Freeze Your Constants in Ruby

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Years ago, I worked on a very large Ruby on Rails codebase that used constants to hold lists of credit card transaction states. For example:

class Txn
  ACTIONABLE_STATES = [:authenticated, :to_settle]
  DONE_STATES = [:settled, :declined]
end

However, we had a bug where a settled transaction would return true when txn.state.in? ACTIONABLE_STATES was called on it.