From: Reimer Behrends <behrends@...>
Date: 2004-08-08T01:51:22+09:00
Subject: Re: Sets are not extensional

Csaba Henk (csaba@phony_for_avoiding_spam.org) wrote:
[...]
>  "Set" can be forced to behave extensionally by doing so:
> 
>  class Set
>    def eql? other
>      subset? other and other.subset? self
>    end
>    def hash
>      to_a.hash
>    end
>  end

Note that to_a.hash is effectively non-deterministic. If @hash == { 1 =>
true, 2 => true }, then to_a can be both [1, 2] and [2, 1], with
different hash values. To generate a correct hash function, you have to
use something like:

	to_a.map{|el| el.hash}.sort.hash

or:

	to_a.inject(0){|a,b| a ^ b.hash}

			Reimer Behrends