From: zdennis <zdennis@...>
Date: 2006-04-11T02:46:24+09:00
Subject: Re: Adding both class and instance/object methods from a Module?

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Sam Roberts wrote:
> On Mon, Apr 10, 2006 at 08:57:04PM +0900, Robert Klemme wrote:
> 
>>There's an easier way:
>>
>>class D
>> extend M
>>end
>>
>>or
>>
>>class D;end
>>D.extend M
> 
> 
> Is there an easy way to extend classes with both class and object
> methods at the same time?
> 
> 	module M
> 	  def self.create
> 	    # would do something special here
> 	    new
> 	  end
> 	  def objmethod
> 	  end
> 	end
> 
> 	class A
> 	  extend M
> 	end
> 
> 	a.create   # <-- nope, doesn't work

This is because you created the 'create' method as a class-like method on the mdoule, a module method. You need to create it as an
instance level method. It seems to do this cleanly you'd want to separate this into two modules, one for instance level methods
and one intended for class-level use:

  irb(main):017:0> module ClassLevelAddons
  irb(main):018:1>   def create
  irb(main):019:2>      new
  irb(main):020:2>   end
  irb(main):021:1> end
  => nil
  irb(main):022:0> module InstanceLevelAddons
  irb(main):023:1>   def foo
  irb(main):024:2>     "bar"
  irb(main):025:2>   end
  irb(main):026:1> end
  => nil
  irb(main):027:0> class A
  irb(main):028:1>   extend ClassLevelAddons
  irb(main):029:1>   include InstanceLevelAddons
  irb(main):030:1> end
  => A
  irb(main):031:0> A.create
  => #<A:0xb7c9dc1c>
  irb(main):032:0> A.new.foo
  => "bar"

For organizational concerns you could always let one module be in charge:

  irb(main):001:0> module M
  irb(main):002:1>   module ClassLevelAddons
  irb(main):003:2>     def create; new ; end
  irb(main):004:2>   end
  irb(main):005:1>
  irb(main):006:1*   def foo
  irb(main):007:2>     "bar"
  irb(main):008:2>   end
  irb(main):009:1>
  irb(main):010:1*   def self.included( clazz )
  irb(main):011:2>      clazz.extend( ClassLevelAddons )
  irb(main):012:2>   end
  irb(main):013:1> end
  => nil
  irb(main):014:0> class A ; include M; end
  => A
  irb(main):015:0> A.create
  => #<A:0x2bfc5e8>
  irb(main):016:0> A.new.foo
  => "bar"
  irb(main):017:0>

The above lets M take care of the work, so your class doesn't have to think about it. This is the method that Robert Klemme was
refering to, Module::included


Zach



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