methods => [:play]

139. >> zoe.singleton_class => #<Class:#<Child:0x007fa404907e50>> 
 >> zoe.singleton_methods => [:play]
 >> zoe.singleton_class.instance_methods(false)
140. >> zoe.singleton_class => #<Class:#<Child:0x007fa404907e50>> 
 >> zoe.singleton_methods => [:play]
 >> zoe.singleton_class.instance_methods(false) => [:play]
141. Child Kernel Object Human zoe #play >> zoe.play Ba Ob
142. Child Kernel Object Human zoe Singleton zoe #play >> zoe.play Ba Ob
143. Child Kernel Object Human zoe Singleton zoe #play #play >> zoe.play Ba Ob
144. >> class Child => end
145. >> Child = Class.new => Child >> class Child => end
146. >> Child.singleton_class => #<Class:Child> >> Child = Class.new => Child >> class Child => end
147. >> class Child
 >> def self.run
 >> puts "Aaah zombies!!"
 >> end
 >> end

148. >> class Child
 >> def self.run
 >> puts "Aaah zombies!!"
 >> end
 >> end
 >> def Child.run
 >> puts “Aaah zombies!!”
 >> end

149. >> class Child
 >> def self.run
 >> puts "Aaah zombies!!"
 >> end
 >> end
 >> def Child.run
 >> puts “Aaah zombies!!”
 >> end
 >> Child.singleton_methods
150. >> class Child
 >> def self.run
 >> puts "Aaah zombies!!"
 >> end
 >> end
 >> def Child.run
 >> puts “Aaah zombies!!”
 >> end
 >> Child.singleton_methods => [:run]

151. >> class Child
 >> def self.run
 >> puts "Aaah zombies!!"
 >> end
 >> end
 >> def Child.run
 >> puts “Aaah zombies!!”
 >> end
 >> Child.singleton_methods => [:run]
 >> Child.singleton_class.instance_methods(false)
152. >> class Child
 >> def self.run
 >> puts "Aaah zombies!!"
 >> end
 >> end
 >> def Child.run
 >> puts “Aaah zombies!!”
 >> end
 >> Child.singleton_methods => [:run]
 >> Child.singleton_class.instance_methods(false) => [:run]
153. Child Kernel Object Human Basi Objec Object Singleton Human Singleton Child Singleton Basic Object Singleton >> Child.run
154. Child Kernel Object Human Basi Objec Object Singleton Human Singleton Child Singleton Basic Object Singleton >> Child.run .run
155. Child Kernel Object Human Basi Objec Object Singleton Human Singleton Child Singleton Basic Object Singleton >> Child.run >> Child.new .run
156. Child Kernel Object Human Basi Objec Object Singleton Human Singleton Child Singleton Basic Object Singleton >> Child.run >> Child.new .run .new
157. Child Kernel Object Human zoe Singleton zoe Ba Ob >> zoe.play #play
158. Child Kernel Object Human Basi Objec Object Singleton Human Singleton Child Singleton Basic Object Singleton >> Child.run
159. >> zoe.singleton_class
160. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>

161. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass
162. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child

163. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child
 >> zoe.singleton_class.superclass
 .superclass
164. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child
 >> zoe.singleton_class.superclass
 .superclass => Human

165. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child
 >> zoe.singleton_class.superclass
 .superclass => Human
 >> zoe.singleton_class.superclass
 .superclass.superclass
166. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child
 >> zoe.singleton_class.superclass
 .superclass => Human
 >> zoe.singleton_class.superclass
 .superclass.superclass => Object

167. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child
 >> zoe.singleton_class.superclass
 .superclass => Human
 >> zoe.singleton_class.superclass
 .superclass.superclass => Object
 >> zoe.singleton_class.superclass
 .superclass.superclass.superclass
168. >> zoe.singleton_class => #<Class:#<Child:0x007f8048b69cf0>>
 >> zoe.singleton_class.superclass => Child
 >> zoe.singleton_class.superclass
 .superclass => Human
 >> zoe.singleton_class.superclass
 .superclass.superclass => Object
 >> zoe.singleton_class.superclass
 .superclass.superclass.superclass => BasicObject
169. >> Child.singleton_class
170. >> Child.singleton_class => #<Class:Child>

171. >> Child.singleton_class => #<Class:Child>
 >> Child.singleton_class.superclass
172. >> Child.singleton_class => #<Class:Child>
 >> Child.singleton_class.superclass => #<Class:Human>

173. >> Child.singleton_class => #<Class:Child>
 >> Child.singleton_class.superclass => #<Class:Human>
 >> Child.singleton_class.superclass
 .superclass
174. >> Child.singleton_class => #<Class:Child>
 >> Child.singleton_class.superclass => #<Class:Human>
 >> Child.singleton_class.superclass
 .superclass => #<Class:Object>

175. >> Child.singleton_class => #<Class:Child>
 >> Child.singleton_class.superclass => #<Class:Human>
 >> Child.singleton_class.superclass
 .superclass => #<Class:Object>
 >> Child.singleton_class.superclass
 .superclass.superclass
176. >> Child.singleton_class => #<Class:Child>
 >> Child.singleton_class.superclass => #<Class:Human>
 >> Child.singleton_class.superclass
 .superclass => #<Class:Object>
 >> Child.singleton_class.superclass
 .superclass.superclass => #<Class:BasicObject>
177. a class method (Child.run) is just a method defined on a class’s singleton class
178. a class method (Child.run) is just a method defined on a class’s singleton class
179. >> katie.class
180. >> katie.class => Child

181. >> katie.class => Child
 >> katie.intelligence
182. >> katie.class => Child
 >> katie.intelligence => 23

183. >> katie.class => Child
 >> katie.intelligence => 23
 >> katie.get_bitten_by_zombie
184. >> katie.class => Child
 >> katie.intelligence => 23
 >> katie.get_bitten_by_zombie => “Why, it’s just a flesh wound!”

185. >> katie.class => Child
 >> katie.intelligence => 23
 >> katie.get_bitten_by_zombie => “Why, it’s just a flesh wound!”
 >> katie.intelligence
186. >> katie.class => Child
 >> katie.intelligence => 23
 >> katie.get_bitten_by_zombie => “Why, it’s just a flesh wound!”
 >> katie.intelligence => 23

187. >> katie.class => Child
 >> katie.intelligence => 23
 >> katie.get_bitten_by_zombie => “Why, it’s just a flesh wound!”
 >> katie.intelligence => 23
 >> katie.singleton_methods
188. >> katie.class => Child
 >> katie.intelligence => 23
 >> katie.get_bitten_by_zombie => “Why, it’s just a flesh wound!”
 >> katie.intelligence => 23
 >> katie.singleton_methods => [:get_bitten_by_zombie]
191. magical_science_magic
192. >> def zoe.get_bitten_by_zombie >> magical_science_magic >> end >> zoe.singleton_methods => [:get_bitten_by_zombie]
193. module ZombieVaccine def get_bitten_by_zombie magical_science_magic end end
194. class Child < Human prepend ZombieVaccine ... def get_bitten_by_zombie horrible_zombie_change end end module ZombieVaccine def get_bitten_by_zombie magical_science_magic end end
195. >> roshni = Child.new => #<Child:0x007fb813064690 @intelligence=1, @creativity=1>
196. >> roshni.method(:get_bitten_by_zombie) => #<Method: Child(ZombieVaccine)#get_bitten> >> roshni = Child.new => #<Child:0x007fb813064690 @intelligence=1, @creativity=1>
197. >> roshni.get_bitten_by_zombie => “Why, it’s just a flesh wound!” >> roshni.method(:get_bitten_by_zombie) => #<Method: Child(ZombieVaccine)#get_bitten> >> roshni = Child.new => #<Child:0x007fb813064690 @intelligence=1, @creativity=1>
199. class Child < Human prepend ZombieVaccine ... def get_bitten_by_zombie horrible_zombie_change end end
200. class Child < Human ... remove_method(:get_bitten_ by_zombie) end
201. >> jonan = Child.new
202. >> jonan = Child.new >> jonan.get_bitten_by_zombie
203. >> jonan = Child.new >> jonan.get_bitten_by_zombie => “Aaaagh!!”
204. >> jonan = Child.new >> jonan.get_bitten_by_zombie => “Aaaagh!!” >> Child.instance_method(:get_bitten_ by_zombie) => #<UnboundMethod: Child(Human)#get_bitten_by_zombie>
205. >> jonan = Child.new >> jonan.get_bitten_by_zombie => “Aaaagh!!” >> Child.instance_method(:get_bitten_ by_zombie) => #<UnboundMethod: Child(Human)#get_bitten_by_zombie> >> Human.instance_methods => [:get_bitten_by_zombie]
206. class Child < Human ... undef_method(:get_bitten_ by_zombie) end
207. >> Child.instance_method(:get_bitten_by_zombie)
208. >> Child.instance_method(:get_bitten_by_zombie) => NameError: undefined method `get_bitten_by_zombie’ for class `Child'
209. “Contrast this with remove_method, which deletes a method from the particular class; Ruby will still search superclasses and mixed-in modules for a possible receiver.” “undef_method prevents the current class from responding to any calls to the named method.” - APIdock http://apidock.com/ruby/Module/undef_method
210. >> roshni.get_bitten_by_zombie
211. >> roshni.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>

212. >> roshni.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
 >> zoe.get_bitten_by_zombie
213. >> roshni.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
 >> zoe.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>

214. >> roshni.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
 >> zoe.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
 >> katie.get_bitten_by_zombie
215. >> roshni.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
 >> zoe.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
 >> katie.get_bitten_by_zombie => NoMethodError: undefined method `get_bitten_by_zombie’ for #<Child: 0x007f8048b69cf0>
219. • ancestors • Object, Kernel, BasicObject • include vs prepend • const_set • const_get • define_method • singleton class + singleton methods • remove_method • undef_method What was covered?
220. That’s all folks Thank you! @zoevkay