From: Massimiliano Mirra <list@...>
Date: 2002-08-04T09:19:40+09:00
Subject: Re: Assoc Class (Hash Pairs)

On Sun, Aug 04, 2002 at 08:47:35AM +0900, Tom Sawyer wrote:
> i recall having a need for it some time ago, but i don't recall off hand
> what that was. but the use i recently ran across was for building
> complex hierachies (for gui models) where sometimes you have a "child"
> object and some times you don't:
> 
> complex_hierarchy = [ parent => child,
>                       childless,
>                       another_parent => [ subchildless,
>                                           subparent => subchild ]
>                     ]
> 
>                                           
> without this, you either have to use hashes with extraneous nils, or
> fall back to pure associative arrays.

This way, the parent won't know it has a child, and the child won't
know it has a parent.  Is this intended?  If so, try out this:

class Assoc
	def Assoc.[](*args)
		new(*args)
	end

	def initialize(from, to nil)
		@from, @to = from, to
	end
	attr_reader :from, :to
end

class Object
	def link(to)
		Assoc[self, to]
	end
end

complex_hierarchy = [ Assoc[parent, child],
                      childless,
                      another_parent.link([
                        subchildless,
		        Assoc[subparent, subchild]
                      ])

Not as nice as a simple `=>', but seems to do what you ask.

(I wonder, however, if `=>' could be made into a method of Object.)

Massimiliano