What is the difference between 'include' and 'prepend' in Ruby?

Question

From the Module

Module#append_features(mod) → mod => When this module is included in another, Ruby calls append_features in this module, passing it the receiving module in mod. Ruby’s default implementation is to add the constants, methods, and module variables of this module to mod if this module has not already been added to mod or one of its ancestors.

Module#prepend_features(mod) → mod => When this module is prepended in another, Ruby calls prepend_features in this module, passing it the receiving module in mod. Ruby’s default implementation is to overlay the constants, methods, and module variables of this module to mod if this module has not already been added to mod or one of its ancestors.

Can anyone help me to understand the below questions:

• What more features of Module are defined as append and prepend except those default?

• How they differ functionally?

• When to use append_features and when prepend_features?

• what is the difference between two bold lines as above?

Answer 1

• What features of Module are defined as append and prepend?

As specified in the text you quoted:

the constants, methods, and module variables

• How they differ functionally?

Both add methods of the mixed-in module to the passed module (class). The difference is in the lookup order of these methods, in case that the target class already has them defined:

include behaves as if the target class inherited mixed-in module:

module FooBar
  def say
    puts "2 - Module"
  end
end

class Foo
  include FooBar

  def say
    puts "1 - Implementing Class"
    super
  end
end

Foo.new.say # =>
            # 1 - Implementing Class
            # 2 - Module

prepend makes the methods from the mixed in module "stronger" and executes them first:

module FooBar
  def say
    puts "2 - Module"
    super
  end
end

class Foo
  prepend FooBar

  def say
    puts "1 - Implementing Class"
  end
end

Foo.new.say # =>
            # 2 - Module
            # 1 - Implementing Class

The example kindly ripped off from here: http://blog.crowdint.com/2012/11/05/3-killer-features-that-are-coming-on-ruby-2-0.html

• When to use append_features and when prepend_features?

Use prepend when you want to keep methods of the target module (class) at the end of the method lookup chain.

Some real-world examples can be found by searching SO for ruby, module and prepend:

• Overriding method by another defined in module
• When monkey patching a method, can you call the overridden method from the new implementation?
• Ruby: Module, Mixins and Blocks confusing?

(Note: I am mentioning only methods, as they are easiest to picture when it comes to inheritance and mixing-in, but the same applies to other features.)