Tricks and tips for meta programming in Ruby

class_eval_trick.rb

# Usage of class_eval method

class Sample

end

s = Sample.new

Sample.class_eval do
  def say_hello
    puts "I am becoming the instance method in Sample class"
  end
end

Sample.class_eval do
  def self.say_bye
    puts "I am becoming the class method in Sample class"
  end
end

# the following method will raise error. Because its not possible to execute class_eval for an object.
begin
  s.class_eval do
    def status
      puts "I am no more executed."
    end
  end
rescue Exception => e
  puts "Exception raised in above method defination"
end

s.say_hello # => I am becoming the instance method in Sample class
Sample.say_bye # => I am becoming the class method in Sample class

s.status # => This method was not created, so will raise NoMethodError

instance_eval_trick.rb

# Usage of instance_eval method

class Sample

end

s = Sample.new

Sample.instance_eval do
  def say_hello # or self.say_hello ; here both will create a class method in "Sample" class. 
    puts "Hello Meta Programming with Ruby"
  end
end

s.instance_eval do
  def status # or self.say_hello ; here both will create an instance method in "Sample" class. 
    puts "I am an instance method"
  end
end

Sample.say_hello # => Hello Meta Programming with Ruby
s.status # => I am an instance method

meta_programming_tricks.rb

# Getting/Setting instance variables

class Sample
  def initialize
    @secret = 99
  end
end

k = Sample.new
k.instance_variable_set('@secret', 21)
puts k.instance_variable_get('@secret') # => 21

mixins_ruby.rb

module EmailReporter
  def send_report
    puts "Email Reporter"
  end
end

module PdfReporter
  def send_report
    puts "Pdf Reporter"
  end
end

class Person
  def send_report
    puts "Person Reporter"
  end
end

class User < Person
end

class Employee < Person
  include EmailReporter
end

class Manager < Person
  include EmailReporter
  include PdfReporter
end

class Guest < Person
  include PdfReporter
  include EmailReporter
  # the sequence of including the module will matter.
  # the methods in EmailReporter will be looked up first when you called the send_reoprt on Guest object.
  # because it is included recently.
end

Person.new.send_report # => Person Reporter
User.new.send_report # => Person Reporter
Employee.new.send_report # => Email Reporter
Manager.new.send_report # => Pdf Reporter
Guest.new.send_report # => Email Reporter