contracts.ruby :: Non-type checking example

x.rb

# Classical defensive programming
def do_something_useful(active_user)
  return unless active_user.last_action_date < 2.weeks.ago
  # do something useful with active user
end

# ----

# With contracts:
Contract ActiveUser => Any
def do_something_useful(active_user)
  # do something useful with active user, but it is already a guarantee, that it is active, you don't need to guard against that
end

# Definition of ActiveUser contract (it is not a class of active_user)
class ActiveUser
  def self.valid?(user)
    active_user.last_action_date < 2.weeks.ago
  end
end

With contracts it will look very elegantly