Modules

A simple mixin with include

module SomeMixin
  def foo
    puts "foo!"
  end
end

class Bar
  include SomeMixin
  def baz
    puts "baz!"
  end
end

b = Bar.new
b.baz         # => "baz!"
b.foo         # => "foo!"
# works thanks to the mixin

Now Bar is a mix of its own methods and the methods from SomeMixin.

Note that how a mixin is used in a class depends on how it is added:

• the include keyword evaluates the module code in the class context (eg. method definitions will be methods on instances of the class),
• extend will evaluate the module code in the context of the singleton class of the object (methods are available directly on the extended object).

Modules and Class Composition

You can use Modules to build more complex classes through composition. The include ModuleName directive incorporates a module's methods into a class.

module Foo
  def foo_method
    puts 'foo_method called!'
  end
end

module Bar
  def bar_method
    puts 'bar_method called!'
  end
end

class Baz
  include Foo
  include Bar

  def baz_method
    puts 'baz_method called!'
  end  
end

Baz now contains methods from both Foo and Bar in addition to its own methods.

new_baz = Baz.new
new_baz.baz_method #=> 'baz_method called!'
new_baz.bar_method #=> 'bar_method called!'
new_baz.foo_method #=> 'foo_method called!'

Module as Namespace

Modules can contain other modules and classes:

module Namespace

    module Child

        class Foo; end

    end # module Child

    # Foo can now be accessed as:
    #
    Child::Foo

end # module Namespace

# Foo must now be accessed as:
# 
Namespace::Child::Foo

A simple mixin with extend

A mixin is just a module that can be added (mixed in) to a class. one way to do it is with the extend method. The extend method adds methods of the mixin as class methods.

module SomeMixin
  def foo
    puts "foo!"
  end
end

class Bar
  extend SomeMixin
  def baz
    puts "baz!"
  end
end

b = Bar.new
b.baz         # => "baz!"
b.foo         # NoMethodError, as the method was NOT added to the instance
Bar.foo       # => "foo!"
# works only on the class itself 

Syntax

Declaration

module Name;

    any ruby expressions;

end

Remarks

Module names in Ruby are constants, so they have to start with a capital letter.

module foo; end # Syntax error: class/module name must be CONSTANT