From: "Ara.T.Howard" <ara.t.howard@...>
Date: 2005-11-11T02:22:13+09:00
Subject: Re: Golf, anyone?

On Fri, 11 Nov 2005, m4dc4p wrote:

> I spent a day figuring out how to solve this problem, and it seems like
> there should be a much better solution. I wanted to write a module
> that, when included in an object, would create a class variable and
> accessor for that object. If another class includes the module, it
> should have its own class variable and accessor. The class variable
> does *not* live in the module - it is not shared by all objects which
> include the module.
>
> A simplified version of what I came up with is below. Module Foo
> defines the class variable and accessor. Class Bar and Baz are related
> by inheritance. Instance variables a and b are of class Bar and Baz.
> Instance variable w is of class Bil, which is not related to either Bar
> or Baz.
>
> When a value is added to the "prop" class variable of either a or
> b, they both show it. However, when a value is added to the "prop"
> class variable in w, it does not show up in either a or b.
>
> I'm convinced there is a better way to do this and what I came up
> with is just a hack on a kludge. Any and all help is appreciated!
>
>
> module Foo
> 	def self.included(mod)
> 		super(mod)
> 		class << mod; self; end.class_eval <<-EOS
> 			@prop = []
> 			def prop
> 				@prop ||= []
> 			end
> 		EOS
>
> 		def mod.inherited(p)
> 			def p.prop
> 				superclass.prop
> 			end
>
> 		end
> 	end
> end
>
> class Bar
> 	include Foo
> end
>
> class Baz < Bar
> end
>
> class Bil
> 	include Foo
> end
>
> a = Bar.new
> b = Bar.new
> x = Baz.new
> w = Bil.new
>
> a.class.prop << "p1"
> puts "a, b, and x share property added to a:
> 	#{a.class.prop.inspect},
> 	#{b.class.prop.inspect},
> 	#{x.class.prop.inspect}"
>
> b.class.prop << "p2"
> puts "and property added to b:
> 	#{a.class.prop.inspect},
> 	#{b.class.prop.inspect},
> 	#{x.class.prop.inspect}"
>
> w.class.prop << "p3"
> puts "but not a property added to w.
> 	a: #{a.class.prop.inspect},
> 	b: #{b.class.prop.inspect},
> 	x: #{x.class.prop.inspect},
> 	w: #{w.class.prop.inspect}"
>
> #>ruby mod_test.rb
> #a, b, and x share property added to a:
> #        ["p1"],
> #        ["p1"],
> #        ["p1"]
> #and property added to b:
> #        ["p1", "p2"],
> #        ["p1", "p2"],
> #        ["p1", "p2"]
> #but not a property added to w.
> #        a: ["p1", "p2"],
> #        b: ["p1", "p2"],
> #        x: ["p1", "p2"],
> #        w: ["p3"]
>
> p.s. The implementation here relies on the append_features method of
> module, and the inherited method of objects. When an object first
> includes the module, the class variable and property are added to the
> object via append_features. I found that this this did NOT extend down
> the inheritance chain for those objects, though. I was forced to
> override the inherrited method on the initial object so I could place
> the 'properties' definition in each subclass. This is my least favorite
> part of my solution.


   harp:~ > cat a.rb
   require "traits"


   class Bar; class_trait "prop" => [];end
   class Baz < Bar; end
   class Bil; class_trait "prop" => [];end

   a, b, x, w = [Bar, Bar, Baz, Bil].map{|c| c::new}

   a.class.prop << "p1"
   puts "a, b, and x share property added to a:
           #{a.class.prop.inspect},
           #{b.class.prop.inspect},
           #{x.class.prop.inspect}"

   b.class.prop << "p2"
   puts "and property added to b:
           #{a.class.prop.inspect},
           #{b.class.prop.inspect},
           #{x.class.prop.inspect}"

   w.class.prop << "p3"
   puts "but not a property added to w.
           a: #{a.class.prop.inspect},
           b: #{b.class.prop.inspect},
           x: #{x.class.prop.inspect},
           w: #{w.class.prop.inspect}"


   harp:~ > ruby a.rb
   a, b, and x share property added to a:
           ["p1"],
           ["p1"],
           ["p1"]
   and property added to b:
           ["p1", "p2"],
           ["p1", "p2"],
           ["p1", "p2"]
   but not a property added to w.
           a: ["p1", "p2"],
           b: ["p1", "p2"],
           x: ["p1", "p2"],
           w: ["p3"]


you could do it with a Foo module too, but it's excess typing and you said 'golf' ;-)

regards.

-a
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