From: Robert Klemme <shortcutter@...>
Date: 2007-06-10T19:50:35+09:00
Subject: Re: Overloading Array Subtraction operator

On 10.06.2007 10:25, Nicko wrote:
> Wow!
> Thank you both!
> 
> I ended up with
> 
> class SuperArray < Array
> 	def -(other)
> 		self.select{|x| !other.include? x}
> 	end
> end
> 
> and it works great :)  I can optimise it later :)

It is usually not such a good idea to inherit base classes like Array 
and Hash.  Here are two more healthy approaches.

1. wrap Array with a class that represents the concept (which one btw?) 
your Array is used for.  Then implement #- (and all the other methods).

2. wrap Hash with a class that represents the concept (which one btw?) 
your Hash is used for.  Then implement #==, #hash and #eql? accordingly.

The basic reason why your code does not work as you would like it to 
work is that Hash does not implement #eql? and #hash in a way that 
considers Hash content (for the reasons please search the archives, the 
topic has come up frequently).  Note:

irb(main):037:0> h={:foo=>:bar}
=> {:foo=>:bar}
irb(main):038:0> h == h.dup
=> true
irb(main):039:0> h.eql? h.dup
=> false
irb(main):040:0> h.hash == h.dup.hash
=> false

Kind regards

	robert