Alternate pattern for Shock costing from Sandi Metz's GoGaRuCo Talk

shock.rb

class Shock
  def intialize(type)
    @type = type
  end

  def cost
    coster_for(type).compute
  end

private

  def coster_for(type)
    coster_implementations.fetch(type) do
      raise ArgumentError, "Unrecoganized cost type: #{type}"
    end.new
  end

  def self.coster_implementations
    @coster_implementations ||= {}
  end

  class BaseCoster
    def self.register_as(type)
      Shock.coster_implementations[type] = self
    end
  end

  class FrontShockCoster < BaseCoster
    def compute
    end

    register_as :front
  end

  class RearShockCoster < BaseCoster
    def compute
    end

    register_as :rear
  end

  class LeftyShockCoster < BaseCoster
    def compute
    end

    register_as :lefty
  end
end