From: Christopher Dicely <cmdicely@...>
Date: 2010-09-02T00:18:45+09:00
Subject: Re: Array#=== as set equality

On Wed, Sep 1, 2010 at 2:03 AM, Intransition <transfire@gmail.com> wrote:
> It looks like Array#== and Array#=== are the same. Is that correct?
> That being so, would there be any serious consequences if Array#===
> were redefined to mean "set equality", e.g.
>
>  [:a,:b] === [:b,:a]    #=> true

Well, it breaks existing code that relies on the existing behavior in
case statements (don't know how common that is), and if it was going
to be redefined, I think it would be more natural to have it reflect
inclusion (like Range#===) so that, e.g.,

[:a,:b] === :a #=> true

OTOH, if you need something that is initialized by a value of a
particular type but provides special match semantics, its pretty
trivial to do; given the number of different things people might want
#=== to do on a particular class, and existing code that depends on
the existing behavior, its usually probably best not to change #===
for existing (especially core) classes, but just to define matcher
classes as needed. E.g.,

class OrderInsensitiveArrayMatcher
  def initialize(array)
    @array = array.dup
  end
  def ===(other)
    identity = ->(x) {x}
    @array.group_by(&identity) == other.group_by(&identity)
  rescue
    nil
  end
end

require 'set'

class SetMatcher
  def initialize(enum)
    @set = Set.new(enum)
  end
  def ===(other)
    @set == other.to_set
  rescue
     nil
  end
end

etc.

If there are common use cases, libraries of useful matchers could be
built so that everyone isn't reinventing the wheel, but without
breaking existing code.