Meta Programming Made Easy

metaprogramming_for_dummies.rb

# Yehuda Katz - (It's all about the self)
#    http://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/


#######################################################################################
# All classes are objects themselves
#######################################################################################

# Defining a class using the "class" keyword is merely syntactic sugar.
# All class definitions are actually instances of the Class object.
class MyClass; end               # MyClass     = Class.new
class ParentClass; end           # ParentClass = Class.new
class ChildClass < ParentClass; end # MyClass  = Class.new(ParentClass)





########################################################################################
# NOTICE: All classes in Ruby are open! Ruby code is interpreted. There is no compiler.
#         Meaning they can be "reopened" and redefined at runtime.
#         Hence why this code is valid if you run this file with the ruby interpreter 
#        (or paste this code into irb)
#######################################################################################

# open MyClass and define a Class (singleton) method in class definition
class MyClass
  def self.my_class_method;end
end

# open String base class and define an instance method in class definition
class String
  def i_can_haz_cheezburgr?
    true
  end
end




########################################################################################
# NOTICE: All objects are open as well.  Methods can be defined on a particular instance (and its class).
#         In this case a method is added to the instance's class; an Eigenclass (ghost, shadow class)
#         is created which duplicates the class of the instance.  This makes the method 
#         available to the instance and that instance ONLY!
#
#         This "Eigenclass" cannot be accessed directly. (e.g. cannot be inherited)
#######################################################################################

# e.g.
my_class_instance = MyClass.new
def my_class_instance.method_for_this_instance_only
  # MyClass is duplicated into an Eigenclass (ghost class)
  # and this method definition is stored there.

  return "I can only be accessed from this instance of MyClass"
end

my_class_instance.method_for_this_instance_only # => "I can only be accessed from this instance of MyClass"
# Notice: The class of my_class_instance is still MyClass. The Eigenclass(MyClass) is a ghost.
my_class_instance.class # => MyClass
another_my_class_instance = MyClass.new
another_my_class_instance.respond_to?(:method_for_this_instance_only) # => false




# Class (singleton) method defined outside of class definition
def MyClass.my_class_method; end
MyClass.my_class_method





# Multiple class (singleton) methods
class MyClass
  # Use this style of metaprogramming to group 2 or more class methods
  # instead of repeating def self.<method_name>; end
  class << self # Define all methods on self's class (here self = MyClass)
    def my_class_method; end

    def my_second_class_method; end
  end

  def my_instance_method; end
end

MyClass.my_class_method
MyClass.my_second_class_method
MyClass.new.my_instance_method





################################################################################################
# ALL methods have a receiver
# To fully grok Ruby one must understand the "dot notation" within an object's method call
# <object>(dot)<method> # is equivelant to:
old_self = self
# self = <object>
# self.send(<method>.to_sym)
# self = old_self

################################################################################################

# e.g.
my_class_instance = MyClass.new
my_class_instance.my_instance_method # is equivelant to:
old_self = self
# self = my_class_instance
# self.send(:my_instance_method)
# self = old_self

# Same works for classes (since they are only object instances of Class.new)
MyClass.my_class_method # is equivelant to:
old_self = self
# self = MyClass
# self.send(:my_class_method)
# self = old_self

################################################################################################


# "This above all; to thine own SELF be true, 
#  and it must follow, as the night the day, 
#  thou canst not then be false to any [class]" - William Shakespeare