From: Robert Klemme <shortcutter@...>
Date: 2006-08-01T00:45:07+09:00
Subject: Re: Weird elusive bug using Comparable

Pedro C�rte-Real wrote:
> I have this class:
> 
> class SimpleDate
>  include Comparable
> 
>  VALS = [:year, :month, :day]
>  attr_accessor *VALS
> 
>  def initialize(year, month, day)
>    @year = year
>    @month = month
>    @day = day
>  end
> 
>  #ideas on making this much shorter are welcome
>  def <=>(other)
>    VALS.each do |key|
>      v1 = self.send(key)
>      v2 = other.send(key)
> 
>      result = (v1 <=> v2)
>      if result != 0
>        return result
>      end
>    end
>    0
>  end
> end

# shorter
SimpleDate = Struct.new :year, :month, :day do
   include Comparable

   def <=> (other)
     self.class.members.each do |field|
       result = send(field) <=> other.send(field)
       return result unless result == 0
     end
     0
   end
end

> that represents a date that might have any of the three parts not
> existant. For some reason date1 == date2 returns nil when done like
> this:
> 
> class SimpleDateTest < Test::Unit::TestCase
>  def test_comparison_with_nils
>    date1 = SimpleDate.new(1,nil,nil)
>    date2 = SimpleDate.new(1,nil,nil)
> 
>    assert_not_equal date1.object_id, date2.object_id
>    assert date1 == date2
>  end
> end
> 
> The code shouldn't even work because nil doesn't implement <=>. What
> the hell is comparable doing behind my back?
> 
> I thought date1 == date2 was the same as (date1 <=> date2> == 0 but
> apparently not.

No, it's differently implemented.

Kind regards

	robert