Metaprogramming with super powers

Metaprogramming in Ruby gets a bad rap, yet Rails itself is chock-full of metaprogramming. So what is it that separates good metaprogramming from bad? Here we will take a look at some of the problems people encounter when they attempt to wield it themselves, and approaches that can make your own metaprogramming code feel as magical as it feels in Rails.

Transcript

1. Metaprogramming with `super` powers Bradley Schaefer @soulcutter
7. What is metaprogramming? “Drawing Hands” by M.C. Escher. [reference]
10. UTC = ActiveSupport::TimeZone["UTC"] class Datapoint < ApplicationRecord def datapoint_at(tz = UTC) tz.at(datapoint_ts) end end class ChartAnnotation < ApplicationRecord def annotation_at(tz = UTC) tz.at(annotation_ts) end end
11. UTC = ActiveSupport::TimeZone["UTC"] class Datapoint < ApplicationRecord def datapoint_at(tz = UTC) tz.at(datapoint_ts) end end class ChartAnnotation < ApplicationRecord def annotation_at(tz = UTC) tz.at(annotation_ts) end end
12. UTC = ActiveSupport::TimeZone["UTC"] class Datapoint < ApplicationRecord def datapoint_at(tz = UTC) tz.at(datapoint_ts) end end class ChartAnnotation < ApplicationRecord def annotation_at(tz = UTC) tz.at(annotation_ts) end end
13. UTC = ActiveSupport::TimeZone["UTC"] class Datapoint < ApplicationRecord def datapoint_at(tz = UTC) tz.at(datapoint_ts) end end class ChartAnnotation < ApplicationRecord def annotation_at(tz = UTC) tz.at(annotation_ts) end end
14. First Attempt
15. module Timify def timify(attr) at_attr = attr.to_s.chomp("ts") + "at" define_method(at_attr) do |tz = UTC| tz.at(public_send(attr)) end end end class Datapoint extend Timify timify :datapoint_ts def datapoint_at(tz = UTC) tz.at(datapoint_ts) end end
16. module Timify def timify(attr) at_attr = attr.to_s.chomp("ts") + "at" define_method(at_attr) do |tz = UTC| tz.at(public_send(attr)) end end end class Datapoint extend Timify timify :datapoint_ts def datapoint_at(tz = UTC) tz.at(datapoint_ts) end end
17. module Timify def timify(attr) at_attr = attr.to_s.chomp("ts") + "at" define_method(at_attr) do |tz = UTC| tz.at(public_send(attr)) end end end class Datapoint extend Timify timify :datapoint_ts end
18. This is why people hate metaprogramming
19. class Datapoint extend Timify timify :datapoint_ts end •"timify" is a verb, it's doing something to "datapoint_ts" - but what? •"datapoint_at" method comes out of nowhere •subtle problem with overriding
20. class Datapoint extend Timify timify :datapoint_ts end •"timify" is a verb, it's doing something to "datapoint_ts" - but what? •"datapoint_at" method comes out of nowhere •subtle problem with overriding
21. class Datapoint extend Timify timify :datapoint_ts end •"timify" is a verb, it's doing something to "datapoint_ts" - but what? •"datapoint_at" method comes out of nowhere •subtle problem with overriding
22. Second Attempt
23. Principle of Least Astonishment “People are part of the system. The design should match the user's experience, expectations, and mental models” –Principles of computer system design: an introduction ISBN 978-0-12-374957-4
24. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end class Datapoint extend TimeAttributeMethods time_reader :datapoint_at, from: :datapoint_ts end
25. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end class Datapoint extend TimeAttributeMethods time_reader :datapoint_at, from: :datapoint_ts end
26. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end class Datapoint extend TimeAttributeMethods time_reader :datapoint_at, from: :datapoint_ts end
28. class AlarmEvent extend TimeAttributeMethods time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end end
29. class AlarmEvent extend TimeAttributeMethods time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end end alarm_event.end_at # What happens?
30. class AlarmEvent extend TimeAttributeMethods time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end end alarm_event.end_at # What happens? NoMethodError: super: no superclass method `end_at' for #<AlarmEvent: 0x00007fba40d4cf40>
31. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end time_reader :end_at, from: :end_ts def end_at(tz = UTC) tz.at(end_ts) end def end_at(tz = UTC) return tz.now if end_ts.nil? super end
32. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end time_reader :end_at, from: :end_ts def end_at(tz = UTC) tz.at(end_ts) end def end_at(tz = UTC) return tz.now if end_ts.nil? super end
33. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end time_reader :end_at, from: :end_ts def end_at(tz = UTC) tz.at(end_ts) end def end_at(tz = UTC) return tz.now if end_ts.nil? super end
34. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end time_reader :end_at, from: :end_ts def end_at(tz = UTC) tz.at(end_ts) end def end_at(tz = UTC) return tz.now if end_ts.nil? super end
35. module TimeAttributeMethods def time_reader(attr, from:) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end time_reader :end_at, from: :end_ts def end_at(tz = UTC) tz.at(end_ts) end def end_at(tz = UTC) return tz.now if end_ts.nil? super end
36. Third Attempt
37. module TimeAttributeMethods def time_reader(attr, from:) include (Module.new do |mod| define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end) end end time_reader :datapoint_at, from: :datapoint_ts Datapoint.ancestors # => [Datapoint, #<Module: 0x00007f9381b73ce0>, Object, PP::ObjectMixin, Kernel, BasicObject]
38. time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end AlarmEvent.ancestors # => [ AlarmEvent, #<Module:0x00007f9381b73ce0>, Object, Kernel, BasicObject ]
39. time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end AlarmEvent.ancestors # => [ AlarmEvent, #<Module:0x00007f9381b73ce0>, Object, Kernel, BasicObject ] super
40. time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end AlarmEvent.ancestors # => [ AlarmEvent, #<Module:0x00007f9381b73ce0>, Object, Kernel, BasicObject ] super
43. pry(main)> ls Datapoint.new #<Module:0x00007fa65d1ef338>#methods: datapoint_at Datapoint#methods: datapoint_ts pry(main)> $ Datapoint.new.datapoint_at From: /Users/soulcutter/super-powers/ time_attribute_methods.rb @ line 4: Owner: #<Module:0x00007fa65d1ef338> Visibility: public Number of lines: 3 define_method(attr) do |tz = UTC| tz.at(public_send(from)) end
44. pry(main)> ls Datapoint.new #<Module:0x00007fa65d1ef338>#methods: datapoint_at pry(main)> $ Datapoint.new.datapoint_at From: /Users/soulcutter/super-powers/ time_attribute_methods.rb @ line 4: Owner: #<Module:0x00007fa65d1ef338> Visibility: public Number of lines: 3 define_method(attr) do |tz = UTC| tz.at(public_send(from)) end
45. Final Attempt
46. The Module Builder Pattern
47. 1.Define a subclass of Module 2.Define an initializer that takes configuration 3.Use configuration to define methods
48. class TimeAttribute < Module def initialize(attr, from) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end module TimeAttributeMethods def time_reader(attr, from:) include TimeAttribute.new(attr, from) end end
49. class TimeAttribute < Module def initialize(attr, from) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end module TimeAttributeMethods def time_reader(attr, from:) include TimeAttribute.new(attr, from) end end
50. class TimeAttribute < Module def initialize(attr, from) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end module TimeAttributeMethods def time_reader(attr, from:) include TimeAttribute.new(attr, from) end end
51. class TimeAttribute < Module def initialize(attr, from) define_method(attr) do |tz = UTC| tz.at(public_send(from)) end end end module TimeAttributeMethods def time_reader(attr, from:) include TimeAttribute.new(attr, from) end end
52. pry(main)> ls Datapoint.new #<TimeAttribute:0x00007fe81cbe3a00>#methods: datapoint_at Datapoint#methods: datapoint_ts pry(main)> $ Datapoint.new.datapoint_at From: /Users/soulcutter/super-powers/ time_attribute_methods.rb @ line 4: Owner: #<Module:0x00007fa65d1ef338> Visibility: public Number of lines: 3 define_method(attr) do |tz = UTC| tz.at(public_send(from))
53. def inspect "#<#{self.class.name}: attr=#{@attr.inspect} \ from=#{@from.inspect}>" end pry(main)> ls Datapoint.new #<TimeAttribute: attr=:datapoint_at from=:datapoint_ts>#methods: datapoint_at Datapoint#methods: datapoint_ts
54. def inspect "#<#{self.class.name}: attr=#{@attr.inspect} \ from=#{@from.inspect}>" end pry(main)> ls Datapoint.new #<TimeAttribute: attr=:datapoint_at from=:datapoint_ts>#methods: datapoint_at Datapoint#methods: datapoint_ts
55. Metaprogramming:
56. Metaprogramming with `super` powers Bradley Schaefer @soulcutter Source: Adventure Time Galactic Fist Bump T-Shirt