Ruby ModulesAlso “open class”

and other languages…

Modules 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

MetamodelObject

Module

Class Numeric

Integer

Fixnum 4

From: Seven Languages in Seven Weeks

Namespace 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++?

Example from textmodule Base64

def self.encode (data) # code hereend

#could be: Base64.decode# cannot just leave off selfdef self.decode (text) # code hereend

end

Call as:text = Base64.encode(data)data = Base64.decode(text)

Mixins 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?

Mixin Best Practices 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?

require vs include 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 end class Bird

include FlyingCreature def initialize @speed = 5end

#... end class Mammal# ...endclass 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.newdaffy.flydracula = Bat.new dracula.flyputs "dracula is a flying creature:

#{dracula.is_a? FlyingCreature}"piglet = Mammal.newputs "piglet is a flying creature:

#{piglet.is_a? FlyingCreature}" TRACE

Open class 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 endend

In another file:require_relative "RubyOpenClasses-1.rb"

class Cat def purr puts "#{name} says Purrrrrrrrrrr" endend

ollie = Cat.new("Ollie", 5)ollie.purr

http://stackoverflow.com/questions/4184821/do-rubys-open-classes-break-encapsulation

Singleton Method Exampleclass Person def initialize(name) @name=name endend

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

Liskov Substitution Principle

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

LSP Example LSP Violation:

puts (1/2) => 0

NOW open file that requires "mathn"puts (1/2) => (1/2)

What class was modified in this example?