Metaprogramming Ruby - Speaker Deck

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Ruby Metaprogramming Tuesday, July 24, 12

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Metaprogramming is writing code that writes code. Tuesday, July 24, 12

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Metaprogramming is writing code that writes code in a galaxy far, far away. Tuesday, July 24, 12

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Metaprogramming is writing code that writes code in a galaxy far, far away. Tuesday, July 24, 12

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Building abstractions to simplify Create nifty Domain Speciﬁc Languages, such as ActiveRecord associations. Tuesday, July 24, 12

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For example, has_many is an abstraction around n-to-many relationships and all the associated duct work. Building abstractions to simplify Tuesday, July 24, 12

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class Feed < ActiveRecord::Base has_many :items belongs_to :user validates_presence_of :user_id before_create do |feed| feed.adopt_user_display_name end end Building abstractions to simplify Tuesday, July 24, 12

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feed = Feed.last feed.items.create(text: "An item!") Building abstractions to simplify Where did that come from? Tuesday, July 24, 12

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Ruby Object Model •classes •objects •class objects •instance methods •class methods •singleton methods •singleton classes •virtual classes? Tuesday, July 24, 12

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Ruby Object Model •classes •objects •class objects •instance methods •class methods •singleton methods •singleton classes •virtual classes? OMG WTF BBQ Tuesday, July 24, 12

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Ruby Object Model objects object_id: 703 obj Tuesday, July 24, 12

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Ruby Object Model classes objects superclass class methods: Object + to_s + nil? + respond_to? object_id: 703 obj obj = Object.new() Tuesday, July 24, 12

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Where am I? Who am I? self Tuesday, July 24, 12

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current object Where am I? Who am I? self Tuesday, July 24, 12

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Where am I? Who am I? class Foo self # self is Foo def self.foo #self is Foo return self end def bar # self is a Foo instance return self end end Tuesday, July 24, 12

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Explicit vs. implicit receiver class Foo def foo "foo" end def bar return self.foo end def baz return bar.upcase end end f = Foo.new f.baz Tuesday, July 24, 12

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Deﬁne instance methods superclass class methods: Person + say_name + say_age class Person attr_attribute :name, :age def initialize(name, age) @name, @age = name, age end def say_name "Hi, my name is #{name}." end def say_age "I'm #{@age} years old." end end Class Deﬁnition Tuesday, July 24, 12

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A class’s class is Class. superclass class methods: Person + say_name + say_age class Person attr_attribute :name, :age def initialize(name, age) @name, @age = name, age end def say_name "Hi, my name is #{name}." end def say_age "I'm #{@age} years old." end end Class Deﬁnition superclass class methods: Class + name + ancestors - include Tuesday, July 24, 12

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Also, Class’s class is Class. superclass class methods: Person + say_name + say_age class Person attr_attribute :name, :age def initialize(name, age) @name, @age = name, age end def say_name "Hi, my name is #{name}." end def say_age "I'm #{@age} years old." end end Class Deﬁnition superclass class methods: Class + name + ancestors - include Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age jeff = Person.new('Jeff', 31) jeff.say_age #=> "I'm 31 years old." jeff.say_name #=> "Hi, my name is Jeff." Class Deﬁnition name: “Jeff” age: 31 jeff Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age jeff = Person.new('Jeff', 31) jeff.say_age #=> "I'm 31 years old." jeff.say_name #=> "Hi, my name is Jeff." jeff.to_s #=> "#" Class Deﬁnition name: “Jeff” age: 31 jeff Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age name: “Jeff” age: 31 jeff Where did that come from? jeff = Person.new('Jeff', 31) jeff.say_age #=> "I'm 31 years old." jeff.say_name #=> "Hi, my name is Jeff." jeff.to_s #=> "#" Class Deﬁnition Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age name: “Jeff” age: 31 jeff superclass class methods: Object + clone + to_s ... Instance Method Lookup jeff.to_s Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age name: “Jeff” age: 31 jeff superclass class methods: Object + clone + to_s ... Instance Method Lookup jeff.to_s Nope! Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age name: “Jeff” age: 31 jeff superclass class methods: Object + clone + to_s ... Instance Method Lookup jeff.to_s Yep! Tuesday, July 24, 12

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superclass class methods: Person + say_name + say_age name: “Jeff” age: 31 jeff superclass class methods: Object + clone + to_s ... Instance Method Lookup jeff.to_s #=> "#" Yep! Tuesday, July 24, 12

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Override the #to_s method superclass class methods: Instructor + to_s class Instructor < Person def to_s "Instructor #{name}." end end jeff = Instructor.new('Jeff', 31) jeff.to_s #=> "Instructor Jeff" Tuesday, July 24, 12

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name: “Jeff” age: 31 jeff Instance Method Lookup jeff = Instructor.new('Jeff', 31) jeff.to_s superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Instructor + to_s Tuesday, July 24, 12

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name: “Jeff” age: 31 jeff Instance Method Lookup jeff = Instructor.new('Jeff', 31) jeff.to_s #=> "Instructor Jeff" superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Instructor + to_s Yep! Tuesday, July 24, 12

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name: “Jeff” age: 31 jeff Instance Method Lookup jeff = Instructor.new('Jeff', 31) jeff.say_name superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Instructor + to_s Tuesday, July 24, 12

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name: “Jeff” age: 31 jeff Instance Method Lookup jeff = Instructor.new('Jeff', 31) jeff.say_name #=> "Hi, my name is Jeff." superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Instructor + to_s Yep! Tuesday, July 24, 12

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Deﬁne a module superclass class methods: Rubyist + write_code module Rubyist def write_code "Ruby!" end end class Person include Rubyist end Module Instance Methods Tuesday, July 24, 12

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A module’s class is Module superclass class methods: Rubyist + write_code module Rubyist def write_code "Ruby!" end end class Person include Rubyist end Module Instance Methods superclass class methods: Module + name + ancestors - include Tuesday, July 24, 12

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name: “chad” age: 33 chad Module Method Lookup class Person include Rubyist end chad = Person.new chad.write_code superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Rubyist + write_code Including a module inserts it into the classe’s superclass chain. Tuesday, July 24, 12

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name: “chad” age: 33 chad Module Method Lookup class Person include Rubyist end chad = Person.new chad.write_code #=> "Ruby!" superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Rubyist + write_code Yep! Tuesday, July 24, 12

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name: “chad” age: 33 chad Module Method Lookup class Person include Rubyist include Lisper end chad = Person.new chad.write_code superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Rubyist + write_code superclass class methods: Lisper + write_code Tuesday, July 24, 12

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name: “chad” age: 33 chad Module Method Lookup class Person include Rubyist include Lisper end chad = Person.new chad.write_code superclass class methods: Person + say_name superclass class methods: Rubyist + write_code superclass class methods: Lisper + write_code Tuesday, July 24, 12

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name: “chad” age: 33 chad Module Method Lookup class Person include Rubyist include Lisper end chad = Person.new chad.write_code superclass class methods: Person + say_name superclass class methods: Rubyist + write_code superclass class methods: Lisper + write_code Nope! Tuesday, July 24, 12

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name: “chad” age: 33 chad Module Method Lookup class Person include Rubyist include Lisper end chad = Person.new chad.write_code #=> "Lithp!" superclass class methods: Person + say_name superclass class methods: Rubyist + write_code superclass class methods: Lisper + write_code Yep! Tuesday, July 24, 12

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name: “chad” age: 33 chad Precedence superclass class methods: Person + write_code superclass class methods: Object ... class Person def write_code "Type, type, type." end end chad = Person.new chad.write_code #=> "Type, type, type." Tuesday, July 24, 12

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name: “chad” age: 33 chad superclass class methods: Person + write_code superclass class methods: Object ... superclass class methods: Rubyist + write_code class Person def write_code "Type, type, type." end end Person.send(:include, Rubyist) chad = Person.new chad.write_code #=> Which? Precedence Tuesday, July 24, 12

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How can one ensure a class provides a method but allow it to be overridden by an included module? Tuesday, July 24, 12

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name: “chad” age: 33 chad class Person module Coder def write_code "Type, type, type." end end include Person::Coder end chad = Person.new chad.write_code superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: P::Coder + write_code Precedence Tuesday, July 24, 12

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name: “chad” age: 33 chad class Person module Coder def write_code "Type, type, type." end end include Person::Coder end Person.send(:include, Rubyist) chad = Person.new chad.write_code superclass class methods: Person + say_name Precedence superclass class methods: Rubyist + write_code superclass class methods: P::Coder + write_code Tuesday, July 24, 12

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Override a method on matt matt = Person.new('Matt', 29) def matt.say_name "I'm Batman." end matt.say_age #=> "I’m 29 years old." matt.say_name #=> "I'm Batman." name: “Matt” age: 29 matt Singleton Methods Tuesday, July 24, 12

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superclass class methods: Person + say_name name: “Matt” age: 29 matt superclass class methods: Object ... Singleton Classes def matt.say_name "I'm Batman." end matt.say_name Tuesday, July 24, 12

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superclass class methods: Person + say_name name: “Matt” age: 29 matt superclass class methods: Object ... Singleton Classes def matt.say_name "I'm Batman." end matt.say_name superclass class methods: Person’ + say_name Tuesday, July 24, 12

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superclass class methods: Person + say_name name: “Matt” age: 29 matt superclass class methods: Object ... Singleton Classes def matt.say_name "I'm Batman." end matt.say_name #=> "I'm Batman." superclass class methods: Person’ + say_name Yep! Tuesday, July 24, 12

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Accessing the singleton class class Foo class << self # self is Class' for Foo end end f = Foo.new class << f # self is Foo' for f # and is equal to f.singleton_class end class << foo Tuesday, July 24, 12

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Class Methods class Person def self.fess_up "There are no class methods." end end Person.fess_up #=> "There are no class methods." superclass class methods: Person + say_name + say_age Tuesday, July 24, 12

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superclass class methods: Object ... superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Person + say_name Class Methods class Person def self.fess_up "There are no class methods." end end Person.fess_up #=> "There are no class methods." Tuesday, July 24, 12

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superclass class methods: Object ... superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Class’ + fess_up superclass class methods: Person + say_name Class Methods class Person def self.fess_up "There are no class methods." end end Person.fess_up #=> "There are no class methods." “Class” methods are methods deﬁned on the class’s singleton class. name: “chad” age: 33 chad Tuesday, July 24, 12

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name: “chad” age: 33 chad Instance extends Module superclass class methods: Person + say_name superclass class methods: Object ... class Person end module Rubyist def write_code "Ruby!" end end chad = Person.new chad.extend Rubyist chad.write_code Tuesday, July 24, 12

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name: “chad” age: 33 chad superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Rubyist + write_code Extending a module inserts it into your class’s superclass chain. Instance extends Module class Person end module Rubyist def write_code "Ruby!" end end chad = Person.new chad.extend Rubyist chad.write_code Tuesday, July 24, 12

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superclass class methods: Object ... superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Person + say_name Class extends Module module Rubyist def write_code "Ruby!" end end class Person extend Rubyist end Person.write_code Tuesday, July 24, 12

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superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Person + say_name Class extends Module module Rubyist def write_code "Ruby!" end end class Person extend Rubyist end Person.write_code superclass class methods: Class’ ... superclass class methods: Rubyist + write_code Tuesday, July 24, 12

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To extend a module is equivalent to an include in your singleton class. Tuesday, July 24, 12

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class Shape attr_reader :dimensions def initialize(dimensions={}) @dimensions = dimensions end [:width, :height, :length].each do |dim| # Fetch value from the dimensions hash define_method(dim) do return dimensions[dim] end end end deﬁne_method Tuesday, July 24, 12

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class Instructor < Person def self.start_pomodoro puts "Ready, set, go" end def self.end_pomodoro puts "RINNGGGGG" end def give_lecture "Pomodoro, pomodoro, pomodoro." end end instance_eval and class_eval superclass class methods: Object ... superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Class’ + start_pomo + end_pomo superclass class methods: Instructor + to_s Tuesday, July 24, 12

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class Instructor < Person def self.start_pomodoro puts "Ready, set, go" end def self.end_pomodoro puts "RINNGGGGG" end def give_lecture "Pomodoro, pomodoro, pomodoro." end end Instructor.class_eval do def give_advice "Bonnie will take care of it!" end end superclass class methods: Object ... superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Class’ + start_pomo + end_pomo instance_eval and class_eval superclass class methods: Instructor + give_lecture + give_advice Tuesday, July 24, 12

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class Instructor < Person def self.start_pomodoro puts "Ready, set, go" end def self.end_pomodoro puts "RINNGGGGG" end def give_lecture "Pomodoro, pomodoro, pomodoro." end end Instructor.instance_eval do def restart_pomodoro end_pomodoro start_pomodoro end end superclass class methods: Object ... superclass class methods: Instructor + to_s superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Class’ + start_pomo + end_pomo + restart_pomo instance_eval and class_eval Tuesday, July 24, 12

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String interpolated class Shape attr_reader :dimensions def initialize(dimensions={}) @dimensions = dimensions end [:width, :height, :length].each do |dim| class_eval <<-MET def #{dim}=(val) @#{dim} = val end MET end end Tuesday, July 24, 12

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class Instructor < Person def self.start_pomodoro puts "Ready, set, go" end def self.end_pomodoro puts "RINNGGGGG" end def give_lecture "Pomodoro, pomodoro, pomodoro." end end Instructor.class_eval <<-MET, __FILE__, __LINE__ def advise_student "Bonnie will take care of it!" end MET superclass class methods: Object ... superclass class methods: Object ... superclass class methods: Class ... superclass class methods: Class’ + start_pomo + end_pomo instance_eval and class_eval superclass class methods: Instructor + give_lecture + give_advice Tuesday, July 24, 12

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instance_exec Tuesday, July 24, 12

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Class “macro” methods module AccessibleAttributes def accessible_attribute(name) define_method(name) do instance_variable_get("@#{name}") end define_method("#{name}=") do |value| instance_variable_set("@#{name}", value) end end end Creating a module that generates methods allows it to be extended into an existing class to provide macro abilities. class Feed extend AccessibleAttributes end Tuesday, July 24, 12

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Testing “macro” methods describe AccessibleAttributes do let(:klass) { Class.new } before(:each) do klass.extend AccessibleAttributes klass.accessible_attribute :foo end describe ".accessible_attribute" do it "allows writing and reading" do k = klass.new.tap{|k| k.foo = "bar" } k.foo.should == "bar" end end end Extending into a new class for each spec prevents specs from interacting with one another. Tuesday, July 24, 12

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Rewrite this class to use a single call to define_method instead of the normal defs. Rewrite it again using a form of eval and a string with interpolation. For either version, add the corresponding reader methods. class Player def name=(name) @name = name end def score=(score) @score = score end def paddle=(paddle) @paddle = paddle end end Task: Deﬁne methods dynamically Tuesday, July 24, 12

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Hooks Tuesday, July 24, 12

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Hash.class_eval do def method_missing(name, *args, &block) if has_key?(name) fetch(name) else super end end end method_missing Tuesday, July 24, 12

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Hash.class_eval do def method_missing(name, *args, &block) if has_key?(name) fetch(name) else super end end def respond_to?(name) has_key?(name) || super end end respond_to? Tuesday, July 24, 12

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class Instructor < Person def give_lecture "Pomodoro, pomodoro, pomodoro." end end jeff = Instructor.new jeff.hand_out_contracts Method Missing name: “Jeff” age: 31 jeff superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Instructor + to_s Tuesday, July 24, 12

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class Instructor < Person def give_lecture "Pomodoro, pomodoro, pomodoro." end end jeff = Instructor.new jeff.hand_out_contracts #NoMethodError: undefined method `hand_out_contracts' for Instructor Jeff Method Missing name: “Jeff” age: 31 jeff superclass class methods: Person + say_name superclass class methods: Object ... superclass class methods: Instructor + to_s #method_missing Yep! Tuesday, July 24, 12

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included is invoked by the Ruby runtime whenever a module is included into another class. It can be used to implement a pattern that gives the appearance that inclusion also imports “class” methods. module AccessibleAttributes def self.included(base) base.extend(ClassMethods) end module ClassMethods def accessible_attribute(name) define_method(name) do instance_variable_get("@#{name}") end define_method("#{name}=") do |value| instance_variable_set("@#{name}", value) end end end end Hooks Tuesday, July 24, 12

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Deﬁne a module Settings that provides a macro method called .setting that takes a name, “foo” and creates #foo_on, #foo_off, and also #foo_on?, #foo_off? instance methods. Next, let .setting take a list of setting names. class Configurator include Settings setting :power end c = Configurator.new c.power_on? #=> false c.power_off? #=> true c.power_on c.power_off? #=> false Task: Deﬁne a macro method Tuesday, July 24, 12

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Define a let macro that takes a name and a block. It creates an instance method of that name, the body of which is the block, but that block should be dynamically evaluated in the context of the instance. Super duper bonus: define a let! that memoizes the result of the block after the first call. Bonus: Macro with instance scope class Computor let(:quotient) { dividend / divisor } def dividend; 1; end def divisor; 0; end end c = Computor.new c.quotient # ZeroDivisionError: divided by 0 Tuesday, July 24, 12

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• class Foo < class_expr() • closure currying • caller • binding • yield (self) • set_trace_function • deﬁned? / const_deﬁned? Misc. Tuesday, July 24, 12

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ﬁn Tuesday, July 24, 12