Modules

Ref. WGR Chapter 4, Modules and program organization

Modules and Mixins

• Any class (or object) can include a Module
  • also known as mixing in
  • so Modules are also called Mixins
• Modules provide behavior but no state
  • more precisely, they affect the extending object's state

Multiple modules

• You can mix in as many modules as you like
• Resembles multiple inheritance, but less confusing

Module definition

@@@ ruby
module Greeter
  def greet
    puts "Hello!"
  end
end

Module usage

@@@ ruby
class Person
  include Greeter
end

alice = Person.new
alice.greet

Modules cannot be instantiated

Greeter.new #=> undefined method `new' for Greeter:Module

Modules have no state

but they can fake it pretty well

@@@ ruby
module Stacky
  def stack
    @stack ||= []
  end
  def push obj
    stack.push obj
  end
  def pop
    stack.pop
  end
end

@stack will become an instance variable on the includer

load vs. require vs. include vs. extend

• They all sound so similar!
• load inserts a file's contents
  • load 'config.rb'
  • The same file can be loaded more than once, last one sticks
• require inserts a file's parsed contents
  • require 'config'
  • requireing the same file more than once has no effect, first one sticks
• include mixes in a module into this class' instances
  • include Config
• extend mixes in a module into self, usually to add class methods
  • extend Config

module lookup gotchas

• If a class defines the same method as a module, then the class wins, even if the module was included later
• If two modules define the same method, then the last one to be included wins

• If you include a module twice, then the second inclusion is ignored, so it's still further back on the lookup chain

    @@@ ruby
    class Foo
      include A
      include B
      include A  #<< ignored!
    end
  

Using Modules for Organization

• Modules are used as namespaces
• Classes are also modules

    class Class < Module
  

Module Organization Example

whiz.rb:

@@@ ruby
module Whiz
  VERSION = "1.0.2"
  class GeeGaw
    # ...
  end
end

whiz/bang.rb:

@@@ ruby
module Whiz
  class Bang
    def geegaw
      # inside the module, its members are in scope
      @geegaw ||= GeeGaw.new
    end
  end
end

client.rb:

@@@ ruby
require "whiz"
require "whiz/bang"

# outside the module, its members are not in scope
# you need to name it explicitly
puts Whiz::VERSION
bang = Whiz::Bang.new