mutable_hash_keys.rb

## Mutable objects as hash keys: probably a bad idea.

silly_array = ['fox', 'cow', 'moose']
another_ref = silly_array

# These variable names refer to the same object
silly_array.object_id == another_ref.object_id  # => true 

# Even though it doesn't seem like a great idea, 
# ruby will let us use mutable objects, like 
# arrays, as keys in a hash.
silly_hash = { silly_array => 3 }

# Both variable names can be used as keys
silly_hash[silly_array]  		# => 3
silly_hash[another_ref]  		# => 3

# But other instances of array with equivalent values can
# also be used to access the hash, so the hash doesn't 
# appear to care about the specific object being rerefenced.
imposter_array = ["fox", "cow", "moose"]
imposter.object_id == silly_array.object_id  # => false
silly_hash[imposter] # => 3   ...still works fine.

# But bad things happen when we mutate the key object, 
# and the key is changed in a way that suggests the specific 
# object referenced as the hash key does have some role in 
# the hash implementation.
silly_array.pop 	# => "moose"
silly_array 		# => ["fox", "cow"]
silly_hash 		# => {["fox", "cow"]=>3}
														
silly_hash[silly_array] 	# => nil
silly_hash.keys 		# => [["fox", "cow"]]
silly_hash[["fox", "cow"]] 	# => nil

# It seems we can no longer access the elements in the 
# hash, by object reference or by value. Damn.

# Addendum: Turns out, there is a method that can help
# us recover from this problem.
silly_hash.rehash		# => {["fox", "cow"]: 3}
silly_hash[["fox", "cow"]] 	# => 3    ...works again
silly_hash[silly_array]  	# => 3