From: Adam Bender <abender@...>
Date: 2008-03-31T11:37:06+09:00
Subject: How does Array#- do comparisons?

How does Array#- (as in, [1, 2, 3, 1, 1] - [1]) compare two objects to
see if they are equal?  I would think the .eql? method, but I've
written a class that implements that method so that two different
objects with the same values are considered equal and '-' isn't
functioning as I would expect.  A contrived example:

irb(main):004:0> class Hash
irb(main):005:1> def eql?(h)
irb(main):006:2> true
irb(main):007:2> end
irb(main):008:1> end
=> nil
irb(main):010:0> h1 = {'a' => 1}
=> {"a"=>1}
irb(main):011:0> h2 = {'a' => 1}
=> {"a"=>1}
irb(main):012:0> h1 == h2
=> true
irb(main):013:0> h1.eql? h2
=> true
irb(main):014:0> [h1] - [h2]
=> [{"a"=>1}]

I would expect [] as a result, just like the following:
irb(main):015:0> ["asdf"] - ["asdf"]
=> []

Thanks,

Adam