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Metaprogramming with `super` powers Bradley Schaefer @soulcutter

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What is metaprogramming? “Drawing Hands” by M.C. Escher. [reference]

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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

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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

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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

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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

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First Attempt

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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

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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

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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

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This is why people hate metaprogramming

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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

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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

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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

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Second Attempt

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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

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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

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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

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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

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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

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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?

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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 #

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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

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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

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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

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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

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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

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Third Attempt

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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, #, Object, PP::ObjectMixin, Kernel, BasicObject]

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time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end AlarmEvent.ancestors # => [ AlarmEvent, #, Object, Kernel, BasicObject ]

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time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end AlarmEvent.ancestors # => [ AlarmEvent, #, Object, Kernel, BasicObject ] super

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time_reader :end_at, from: :end_ts def end_at(tz = UTC) return tz.now if end_ts.nil? super end AlarmEvent.ancestors # => [ AlarmEvent, #, Object, Kernel, BasicObject ] super

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pry(main)> ls Datapoint.new ##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: # Visibility: public Number of lines: 3 define_method(attr) do |tz = UTC| tz.at(public_send(from)) end

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pry(main)> ls Datapoint.new ##methods: datapoint_at pry(main)> $ Datapoint.new.datapoint_at From: /Users/soulcutter/super-powers/ time_attribute_methods.rb @ line 4: Owner: # Visibility: public Number of lines: 3 define_method(attr) do |tz = UTC| tz.at(public_send(from)) end

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Final Attempt

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The Module Builder Pattern

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1.Define a subclass of Module 2.Define an initializer that takes configuration 3.Use configuration to define methods

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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

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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

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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

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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

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pry(main)> ls Datapoint.new ##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: # Visibility: public Number of lines: 3 define_method(attr) do |tz = UTC| tz.at(public_send(from))

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def inspect "#<#{self.class.name}: attr=#{@attr.inspect} \ from=#{@from.inspect}>" end pry(main)> ls Datapoint.new ##methods: datapoint_at Datapoint#methods: datapoint_ts

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def inspect "#<#{self.class.name}: attr=#{@attr.inspect} \ from=#{@from.inspect}>" end pry(main)> ls Datapoint.new ##methods: datapoint_at Datapoint#methods: datapoint_ts

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Metaprogramming:

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Metaprogramming with `super` powers Bradley Schaefer @soulcutter Source: Adventure Time Galactic Fist Bump T-Shirt