What is the difference between extend and include in Ruby?

[geekelo]

In Ruby, both extend and include are used to mix in modules into classes, but they are used in different contexts and have different effects.

1. include: This is used to mix a module's methods as instance methods in a class. When you include a module in a class, the methods of that module become available to instances of the class, but not to the class itself.

   module MyModule
     def my_method
       puts "This is a method from MyModule"
     end
   end
   
   class MyClass
     include MyModule
   end
   
   obj = MyClass.new
   obj.my_method  # This works

2. extend: This is used to mix a module's methods as class methods in a class. When you extend a module in a class, the methods of that module become available at the class level, but not to instances of the class.

   module MyModule
     def my_method
       puts "This is a method from MyModule"
     end
   end
   
   class MyClass
     extend MyModule
   end
   
   MyClass.my_method  # This works
   obj = MyClass.new
   # obj.my_method    # This would raise a NoMethodError since my_method is not available to instances

So, in summary:

• include is for mixing in instance methods.
• extend is for mixing in class methods.

In many cases, you might see extend self used within a module, which effectively makes all its methods both class methods (accessible at the class level) and instance methods (accessible at the instance level).