ruby cookbook -- Using an Array or Other Modifiable Object as a Hash

Recipe 5.5. Using an Array or Other Modifiable Object as a Hash Key
Suppose you want a reliably hashable Array class. If you want this behavior universally, you can reopen the Array class and redefine hash to give you the new behavior. But it's safer to define a subclass of Array that implements a reliable-hashing mixin, and to use that subclass only for the  Arrays you want to use  as hash keys:

module ReliablyHashable
def hash
    return object_id
  end
end

class ReliablyHashableArray < Array
  include ReliablyHashable
end

It's now possible to keep track of the jewels:

coordinates = ReliablyHashableArray.new([10,5])
treasure_map = { coordinates => 'jewels' }
treasure_map[coordinates]                       # => "jewels"

# Add a z-coordinate to indicate how deep the treasure is buried.
coordinates.push(-5)

treasure_map[coordinates]                       # => "jewels"

Ruby performs hash lookups using not the key object itself but the object's hash code (an integer obtained from the key by calling its hash method). The default implementation of hash, in Object, uses an object's internal ID as its hash code. Array, Hash, and String override this method to provide different behavior.