Metaprogramming Ruby for Greater Good

My talk at Reaktor Dev Day 2014

Transcript

1. Metaprogramming RUBY for Greater Good
2. Who?
3. @jarkko jlaine.net [en-us] notkea.fi [fi-fi]
4. bearmetal.eu
5. What?
6. Code that writes code
7. “Writing code that manipulates language constructs at runtime.” Paolo Perrotta
8. Why?
9. Magic
10. Not clever, Flexible
11. Why Ruby?
12. In Ruby Metaprogramming is just Programming
13. Advanced OO concepts in Ruby
14. Prototypes vs Classes
15. speaker = Object.new def speaker.talk_length @talk_length ||= 30 end def speaker.talk_length=(length) @talk_length = length end
16. gary = speaker.clone gary.talk_length # => 30 gary.talk_length = 60 gary.talk_length # => 60 scott = gary.clone scott.talk_length # => 60 scott.talk_length = 45 scott.talk_length # => 45
18. Everything is executable
19. class Conference puts "Hello world (open)" def venue end # … end # Hello world (open) # => nil
20. class Conference < Event end > c = Conference.new > c.class => Conference > c.class.superclass
21. class Conference < Event end > c = Conference.new > c.class => Conference > c.class.superclass => ChunkyBacon
22. class ChunkyBacon end Event = ChunkyBacon class Conference < Event end > c = Conference.new > c.class => Conference > c.class.superclass => ChunkyBacon
23. def gimme_a_class [Array, Hash, String][rand(3)] end class Conference < gimme_a_class() end > c = Conference.new > c.class => Conference > c.class.superclass => ChunkyBacon
24. Open classes class String alias_method :old_reverse, :reverse def reverse old_reverse.upcase end end > speaker = "Gary" > speaker.reverse => "YRAG"
25. Everything is an object > String.instance_method(:reverse) => #<UnboundMethod: String#reverse>
26. Even classes
27. Wait, what?
28. But, classes have class methods. They are special.
29. No.
30. class Conference def self.in_finland # return conferences in Finland end end
31. Singleton methods def speaker.talk_length @talk_length ||= 30 end Defined for a single object
32. Ruby method lookup Right, then up
33. Where are singleton methods sitting?
34. Singleton class aka ghost class, eigenclass, metaclass anonymous class below object's own class
36. Singleton class Just a regular class, except » Hidden from the generic class hierarchy » Cannot be inherited directly » Only ever has a single instance of itself
37. So, what are class methods?
38. So, what are class methods? Singleton methods for the class object itself
39. So, what are class methods? Singleton methods for the class object itself Residing in the singleton class of the class object
41. Singleton classes are classes and classes are objects, so…
42. …a singleton class must have its own singleton class
43. Singleton classes all the way down
44. Metaprogramming, anyone?
45. Creating code dynamically in Ruby
46. Creating code dynamically in Ruby » eval » instance_eval & class_eval » define_method & Class.new » method_missing
47. eval meth = "my_method" eval <<-END def #{meth} "foo" end END
48. eval » Just executes a line of code » Super powerful… » …and dangerous in wrong hands » Code injection hazard
49. instance_eval » Can also take a string, but also a code block… » …which it then executes in the scope of the receiver. » Methods will become class methods (because self == the class) obj.instance_eval do self # => obj @v # => obj's instance var end
50. instance_eval class Turtle def move(&block) instance_eval(&block) end def right(n); end; # … end t = Turtle.new t.move do right(3) up left(2) down end
51. class_eval » Can only be called for classes and modules » creates instance methods » can be used to do stuff inside the class definition [String, Array, Hash].each do |cls| cls.class_eval { include HelloWorld } end
52. define_method » Create a method dynamically » Can be put inside other methods class Cat < Animal [:leg, :head, :tail].each do |part| define_method(part) do # … end end end
53. method_missing The catch-all method » Called whenever a method is not found in the inheritance chain. » By default throws a NoMethodError. » Can be modified to catch any method call, thus “creating” a ghost method » The name of which you don't have to even know.
54. method_missing class Speaker def method_missing(met, *args) if met.to_s == "speak" "I might as well say something: #{args[0]}" else super end end end gary = Speaker.new gary.talk("Destroy it") # => NoMethodError gary.speak("Just destroy it!") # => "I might as well say something: Just destroy it!"
55. Example Rewrite attr_accessor in Ruby
56. class Animal attr_accessor :legs, :diet end cat = Animal.new cat.legs = 4 cat.legs # => 4 cat.diet = "Small birds" cat.diet # => "Small birds"
57. What is attr_accessor there?
58. A class method errrrr... A singleton method of the class object
59. class Animal def self.nattr_accessor(*meths) meths.each do |meth| # getter define_method(meth) do instance_variable_get("@#{meth}") end # setter define_method("#{meth}=") do |wut| instance_variable_set("@#{meth}", wut) end end end end
60. class Cat < Animal nattr_accessor :legs, :diet end c = Cat.new c.legs = 4 c.diet = "Small birds" c.legs # => 4 c.diet # => "Small birds"
61. # Making it more reusable class Object def self.nattr_accessor(*meths) meths.each do |meth| # getter define_method(meth) do instance_variable_get("@#{meth}") end # setter define_method("#{meth}=") do |wut| instance_variable_set("@#{meth}", wut) end end end end
62. module Nattr def nattr_accessor(*meths) meths.each do |meth| # getter define_method(meth) do instance_variable_get("@#{meth}") end # setter define_method("#{meth}=") do |wut| instance_variable_set("@#{meth}", wut) end end end end
63. class Animal include Nattr end Won't work (creates instance methods)
64. class Animal extend Nattr end class Cat < Animal nattr_accessor :legs, :diet end c = Cat.new c.legs = 4 c.diet = "Mice" c.legs c.diet
65. Problems with metaprogramming
66. Abstraction and Indirection Code can become harder to » Read » Debug » Search for
67. Powertool Apply with care Use when you need it, not when you can
68. Where now? Only scratched the surface
69. Dave Thomas's screencasts at Prag Prog A bit dated (1.8) but lay a solid foundation Paolo Perrotta's Metaprogramming Ruby
70. Read the source, Luke Any non-trivial Ruby application is bound to have a bunch of metaprogramming sprinkled in.
71. In Ruby Metaprogramming is just programming
72. Thank you
73. ?