From: Warren Brown <wkb@...>
Date: 2003-02-21T03:27:27+09:00
Subject: Inconsistent behavior in Array methods

    I recently tracked down a bug in my code to a line that read:

        cards = cards.sort.uniq

    This seemed perfectly reasonable - the cards array contained Card
objects which were Enumerable.  However, the bug was due to the fact that
Array#uniq does not use the object's == method to do comparisons - even
though Array#sort uses the object's <=> method.  Upon investigation, I found
that the implementation of Array#uniq uses a hash, and since the key to the
hash is the object ID, uniqueness ends up being defined as (obj1.id ==
obj2.id).  This was *surprising* to me.

    Further investigation shows that the following methods of Array ignore
the object's == method:

        Array#&
        Array#|
        Array#uniq
        Array#uniq!

    On the other hand, the following methods honor the object's == method:

        Array#-
        Array#==
        Array#===
        Array#assoc
        Array#delete
        Array#include?
        Array#index
        Array#rassoc
        Array@rindex

    In addition, the following functions honor the object's <=> method:

        Array#<=>
        Array#sort
        Array#sort!

    Not only does this cause an inconsistency between the definition of
equality between "set intersection" (Array#&) and "set difference"
(Array#-), it also leads to different behavior between Numerics (and
Strings) and other objects:

irb(main):001:0> RUBY_VERSION
=> "1.6.8"
irb(main):002:0> class A
irb(main):003:1>   def initialize(i) @i = i end
irb(main):004:1>   def ==(j) @i == j end
irb(main):005:1> end
=> nil
irb(main):006:0> [1,1,2].uniq
=> [1, 2]
irb(main):007:0> ['1','1','2'].uniq
=> ["1", "2"]
irb(main):008:0> [A.new(1),A.new(1),A.new(2)].uniq
=> [#<A:0x810190c @i=1>, #<A:0x81018f8 @i=1>, #<A:0x81018e4 @i=2>]

    It seems to me that uniqueness should be determined by the object's ==
method instead of the object's ID as in Array#&, Array#|, Array#uniq, and
Array#uniq!.  This seems more in line with methods like Array#- and
Array#include?.

    Any other thoughts?

    - Warren Brown