From: "Marcel Molina Jr." <marcel@...>
Date: 2005-03-08T05:16:17+09:00
Subject: Customizing Array#uniq by defining eql?

PickAxe II's documentation for Array#uniq says, 

    Returns a new array by removing duplicate values in _arr_, where
    duplicates are detected by comparing using +eql?+.

Given that say I have a simple class:

    class PossibleTime
      attr_reader :day, :start_time, :end_time

      def initialize(day, start_time, end_time)
        @day, @start_time, @end_time = day, start_time, end_time
      end

      def hash
        "#@day #@start_time #@end_time"
      end

      def ==(other_pt)
        self.hash == other_pt.hash
      end

      def eql?(other_pt)
        self == other_pt
      end
    end

Here +eql?+ is defined in terms of +==+. Creating two example instances shows
these methods working:

    > a = PossibleTime.new('M', '9:30', '10:30')
    > b = PossibleTime.new('M', '9:30', '10:30')
    > c = PossibleTime.new('T', '9:30', '12:00')
    > a == b
    => true
    > a.eql? b
    => true
    > a == c
    => false
    > [a,b].uniq.size
    => 2
    > # I was expecting 1, not 2

I'll add debugging to see if +eql?+ was even called:

    class PossibleTime
      def eql?(other_pt)
        puts 'eql? called'
        self == other_pt
      end
    end

    > a.eql? b
    eql? called
    => true
    > a.eql? c
    eql? called
    => false
    > [a,b].uniq.size
    => 2

So eql? isn't even being called. 

Am I overlooking something? Specifying something incorrectly? Not
understanding the fundamental interaction between eql? and uniq?

Thanks for your time,
marcel
-- 
Marcel Molina Jr. <marcel@vernix.org>