From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2003-04-05T06:21:35+09:00
Subject: Re: To inherit or to include? That is the question.

In article <b6k4k5$647i1$1@ID-52924.news.dfncis.de>,
Robert Klemme <bob.news@gmx.net> wrote:
>
>"Jim Freeze" <jim@freeze.org> schrieb im Newsbeitrag
>news:20030404085816.A61099@freeze.org...
>> For example:
>>
>>   class AppMaker
>>     def main; <stuff>; end
>>   end
>>   class MyApp < AppMaker
>>   end
>>   MyApp.new.main
>>
>> or
>>
>>   module AppMaker
>>     def main; <stuff>; end
>>   end
>>   class MyApp
>>     include AppMaker
>>   end
>>   MyApp.new.main
>>
>> I cannot see that the two behave that differently.
>> I'm sure there are some differences. Maybe someone
>> can help me enumerate them.
>
>Currently I can't see why you want to inherit a main function - if it's
>what I guess, then main is very application specific and does not lend
>itself easily for inheritance at all.
>
>> Ideally, I would like some set of guidelines to help
>> me make a choice between these two.
>>
>> Right now, it's fuzzy to me which one I should use.
>
>The simples rule is that you have to use inheritance if you want to reuse
>state.  Modules can only provide methods.  So, if you have a set of
>related methods that can be expected to be used in different classes, put
>them together in a module.

But this isn't quite true.  You can create/manipulate instance variables 
withing a modules's methods:

module Foo
  def set_var(v)
    @var = v
  end
  def get_var
    @var
  end
end

class Bar 
  include Foo 
  attr_accessor :var
end

b = Bar.new
b.set_var(42)
puts b.var #-> 42


So, the module Foo was able to introduce some state into the class Bar.

Phil