1. Also “open class” and other languages…

2.  A module is a named group of methods, constants, and class variables  All classes are modules  Modules are not classes Can’t be instantiated Can’t be subclassed  Purpose: namespaces and mixins

3. Object Module Class Numeric Integer Fixnum4 From: Seven Languages in Seven Weeks

4.  Example: want to provide utility functions that don’t require an object. Maybe encode/decode functions, conversions, math functions, etc.  Can add global methods, but then need to worry about naming conflicts. Remember namespaces from C++?

5. module Base64 def self.encode (data) # code here end #could be: Base64.decode # cannot just leave off self def self.decode (text) # code here end  Call as: text = Base64.encode(data) data = Base64.decode(text)

6.  If a module defines instance methods, those can be “mixed into” other classes  Essentially eliminates need for multiple inheritance  Examples: Enumerable. defines iterators that make use of each method Comparable. defines comparison operators (, >=, between?) in terms of (like compareTo)  Module.include function does the “mixing” How does this compare to Java Interface?

7.  Quick Exercise: take a look at: http://matt.aimonetti.net/posts/2012/07/30/ruby-class-module- mixins/ http://stackoverflow.com/questions/575920/is-a-ruby-module- equivalent-to-a-java-interface  With your partner: List a best practice for classes When does it make sense to write a module instance method?

8.  require brings in the contents of another file (like “include” in C++)  include “mixes in” a module module FlyingCreature def fly puts "Flying! with speed #@speed" end class Bird include FlyingCreature def initialize @speed = 5 end #... end class Mammal #... end class Bat < Mammal include FlyingCreature #... end RubyModules-2.rb demo code, example from: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/16071 daffy = Bird.new daffy.fly dracula = Bat.new dracula.fly puts "dracula is a flying creature: #{dracula.is_a? FlyingCreature}" piglet = Mammal.new puts "piglet is a flying creature: #{piglet.is_a? FlyingCreature}" TRACE

9.  Methods and variables can be added to classes  In RubyOpenClasses-1.rb: class Cat attr_accessor :name, :age def initialize(name, age) @name, @age = name, age end  In another file: require_relative "RubyOpenClasses-1.rb" class Cat def purr puts "#{name} says Purrrrrrrrrrr" end ollie = Cat.new("Ollie", 5) ollie.purr http://stackoverflow.com/questions/4184821/do-rubys-open-classes-break-encapsulation

10. class Person def initialize(name) @name=name end shugo = Person.new("Shugo") matz = Person.new("Matz") def matz.design_ruby puts "I am #{@name} and I designed Ruby!" end matz.design_ruby shugo.design_ruby # error! From: http://www.slideshare.net/ShugoMaeda/rc2010-refinements

11.  Helps to guide best practices for Ruby open classes  LSP (as we know it): subtypes must be substitutable for base class Goal: don’t break program when call a method that expects a parent object but receives a child object Remember: square vs rectangle example.  LSP: instances of a class after reopen must behave like instances of a class before From: http://www.slideshare.net/ShugoMaeda/rc2010-refinements

12.  LSP Violation: puts (1/2) => 0 NOW open file that requires "mathn" puts (1/2) => (1/2) What class was modified in this example?