The Curious Incident of the Shadow in the Run-Time

A presentation about the perils of shadowing in Ruby. It is an adaptation of my blog post The Curious Incident of the Shadow in the Run-Time (https://paulfioravanti.com/blog/2018/08/20/the-curious-incident-of-the-shadow-in-the-run-time/)

Presented at:

- Ruby on Rails Oceania Sydney meetup on 9 July 2019.
- Melbourne Ruby meetup on 27 May 2020 (via Zoom)

Presentation slide deck markdown and speaker notes (useable in Deckset 2):
https://github.com/paulfioravanti/presentations/tree/master/the_curious_incident_of_the_shadow_in_the_run_time

Transcript

1. The Curious Incident of the SHADOW in the Run-Time
4. The
5. The Curious
6. The Curious Incident
7. The Curious Incident of the SHADOW
8. The Curious Incident of the SHADOW in the Run-Time
9. SHADOW
10. VARIABLE SHADOWING
11. shadow.rb
12. shadow.rb x = 42
13. shadow.rb x = 42 3.times { |x| puts "x is #{x}" }
14. shadow.rb x = 42 3.times { |x| puts "x is #{x}" } $ ruby shadow.rb x is 0 x is 1 x is 2
15. shadow.rb x = 42 3.times { |x| puts "x is #{x}" } $ ruby shadow.rb x is 42 x is 42 x is 42
16. shadow.rb x = 42 3.times { |x| puts "x is #{x}" } $ ruby shadow.rb x is 0 x is 1 x is 2
17. DOES RUBY CARE ABOUT SHADOWING?
18. shadow.rb x = 42 3.times { |x| puts "x is #{x}" }
19. shadow.rb x = 42 3.times { |x| puts "x is #{x}" } $ ruby -w shadow.rb
20. shadow.rb x = 42 3.times { |x| puts "x is #{x}" } $ ruby -w shadow.rb shadow.rb:2: warning: shadowing outer local variable - x shadow.rb:1: warning: assigned but unused variable - x x is 0 x is 1 x is 2
23. BUT WHY THOUGH?
24. shadow.rb x = 42 3.times { |x| puts "x is #{x}" }
25. shadow.rb x = 42 3.times do |x| puts( "Local x is #{x} and "\ "outer x is #{'What goes here??'}" ) end
26. shadow.rb y = 42 3.times { |x| puts "x is #{x} and y is #{y}" }
27. shadow.rb y = 42 3.times { |x| puts "x is #{x} and y is #{y}" } $ ruby -w shadow.rb x is 0 and y is 42 x is 1 and y is 42 x is 2 and y is 42
28. "[Matz] thinks that shadowing by itself is a bad habit and should be discouraged." — Shouhei Urabe, quo/ng Matz
29. ShadowingOuterLocalVariable
32. INSTANCE METHOD SHADOWING
33. "In Ruby, local variable names and method names are nearly iden0cal."
34. "If you have not assigned to one of these ambiguous names, Ruby will assume you wish to call a method."
35. "Once you have assigned to the name, Ruby will assume you wish to reference a local variable."
36. "The local variable is created when the parser encounters the assignment, not when the assignment occurs." — Local Variables and Methods Assignment sec9on of Ruby's syntax documenta9on
37. TOP ‑ BOTTOM
38. NAMES CAN CHANGE
39. METHOD ‑ VARIABLE
41. person.rb
42. class Person end
43. class Person def initialize(name = nil) @name = name end end
44. class Person attr_accessor :name def initialize(name = nil) @name = name end end
45. class Person def name @name end def name=(new_name) @name = new_name end def initialize(name = nil) @name = name end end
46. class Person attr_accessor :name def initialize(name = nil) @name = name end end
47. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
48. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
49. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
50. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
51. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
52. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
53. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
54. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
55. $ irb irb(main):001:0> require "./person.rb" true irb(main):002:0>
56. $ irb irb(main):001:0> require "./person.rb" true irb(main):002:0> Person.new("Matz").say_name
57. $ irb irb(main):001:0> require "./person.rb" true irb(main):002:0> Person.new("Matz").say_name My name is nil nil
58. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
59. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
60. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
61. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
62. $ irb irb(main):001:0> require "./person.rb" true irb(main):002:0> Person.new("Matz").say_name My name is nil nil irb(main):003:0>
63. $ irb irb(main):001:0> require "./person.rb" true irb(main):002:0> Person.new("Matz").say_name My name is nil nil irb(main):003:0> Person.new.say_name
64. $ irb irb(main):001:0> require "./person.rb" true irb(main):002:0> Person.new("Matz").say_name My name is nil nil irb(main):003:0> Person.new.say_name My name is "Unknown" nil
65. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
66. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
67. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
68. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
69. class Person attr_accessor :name def initialize(name = nil) @name = name end def say_name if name.nil? name = "Unknown" end puts "My name is #{name.inspect}" end end
72. irb(main):002:0> Person.new("Matz").say_name
73. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: if name.nil? 14: name = "Unknown" 15: end 16: 17: puts "My name is #{name.inspect}" 18: end [1] pry(#<Person>)>
74. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: if name.nil? 14: name = "Unknown" 15: end 16: 17: puts "My name is #{name.inspect}" 18: end [1] pry(#<Person>)> name nil [2] pry(#<Person>)>
75. ERR... WHAT?
76. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: if name.nil? 14: name = "Unknown" 15: end 16: 17: puts "My name is #{name.inspect}" 18: end [1] pry(#<Person>)> name nil [2] pry(#<Person>)>
77. [1] pry(#<Person>)> name nil [2] pry(#<Person>)> next
78. [1] pry(#<Person>)> name nil [2] pry(#<Person>)> next From: /person.rb @ line 17 Person#say_name: 10: def say_name 11: binding.pry 12: 13: if name.nil? 14: name = "Unknown" 15: end 16: => 17: puts "My name is #{name.inspect}" 18: end [2] pry(#<Person>)>
79. [2] pry(#<Person>)> next From: /person.rb @ line 17 Person#say_name: 10: def say_name 11: binding.pry 12: 13: if name.nil? 14: name = "Unknown" 15: end 16: => 17: puts "My name is #{name.inspect}" 18: end [2] pry(#<Person>)> name nil [3] pry(#<Person>)> exit My name is nil
80. SPOOKY
81. SPOOKY QUANTUM
82. SPOOKY QUANTUM RUBY
83. SPOOKY QUANTUM RUBY?
85. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts name.inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end [1] pry(#<Person>)>
86. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts name.inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end [1] pry(#<Person>)> name nil
87. [1] pry(#<Person>)> name nil [2] pry(#<Person>)> next "Matz" From: /person.rb @ line 14 Person#say_name: 10: def say_name 11: binding.pry 12: 13: puts name.inspect => 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
89. defined?
90. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end [1] pry(#<Person>)>
91. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end [1] pry(#<Person>)> next "method"
92. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end [1] pry(#<Person>)> defined?(name) "local-variable"
94. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
95. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
96. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
97. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
98. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
99. irb(main):002:0> Person.new("Matz").say_name From: /person.rb @ line 13 Person#say_name: 10: def say_name 11: binding.pry 12: => 13: puts defined?(name).inspect 14: if name.nil? 15: name = "Unknown" 16: end 17: 18: puts "My name is #{name.inspect}" 19: end
101. DON'T
102. DON'T SHADOW
103. DON'T SHADOW
104. def say_name if name.nil? self.name = "Unknown" end puts "My name is #{name.inspect}" end
105. def say_name name = self.name || "Unknown" puts "My name is #{name.inspect}" end
106. def say_name name = self.name || "Unknown" puts "My name is #{name.inspect}" end def say_name name = name() || "Unknown" puts "My name is #{name.inspect}" end
108. THANKS! @paulfioravanti