Is it a hash? Or is it a hash?

hash = {"Idiosyncratic" => "Ruby"}
hash["Idiosyncratic"] # => "Ruby"

idiosyncratic_in_variable = "Idiosyncratic"
hash[idiosyncratic_in_variable] # => nil
hash["Idiosyncratic"] # => "Ruby"

Hash#compare_by_identity changes the semantics of what is equal in a hash and what not. Only if exact the same object is passed in, the value will be retrieved.

Please note: The above example works for the key of"Idiosyncratic", because every string literal returns the same object. This behaviour has been introduced with Ruby 2.2, so running it in 2.1 or lower will return nil for the last line.

Hash Surprise

Identity hashes can feel very unnatural as the following snippet illustrates:

hash = {}.compare_by_identity


hash[a] = "Ruby1"
hash[b] = "Ruby2"
hash[c] = "Ruby3"

hash #=> {"Idiosyncratic"=>"Ruby1",
     #    "Idiosyncratic"=>"Ruby2",
     #    "Idiosyncratic"=>"Ruby3"}

# Quiz: What is the result of: hash["Idiosyncratic"]

Explicit Identity Hashes

What can be done to improve readability and reduce surprise? A starting point can be using an extra class:

class IdentityHash < Hash
  def initialize(*)

  def inspect
    "#<IdentityHash: #{super}>"

For this class to be really useful it would need some more Hash-like features, like ActiveSupport does with HashWithIndifferentAccess. Still, it is already less confusable than some hash that looks like a normal hash and that you haven't checked compare_by_identity? for.

More Idiosyncratic Ruby