Ruby MetaProgramming Samples

meta_define_method.rb

class Testt
  define_method :foo do
    "welcome"
  end
end
Testt.new().foo
=> "welcome"

module Hello
  define_method :foo do |name|
    "welcome #{name}"
  end
end

include Hello
foo "Dave"
=> "welcome Dave"

meta_instance_eval.rb

Testt.instance_eval do
  def foo
    "I'll be available as class method"
  end

  def self.bar
    "I'll be available as class method"
  end
end

Testt.foo
=>"I'll be available as class method"
Testt.bar
=>"I'll be available as class method"

metaprogramming_class_eval.rb

class Testt

end

Testt.class_eval do
  def foo
    "I'll be available as instance method"
  end

  def self.foo
    "I'll be available as class method"
  end
end

Testt.new().foo
=> "I'll be available as instance method"
Testt.foo
=>"I  be available as class method"

metaprogramming_method_missing.rb

class User < ActiveRecord::Base
  #user has one role so association is
  has_one :role
  #First time when there is no method found; this method will catch that
  def method_missing(method_sym, *params)
    #Here we match the pattern of method and see if we can process it or not
    if method_sym.to_s =~ /^is_(.*)\?$/
      function = $1

      self.class.class_eval <<-END
	def #{method_sym.to_s}
	  #{role.name == function.to_s.upcase}
        end
      END
      #Let's register this method with user class when it is invoked first time.
      send(method_sym)
    else
      super
    end
  end

  #Let this class respond for these methods
  def self.respond_to?(method_sym, *args, &block)
    method_sym.to_s =~ /^is_(.*)\?$/ ? true : super
  end
end

user.rb

class User < ActiveRecord::Base
  #user has one role so association is
  has_one :role
end