gistfile1.rb

class CalculatesTax
  include LightService::Organizer

  def self.for_order(order)
    with(:order => order).reduce(
        [FetchesTaxRangesAction, FetchesCachedTaxRangesAction],
        LooksUpTaxPercentageAction,
        CalculatesOrderTaxAction,
        ProvidesFreeShippingAction
        fallback: GenericErrorHandler
      )
  end
end

class GenericErrorHandler
  include LightService::Action
  expects :errors
  promises :error_messaging

  executed do |context|
    error_messaging = errors.map do |error|
      log(:error, error.message, error.backtrace)
      error.message
    end.join("; ")
  end
end

class FetchesTaxRangesAction
  include LightService::Action
  expects :order
  promises :tax_ranges

  executed do |context|
    tax_ranges = TaxRange.for_region(context.order.region) # Raises exceptions when things go wrong
  end
end

class FetchesCachedTaxRangesAction
  include LightService::Action
  expects :order
  promises :tax_ranges

  executed do |context|
    tax_ranges = CachedTaxRanges.for_region(context.order.region) # Raises exceptions when things go wrong
  end
end

class LooksUpTaxPercentageAction
  include LightService::Action
  expects :order, :tax_ranges
  promises :tax_percentage

  executed do |context|
    tax_percentage = tax_ranges.for_total(order.total) # Raises an exception when there are no ranges, or no applicable range for the total
  end
end

class CalculatesOrderTaxAction
  include ::LightService::Action
  expects :order, :tax_percentage

  executed do |context|
    order.tax = (order.total * (tax_percentage/100)).round(2)
  end
end

class ProvidesFreeShippingAction
  include LightService::Action
  expects :order

  executed do |context|
    if order.total_with_tax > 200
      # Let's say the bang indicates a method that may raise an exception, such as:
      # FreeShippingNotAllowedInRegion
      order.provide_free_shipping!
    end
  end
end